Principles of infectious disease control: overview
Infectious diseases remain a leading cause of morbidity, disability, and mortality worldwide. Lower respiratory infections are the third leading causes of death worldwide (World Health Organization (WHO) 2008). Their control is a constant challenge that faces health workers and public health officials in both industrialized and developing countries. Only one infectious disease, smallpox, was eradicated and stands as a landmark in the history of the control of infectious diseases. The international community is now well down the path towards eradication of poliomyelitis and dracunculiasis (Guinea worm infection). Other infectious diseases, like malaria and tuberculosis, foiled eradication attempts or control efforts and are re-emerging as increasing threats in many countries, including both developing and developed countries. Some infectious diseases, such as tetanus, will always be a threat if control measures are not maintained. Newer infectious diseases, like AIDS, demonstrate the truth of McNeill’s statement that infectious disease will remain ‘one of the fundamental parameters and determinants of human history’. The history of infectious diseases is an exciting story in itself and readers interested in the subject are referred to McNeill (1976) or the comprehensive work on the history of human diseases (Kiple 1993).
This chapter provides a global and comprehensive view of infectious disease control through examination of the magnitude of disease burden, the chain of infection (agent, transmission, and host) of infectious diseases, the varied approaches to their prevention and control, and the factors conducive to their eradication as well as emergence and re-emergence. Although this chapter provides many examples of infectious diseases that illustrate modes of transmission and approaches to infectious disease control, this chapter does not attempt to be comprehensive in listing all infectious diseases. Detailed recommendations on control measures for any specific disease are outlined periodically in the updated reports of the American Public Health Association, Control of Communicable Diseases Manual (Heymann 2010), the comprehensive two-volume work Mandell, Douglas and Bennett’s Principles and Practice of Infectious Diseases (Mandell et al. 2009), and the textbook Infectious Diseases (Gorbach et al. 2004). For readers specifically interested in paediatric infectious diseases there is the comprehensive two-volume Feigin and Cherry’s Textbook of Pediatric Infectious Diseases (Feigin et al. 2009); for infectious diseases in emergency medicine settings there is the textbook Infectious Diseases: Emergency Department Diagnosis & Management (Red and White Emergency Medicine Series) (Slaven et al. 2006); and for tropical infectious diseases there is Tropical Infectious Diseases: Principles, Pathogens, and Practice (Guerrant et al. 2011). A comprehensive treatment of the worldwide distribution of infectious diseases is provided in Atlas of Human Infectious Diseases (Wertheim et al. 2012). The Centers for Disease Control and Prevention (CDC) publishes up-to-date disease surveillance information for the United States and recommendations for control measures in the Morbidity and Mortality Weekly Reports and provides annual summaries of notifiable infectious diseases in the Summary of Notifiable Diseases, United States (CDC 2011). Many other countries have similar types of publications. The WHO publishes worldwide surveillance information and recommendations for control measures in the Weekly Epidemiological Record. A more detailed background on infectious agents as determinants of health and disease is provided in Chapters 8.11–8.15.
Definitions of infectious diseases and their control
Infection occurs when an infectious agent enters a body and develops or multiplies. Infectious agents are organisms capable of producing inapparent infection or clinically manifest disease and include bacteria, rickettsia, chlamydiae, fungi, parasites, viruses, and prions. An infectious disease, or communicable disease, is an infection that results in clinically manifest disease. Infectious disease may also be due to the toxic product of an infectious agent, such as the toxin produced by Clostridium botulinum causing classical botulism. As this is a textbook of public health, the infectious diseases considered are those that manifest in human hosts and are a result of the interaction of people, animals, and their environment. Infectious diseases may be due to infectious agents exclusively found in human hosts such as rubella virus, in the environment such as Legionella pneumophila, or primarily in animals such as Brucella abortus.
Control of infectious diseases refers to the actions and programmes directed towards reducing disease incidence (new infections), reducing disease prevalence (infections in the community at any given point in time), or completely eradicating the disease. Control aimed at reducing the incidence of infectious disease or their risk factors can be considered as primary prevention of infectious disease. Primary prevention protects health through individual and community-wide measures, including such actions as maintaining good nutritional status, keeping physically fit, immunizing against infectious diseases, providing safe water, and ensuring the proper disposal of faeces.
Control aimed at reducing the prevalence by shortening the duration of infectious disease can be considered as secondary prevention of infectious disease. Secondary prevention corrects departures from good health through individual and community-wide measures, including such actions as screening that result in early detection of disease, prompt antibiotic treatment, and ensuring adequate nutrition. It should be noted that such control efforts in secondary prevention in a group of infected individuals may also result in primary prevention in uninfected people. For example, prompt and specific drug therapy for tuberculosis patients resulting in sputum conversion to culture-negative status renders them no longer a source of infection to others and treatment of HIV-positive pregnant women reduces transmission of HIV to their newborns.
Control aimed at reducing or even eliminating long-term impairments of infectious disease can be considered as tertiary prevention of infectious disease. Tertiary prevention reduces or eliminates disabilities, minimizes suffering, and promotes adjustment to permanent disabilities through such actions as providing orthopaedic appliances and its associated rehabilitation for victims of poliomyelitis, counselling and vocational training, and prevention of opportunistic infections. For example, the prevention of opportunistic infections in HIV infection can be considered as tertiary prevention (Osterholm and Hedberg 2005).
Global burden of infectious diseases
A WHO analysis of the global burden of disease estimates that infectious diseases caused 12.3 million deaths, accounting for 21 per cent of total global mortality of 56.8 million deaths in 2008 (WHO 2008). Five diseases—lower respiratory infections, AIDS, diarrhoeal diseases, tuberculosis, and malaria—account for some 81 per cent of the total infectious disease burden. Most infectious disease (some 97 per cent) deaths occur in the economically developing group of countries where approximately a quarter of deaths are due to an infectious cause (WHO 2008). However, infectious diseases are also of importance in more developed countries. In the United States, for example, influenza still ranks as an important cause of death.
The current magnitude of morbidity and mortality due to infectious diseases worldwide is highlighted by the WHO as follows (WHO 2012a):
◆ Acute lower respiratory infections, including pneumonia and influenza, cause some 447 million episodes of illness each year and result in 3.5 million deaths annually. These infections are the highest cause of infant and child mortality in developing countries.
◆ HIV newly infects about 2.7 million people each year and causes an estimated 1.8 million AIDS-related deaths annually, down from a peak of 2.2 million that occurred in the mid 2000s. There are now about 34 million people living with HIV as of the end of 2011 (Joint United Nations Programme on HIV/AIDS 2011).
◆ Diarrhoeal diseases are also a major cause of morbidity and mortality in infants and children in developing countries. Each year there are some 4.6 billion episodes with 2.5 million deaths due to diarrhoeal disease, of which the vast majority are among children under 5 years of age.
◆ Tuberculosis, caused by Mycobacterium tuberculosis, infects about one-third of the world’s population. It is estimated that there are approximately 8.8 million people who develop clinical disease and 1.1 million who die of tuberculosis each year.
◆ Malaria is estimated to cause some 241 million cases of acute illness. An estimated 827,000 deaths per year worldwide are attributable to malaria, mainly in children under the age of 5 years.
◆ Influenza epidemics, caused by influenza A and B viruses, are estimated to kill from 500,000 to 1 million people worldwide each year. Influenza pandemics, of which there were 3 in the last century, have killed as many as 40–50 million people (as occurred during the 1918–1919 ‘Spanish flu’ pandemic) and, as with the current concerns regarding avian influenza (H5N1), continue to threaten to re-emerge as a major public health emergency.
Chain of infection: agent, transmission, and host
The chain of infection is the relationship between an infectious agent, its routes of transmission, and a susceptible host. The prevention and control of infectious diseases depend upon the interaction of these three factors that may result in the human host clinically manifesting disease.
The infectious agent is the first link in the chain of infection and is any microorganism whose presence or excessive presence is essential for the occurrence of an infectious disease. Examples of infectious agents include the following:
◆ Bacteria: for example, spirochetes and curved bacteria such as Borrelia burgdorferi causing Lyme disease; Gram-negative rods such as Yersinia pestis causing plague; Gram-positive cocci such as Streptococcus pyogenes, group A causing erysipelas; and Gram-positive rods such as Mycobacterium tuberculosis causing tuberculosis.
◆ Rickettsiae: for example, Rickettsia ricketsii causing Rocky Mountain spotted fever, and Rickettsia prowazekii causing epidemic louse-borne typhus fever.
◆ Chlamydiae: for example, Chlamydia psittaci causing psittacosis (ornithosis), and Chlamydia trachomatis causing trachoma and genital infections.
◆ Fungi: for example, Trichophyton schoenleinii and Microsporum canis causing tinea capitis (ringworm of the scalp), and Trichophyton rubrum, Trichophyton mentagrophytes, and Epidermophyton floccosum causing tinea pedis (ringworm of the foot, or athlete’s foot).
◆ Parasites: for example, helminths such as Trichinella spiralis causing trichinosis, filaria such as Brugia malayi causing filariasis (which may lead to the symptom of elephantiasis), nematodes such as Enterobius vermicularis causing enterobiasis (pinworm disease), trematodes such as Clonorchis sinensis causing clonorchiasis (oriental liver fluke disease), cestodes such as Taenia solium causing taeniasis (beef tapeworm disease), and protozoa such as Trypanosoma cruzi causing American trypanosomiasis (Chagas’ disease).
◆ Viruses: for example, Paramyxoviridae such as measles virus which causes measles, Togaviridae such as rubella virus which causes rubella, Picornaviridae such as the hepatovirus which causes hepatitis A, and arthropod-borne viruses (arboviruses) such as dengue viruses which cause dengue fever.
◆ Prions: small proteinaceous infectious particles which cause such diseases as kuru, Creutzfeldt–Jakob disease (and its variant associated with exposure of humans to the bovine spongiform encephalopathy, or ‘mad cow’, agent), and the Gerstmann–Straussler–Scheinker syndrome.
There is increasing evidence that some infectious agents, often with cofactors, are associated with human tumours. Examples include: Schistosoma haematobium with bladder cancer, Schistosoma japonicum with colorectal cancer, Clonorchis sinensis with cholangiocarcinoma, hepatitis B and C viruses with hepatocellular carcinoma, Helicobacter pylori with gastric cancer, and human papillomaviruses with cervical cancer. Cancers attributed by the International Agency for Research on Cancer (IARC) to infectious agents are considered to account for some 22 per cent of cancer deaths in the developing world and 6 per cent in industrialized countries (WHO 2003).
Agents can be described by their ability to infect (infectivity), be transmitted (infectiousness), and cause disease (pathogenicity) as well as their ability to cause serious disease (virulence). The infectivity of an infectious agent is the extent to which the agent can enter, survive, and multiply in a host and is measured by the ratio of the number of people who become infected to the total number exposed to the agent. Rhinoviruses that can cause the common cold are examples of infectious agents with high infectivity.
The infectiousness, or transmissibility, of an infectious agent is the extent to which the agent can be transmitted from one host to another. It is measured by the basic reproduction number (known as R0) which is determined by the number of secondary infections in a susceptible population that result from a primary infection. Epidemic-prone diseases such as measles and pertussis caused by measles virus and Bordatella pertussis bacteria, respectively, are examples of diseases due to highly infectious agents with an R0 of 10 or greater.
The pathogenicity of an infectious agent is the extent to which clinically manifest disease is produced in an infected population and is measured by the ratio of the number of people developing clinical illness to the total number infected. Examples of highly pathogenic infectious agents are the measles virus and the human (α) herpesvirus 3 (varicella-zoster) causing measles and chickenpox, respectively, in which most infected susceptible people will manifest disease.
The virulence of an infectious agent is the extent to which severe disease is produced in a population with clinically manifest disease. It is the ratio of the number of people with severe and fatal disease to the total number of people with disease. An example of a highly virulent infectious agent is HIV whereby nearly all untreated people with AIDS will die.
Characteristics of infectious agents that affect pathogenicity include their ability to invade tissues (invasiveness), produce toxins (intoxication), cause damaging hypersensitivity (allergic) reactions, undergo antigenic variation, and develop antibiotic resistance. An example of an infectious agent with high invasiveness is the Shigella organism which can invade the submucosal tissue of the intestine and cause clinically manifest shigellosis (bacillary dysentery). An example of an infectious agent that has a high degree of ability to produce toxins is the Clostridium botulinum organism which can elaborate toxins in inadequately prepared food and cause classical botulism. An example of an infectious agent that is highly allergenic is the Mycobacterium tuberculosis organism which can cause tuberculosis. An example of an infectious agent that has a high degree of antigenic variation is the type A influenza virus which frequently experiences minor antigenic changes—antigenic ‘drift’. Influenza A viruses, on an irregular basis, may also undergo a major antigenic change creating an entirely new subtype—antigenic ‘shift’. Antigenic shifts that have the characteristic of high transmissibility between people may result in an influenza pandemic when large numbers of individuals are exposed to the new subtype for which they have no prior immunity. An example of an infectious agent that can develop antibiotic resistance that challenges control efforts is Neisseria gonorrhoeae that has both chromosomally mediated and resistance transfer plasmid-mediated genetic factors for antibiotic resistance.
The infective dose of an infectious agent is the number of organisms needed to cause an infection. The infective dose may vary depending upon the route of transmission and host susceptibility. An example of an infectious agent that needs only a very low infective dose (as few as ten organisms) is Escherichia coli O157:H7 which causes enterohaemorrhagic diarrhoea. An example of an infectious agent that needs a relatively higher infective dose is Vibrio cholerae which requires around 100 million bacteria to be ingested to cause cholera disease in a healthy adult.
Control measures for infectious diseases directed at inactivating the agent are designed according to the type of agent and its reservoirs and sources. For example, an agent like Vibrio cholerae can be inactivated through adequate chlorination of the water supply. This is a chemical method for provision of safe water to control cholera. An agent like hepatitis B virus can be inactivated through adequate autoclaving of injection and surgical equipment. This is a sterilization method to control hepatitis B. Details of these and other methods of control directed at the agent are provided in the sections in this chapter on control measures applied to the agent and the environment.
Routes of transmission
Control efforts are often designed to break the routes of transmission, the mechanisms by which infectious agents are spread from reservoirs or sources to human hosts. A reservoir of an infectious agent is any person, other living organism, or inanimate material in which the infectious agent normally lives and grows. The source of infection for a host is the person, other living organism or inanimate material from which the infectious agent came. Horizontal transmission refers to transmission between individuals whereas vertical transmission refers to the specific situation of transmission between parent to offspring (e.g. transplacentally in utero, during passage through the birth canal, or through breast milk). The routes of transmission have been summarized in the Control of Communicable Diseases Manual (Heymann 2010) as follows:
Direct and essentially immediate transfer of infectious agents to a receptive portal of entry through which human or animal infection may take place. This may be by direct contact such as touching, biting, kissing or sexual intercourse, or through direct projections (droplet spread) of droplet spray onto the conjunctiva or onto the mucous membranes of the eye, nose or mouth during sneezing, coughing, spitting, singing or talking (risk of transmission in this manner is usually limited to a distance of about 1 meter or less from the source of infection). Direct transmission may also occur through direct exposure of susceptible tissue to an agent in soil, through the bite of a rabid animal, or trans-placentally. (Heymann 2010)
Contaminated inanimate materials or objects (fomites) such as toys, handkerchiefs, soiled clothes, bedding, cooking or eating utensils, surgical instruments or dressings; water, food, milk, and biological products including blood, serum, plasma, tissues or organs; or any substance serving as an intermediate means by which an infectious agent is transported and introduced into a susceptible host though a suitable portal of entry. The agent may or may not have multiplied or developed in or on the vehicle before being transmitted. (Heymann 2010)
Mechanical: includes simple mechanical carriage by a crawling or flying insect through soiling of its feet or proboscis, or by passage of organisms through its gastrointestinal tract. This does not require multiplication or development of the organism.
Biological: propagation (multiplication), cyclic development, or a combination of these (cyclopropagative) is required before the arthropod can transmit the infective form of the agent to humans. An incubation period (extrinsic) is required following infection before the arthropod becomes infective. The infectious agent may be passed vertically to succeeding generations (transovarian transmission); trans-stadial transmission indicates its passage from one stage of the life cycle to another, as from nymph to adult. Transmission may be by injection of salivary gland fluid during biting, or by regurgitation or deposition on the skin of feces or other material capable of penetrating through the bite wound or through an area of trauma, often created by scratching or rubbing. This transmission is by an infected nonvertebrate host and not simple mechanical carriage by a vector as a vehicle. An arthropod in either role is termed a vector. (Heymann 2010)
The dissemination of microbial aerosols to a suitable portal of entry, usually the respiratory tract. Microbial aerosols are suspensions of particles in the air consisting partially or wholly of microorganisms. They may remain suspended in the air for long periods of time, some retaining and others losing infectivity or virulence. Particles in the 1- to 5-micrometer range are easily drawn into the alveoli of the lungs and may be retained there. Not considered as airborne are droplets and other large particles that promptly settle out (see ‘Direct transmission’).
Droplet nuclei: usually the small residues that result from evaporation of fluid from droplets emitted by an infected host (see above). They also may be created purposely by a variety of atomizing devices, or accidentally as in microbiology laboratories, abattoirs, rendering plants or autopsy rooms. They usually remain suspended in the air for long periods.
Dust: the small particles of widely varying size that may arise from soil (e.g., fungus spores), clothes, bedding, or contaminated floors. (Heymann 2010)
Control measures for infectious diseases directed at interrupting transmission are designed according to the type of transmission for the agent. Direct transmission of an agent like Neisseria gonorrhoeae, for example, can be reduced by using condoms as a barrier method of control of gonorrhoea. Vector-borne transmission of an agent like Plasmodium falciparum can be reduced by using a residual insecticide against Anopheles mosquitoes as a chemical vector control method for malaria. Airborne transmission of an agent like Mycobacterium tuberculosis from sputum-positive pulmonary tuberculosis patients in hospital can be reduced by the use of special ventilation in the patient’s room as an environmental method of control of tuberculosis. It should be recognized that some infectious agents may have more than one route of transmission. Poliovirus, for example, can be spread via direct transmission through the faecal–oral route and pharyngeal spread, or indirect transmission through contaminated food or other ma- terials. Details of these and other methods of control directed at interrupting transmission are provided in the section on ‘Tools for control of infectious diseases’ in this chapter.
The human host is the final link in the chain of infection. The infectious agent may enter the host through the following portals of entry.
◆ Respiratory tract: infectious agents can be inhaled into the respiratory tract and will be deposited at different levels of the pulmonary tree according to the size of the aerosol, droplet nuclei, or dust particles. For example, Mycobacterium tuberculosis in airborne droplet nuclei, 1–5 μm in diameter, may be inhaled into the alveoli of the lungs of a vulnerable host and result in tuberculosis.
◆ Intact skin: some infectious agents, such as Necator americanus which causes hookworm disease, can penetrate the intact skin.
◆ Gastrointestinal tract: an infectious agent, such as Vibrio cholerae which causes cholera, may enter via the gastrointestinal tract. People who have a compromised gastric function, such as gastric achlorhydria, may be at increased risk of disease.
◆ Mucous membranes: infectious agents, such as measles viruses, may be deposited on mucous membranes, including the conjunctiva of the eye, by droplet spread or by direct contact with infected people or contaminated objects.
◆ Genitourinary system: some infectious agents, such as Escherichia coli which causes urinary tract infections, can enter the urinary tract via an ascending route from the urethra colonized with the organism. Structural abnormalities of the urinary tract and procedures such as urinary catheterization may predispose the host to disease. Sexually transmitted infectious agents, such as Neisseria gonorrhoeae which causes gonorrhoea, may enter the male or female genital orifices.
◆ Placenta: certain infectious agents, such as rubella virus which causes congenital rubella syndrome, can cross the placenta resulting in transplacental transmission, a direct route of transmission from the mother to the fetus that is a form of vertical transmission.
Infectious agents also enter the host though mechanisms that get past the body’s natural barriers, including wounds that break the integrity of the skin or mucous membranes; invasive procedures, parenteral injections, parenteral infusions, blood transfusions, or organ transplants that may introduce an agent into the body; or insect vectors that may inject agents through the skin.
The most important host factors regarding developing clinically manifest disease and the severity of disease are immune status and age. Infants, young children, and the elderly are at generally higher risk from more severe disease due to immaturity or deterioration of their immune systems, respectively. Host co-infections or other health conditions that diminish the immune status, for example, HIV infection or malnutrition, respectively, are also risk factors.
Many host defence mechanisms help prevent infection or disease. Non-specific host defence mechanisms include the intact skin (by providing a natural barrier), sinus and nasal cilia (by moving mucus and particles towards the nasal cavity), tears, saliva, and mucus (by preventing drying and containing lysozymes and other components with antimicrobial properties), and gastric acid (by the antimicrobial and antiparasitic properties of hydrochloric acid). Specific host defence mechanisms include naturally acquired immunity from previous infection, transplacentally acquired passive immunity in the newborn from the mother, artificially acquired active immunity from immunization, and artificially acquired passive immunity from immunoglobulins and antitoxins.
Host responses to infection that prevent or reduce the severity of infectious disease include: (1) polymorphonuclear leucocytosis stimulated by some bacterial infections that increases the number of phagocytic white blood cells, (2) fever that may slow the multiplication of some infectious agents, (3) antibody production that may neutralize some infectious agents or their toxins, (4) interferon production that may block intracellular replication of viruses, and (5) cytotoxic immune cell responses that kill cells infected with viruses
The manifestation of infection in the host may vary from inapparent (subclinical) infection to severe disease that may even result in death. The interaction between an infectious agent, routes of transmission, and host factors determines the spectrum of signs and symptoms. Sometimes the host may become an asymptomatic carrier of the infectious agent and be a source of infection for others.
Control measures for infectious diseases directed at the host are designed according to the immune status of the host and the likelihood of host exposure to certain infectious agents. For example, measles disease can be prevented by active immunization with measles vaccine to develop host immunity. Pneumonic plague can be prevented in those in close contact with patients with plague pneumonia by chemoprophylaxis using tetracycline or sulphonamide. Details of these and other methods of control directed at the host are provided in the section on control measures applied to the host in this chapter.
Tools for control of infectious diseases
The primary concern of infectious disease control in public health, whether in developing or industrialized countries, is the reduction, elimination, or even eradication of infectious disease. This is accomplished by directing control measures to the agent, the routes of transmission, or the host. Such control measures include: (1) identifying and then reducing or eliminating infectious agents at their sources and reservoirs, (2) breaking or interfering with the routes of transmission of infectious agents, and (3) identifying susceptible populations and then reducing or eliminating their susceptibility.
The tools for control of infectious diseases are related to recognition and evaluation of the patterns of diseases and the results of interventions to control them. In infectious disease control, the most important tool for recognition and evaluation is surveillance of disease defined as:
the process of systematic collection, orderly consolidation and analysis, and evaluation of pertinent data with prompt dissemination of the results to those who need to know them, and particularly those who are in a position to take action. It includes the systematic collection and evaluation of:
1. Morbidity and mortality reports
2. Special reports of field investigations of epidemics and of individual cases
3. Isolation and identification of infectious agents by laboratories
4. Data concerning the availability, use and untoward effects of vaccines and toxoids, immune globulins, insecticides and other substances used in control
5. Information regarding immunity levels in segments of the population
6. Other relevant epidemiologic data
A report summarizing the above data should be prepared and distributed to all cooperating persons and others with a need to know the results of the surveillance activities. The procedure applies to all jurisdictional levels of public health from local to international. (Heymann 2010)
Surveillance, therefore, is ‘information for action’. More detailed information on surveillance or on field investigations is given in Chapter 5.19.
Tools for control related to interventions include:
◆ Control measures applied to the host: active immunization, passive immunization, chemoprophylaxis, behavioural change, reverse isolation, barriers, and improving host resistance.
◆ Control measures applied to vectors: chemical, environmental, and biological.
◆ Control measures applied to infected humans: chemotherapy, isolation, quarantine, restriction of activities, and behavioural change.
◆ Control measures applied to animals: active immunization, isolation, quarantine, restriction or reduction, chemoprophylaxis, and chemotherapy.
◆ Control measures applied to the environment: provision of safe water, proper disposal of faeces, food and milk sanitation, and design of facilities and equipment.
◆ Control measures applied to infectious agents: cleaning, cooling, pasteurization, disinfection, and sterilization.
Achieving maximum impact on control of a specific infectious disease may involve more than one of these interventions. For example, the control of hepatitis A infection can be achieved through interventions that may include: active immunization, passive immunization, food preparation and hand-washing behaviours, provision of safe water, food sanitation, and proper disposal of faeces.
The tools for control can also be considered according to the level at which they are applied: individual, institutional, or community based. At the individual level, control measures, usually initiated by a clinician, are directed towards the specific infectious disease threats to the particular individual. Examples include chemoprophylaxis to prevent wound infection, pre-exposure prophylactic immunization against rabies for a veterinarian, and use of diphtheria antitoxin in a patient with diphtheria.
At the institutional level, control measures, usually initiated by the officials of the institution, are directed to a group of people who are in close contact with each other, such as people in day-care centres, schools, military barracks, hospital wards, nursing homes, and correctional facilities. Examples include: (1) administering amantadine hydrochloride or rimantadine for chemoprophylaxis or chemotherapy of influenza A in a high-risk institutional population, (2) quarantining institutionalized young children during a measles outbreak, and (3) hepatitis B immunization of staff and clients of institutions for the developmentally disabled.
At the community level, control measures, usually initiated by local, state or national public health agencies, are directed to the community at large. Examples include childhood immunization programmes, provision of safe water, regulation of food supplies, and recall of contaminated food products.
It should be noted that some control measures, such as immunization, may take place at all levels while others, such as the provision of safe water to a community, are more specifically applied at a particular level.
The tools for the control of infectious diseases and their relationship to the chain of infection are the main focus of the remainder of this chapter.
Control measures applied to the host
Control measures applied to the host range from relatively easily administered immunization to behavioural changes that may be extremely difficult for an individual to accept and practise. This section details the types of control measures applied to susceptible hosts and gives examples of their application in the control of selected infectious diseases.
One of the most efficient control measures applied to a host is one that renders the host immune from infectious disease by an infectious agent. Active immunization is a cornerstone of public health measures for the control of many infectious diseases and is considered one of the most cost-effective methods of individual, institutional, and community protection for vaccine-preventable infectious diseases. The most powerful example of the potential impact of active immunization against an infectious disease is that of smallpox vaccination. Mobilization of political will on a worldwide basis, coupled with full application of the strategies of active surveillance and containment immunization against smallpox, ultimately resulted in the complete global eradication of the disease and cessation of transmission of the infectious agent, variola virus.
Active immunization is usually considered synonymous with the term vaccination, and is the process of administration of an antigen that can induce a specific immune response that protects a susceptible host from an infectious disease. Some draw a distinction between the two terms. Narrowly defined, vaccination is the process of administration of an antigen and immunization is the development of a specific immune response. Administering an antigen without evoking an immune response is possible, since no vaccine is 100 per cent effective. Conversely, someone can become immunized even if they were not administered an antigen. For example, the live, attenuated oral polio vaccine viruses can be transmitted from the recipient to other close contacts.
Active immunization can be accomplished through different types of antigens, including the following:
◆ Inactivated toxins: diphtheria toxoid is an example of a formaldehyde-inactivated preparation of diphtheria toxin that protects against clinically manifest disease, although the immunized person may still become infected with toxin-producing strains of Corynebacterium diphtheria. Tetanus toxoid and Clostridium perfringens toxoid (pig bel vaccine) are other examples of inactivated toxin preparations.
◆ Inactivated complex antigens: whole-cell pertussis vaccine is an example of a heat or chemically treated preparation of killed whole pertussis bacteria that protects against clinically manifest disease, although the immunized person may still become infected with Bordetella pertussis. Inactivated polio vaccine and inactivated influenza vaccine are other examples of inactivated vaccines.
◆ Purified antigens: acellular pertussis vaccine is an example of a vaccine composed of isolated and purified immunogenic pertussis antigens. Other vaccines with purified components include polyvalent capsular polysaccharide pneumococcal, polysaccharide meningococcal, protein-polysaccharide conjugate Haemophilus influenzae type b, and plasma-derived hepatitis B vaccines.
◆ Recombinant antigens: hepatitis B recombinant vaccine is an example of a vaccine composed of hepatitis B surface antigen subunits made through recombinant DNA technology. Human papillomavirus vaccine is another example of a DNA recombinant vaccine.
◆ Live, attenuated vaccines: measles vaccine is an example of a vaccine containing live infectious agents that are of reduced virulence, but induce protective antibodies against measles viruses. Other live, attenuated vaccines include oral polio, mumps, rubella, yellow fever, and bacille Calmette–Guérin (BCG) vaccines.
Protective antibody responses usually take 7–21 days to develop. Although most vaccines must be given before exposure to be effective, some vaccines may protect even if administered after exposure to an infectious agent. For example, measles vaccine may provide protection against measles disease if given within 72 hours of exposure. This occurs since the percutaneous route of administration of the antigen evokes an immune response faster than the measles virus results in disease through the respiratory route of natural exposure.
Duration of protection varies from only months, such as with killed whole-cell cholera vaccine, to years or even life with some live attenuated vaccines, such as measles vaccine. Some inactivated toxoids and vaccines, such as tetanus toxoid, may require a priming series of doses to be optimally effective and additional booster doses to maintain protective antibodies. Many new technologies are being explored that may increase the number and efficacy of vaccines available against infectious disease, including immune stimulating complexes, live viral or bacterial vector vaccines, and timed-release vaccines.
It should be recognized that vaccines vary in their efficacy and no vaccine is 100 per cent effective. Vaccine efficacies vary with type of vaccine, manufacturing techniques, storage and handling conditions, skill of administration, age of vaccination, and other host factors. Vaccines for routine use are safe. However, no vaccine is 100 per cent safe. Potential vaccinees, or their parents or guardians, should be screened for contraindications and be informed of potential side effects.
Immunization schedules for the routine control of infectious diseases preventable by immunization vary between countries and are usually based on expert advice to governments and physicians. For example, recommended policies for immunization in the United States are provided by the Advisory Committee on Immunization Practices (ACIP) and are published in the Morbidity and Mortality Weekly Report (ACIP 2012). In addition, the American Academy of Pediatrics periodically publishes comprehensive immunization recommendations in its Report of the Committee on Infectious Diseases (Committee on Infectious Diseases 2012). At the global level, the WHO publishes recommended immunization schedules and control strategies for vaccine-preventable diseases that are periodically updated by expert advisory groups in the WHO Weekly Epidemiological Record.
In outbreak settings, immunization schedules may be modified. For example, the age of immunization for measles may be lowered to 6 months of age during a measles outbreak. In such situations, people receiving vaccine before the routinely recommended age of immunization should be immunized again at the recommended age since immunization at the earlier age may not have been optimally effective.
Immunization programmes include vaccines for routine child, routine adult, travel, selected high-risk populations, and occupational settings. For example, tetanus toxoid is universally recommended; yellow fever vaccine is only recommended in geographic areas of epidemiological risk (i.e. in certain areas of Africa and South America); typhoid fever vaccine is only recommended for individuals subject to unusual exposure to typhoid, including people living in the same household as known carriers; and anthrax vaccine is only recommended for veterinarians and people occupationally exposed to possibly contaminated industrial raw materials.
Beyond protection of the individual, vaccination may also provide a degree of community protection. This phenomenon is known as herd immunity. Herd immunity is the relative protection of a population group achieved by reducing or breaking the chains of transmission of an infectious agent because a sufficient percentage of the population is immune, that is, resistant to infection through immunization or prior natural infection. When the immunity level in a population becomes such that the effective reproduction number of the disease in that population, R, is less than 1, the disease will eventually die out. Herd immunity is a complex phenomenon and varies according to the infectious agent, its routes of transmission, the degree to which immunization protects against infection versus only clinically manifest disease, and the distribution of immunization in the population. Typically, for childhood vaccine-preventable diseases, the immunity levels in the population to achieve herd immunity range from around 84 per cent (for rubella) to over 90 per cent (for pertussis). The mechanisms of herd immunity are several, including ‘direct protection of vaccinees against disease or transmissible infection and indirect protection of non-recipients by virtue of surreptitious vaccination, passive antibody, or just reduced sources of transmission and, hence, risks of infection in the community’ (Fine 1993).
A particularly difficult problem for vaccine-preventable infectious disease control programmes is complacency by the population that can result from the very successes of the programmes and highlights the continuing need to educate the public and decision-makers. Low rates of vaccine-preventable infectious disease may mistakenly lead parents to consider that vaccination is no longer important for maintaining their children’s health and may result in political leaders reducing funding for immunization programmes. Low disease rates may also focus undue attention on the relatively rare serious side effects of vaccination in relation to current rates of disease. Such side effects should only be compared in relation to rates of disease and its complications that would occur without immunization programmes.
A comprehensive treatment of active immunization is given by Plotkin et al. (2008).
Passive immunization is a temporary immunity in a host due to the protection afforded by antibody produced in another host. Passive immunity may be acquired either naturally or artificially.
Naturally acquired passive immunity is achieved through transfer of maternal antibodies via the placenta. It is the way that newborn infants are provided with a temporary immunity against many infectious diseases for which the mother is immune. This immunity wanes over time and eventually leaves the infant susceptible to these diseases.
An important use of transplacental immunity as a control measure is in the prevention of tetanus neonatorum (neonatal tetanus) by immunization of women before or during pregnancy with tetanus toxoid. This is especially important in developing countries where the disease typically occurs when the umbilical cord is cut with an unclean instrument contaminated with tetanus spores or when substances contaminated with tetanus spores are placed on the umbilical stump after delivery. Control by active immunization of the infant cannot be achieved in sufficient time since the average incubation period is only 6 days (with a range from 3 to 28 days). An adequately immunized mother, however, will usually effectively transfer maternal antibodies against tetanus across the placenta to her newborn and prevent tetanus neonatorum.
Another example of naturally acquired passive immunity is the relative protection against measles disease in a young infant born to a mother who previously had the disease. Typically, such infants are immune for approximately 6–9 months or more after birth, depending upon how many residual maternal antibodies are present at the time of pregnancy. Other diseases for which there is usually an effective transplacental immunity in infants, for variable amounts of time, include diphtheria, mumps, poliomyelitis, rubella, and varicella (chickenpox). It should be noted that if the mother is not immune, or if residual maternal antibodies have significantly waned, then the infant may be susceptible to disease.
Research is ongoing as to other infectious diseases that may be preventable in the neonate or infant through immunization of the mother before or during pregnancy. Examples include Haemophilus influenzae type b and group B streptococcal and meningococcal diseases (Insel et al. 1994). Many diseases, however, are not prevented by transplacental immunity.
Breastfeeding is a form of naturally acquired passive antibody transfer to neonates and infants. Breast milk and colostrum contain secretory immunoglobulin A (IgA) antibodies that may play a protective role in the prevention of infections with such agents as respiratory syncytial virus, rotavirus, and Haemophilus influenzae type b.
Artificially acquired passive immunity is acquired through administration of an antibody-containing preparation, antiserum, or immune globulin. It has a place in the control of certain infectious diseases in special situations. This immunity also wanes over a relatively short period of time.
Examples of the use of artificially acquired passive immunity to control infectious disease include the following:
◆ Rabies: natural immunity to rabies in humans does not exist. Susceptible individuals bitten by an animal known or suspected to be rabid should receive rabies immune globulin to neutralize the rabies virus in the wound. It should be noted that, besides passive immunization with rabies immune globulin, such individuals should also receive active immunization with rabies vaccine.
◆ Hepatitis A: in areas where sanitation is poor, hepatitis A infection commonly occurs at an early age and therefore most adults in developing countries are already immune. However, epidemics may occur in industrialized countries. Passive immunization with immune globulin may be given to: (1) all household and sexual contacts of patients with hepatitis A, (2) other food handlers in an establishment where hepatitis A has occurred in a food handler, (3) all individuals in an institution where a focal outbreak of hepatitis A has occurred, and (4) people from industrialized countries travelling to highly endemic areas. It should be noted that vaccines for active immunization for hepatitis A are now available.
◆ Diphtheria: treatment of this disease is an example of the use of an antibody containing product (diphtheria antitoxin) produced in an animal (only diphtheria antitoxin from horses is available) administered as part of the treatment regimen for secondary prevention of disease. In suspected cases of diphtheria, the antitoxin must be given as soon as possible because it is only effective in neutralizing diphtheria toxins not yet bound to cells.
◆ Other important infectious diseases, including hepatitis B, measles, tetanus, varicella: depending upon the circumstances of exposure, susceptibility of the host, and status of the host’s general immune system there are circumstances under which hepatitis B immune globulin, tetanus immune globulin, varicella-zoster immune globulin, or immune globulin may be warranted.
Chemoprophylaxis is the prevention of infection or its progression to clinically manifest disease through the administration of chemical substances, including antibiotics. Chemoprophylaxis can also consist of the treatment of a disease to prevent complications of that disease. Chemoprophylaxis may be specifically directed against a particular infectious agent or it may be non-specifically directed against many infectious agents. The use of antibiotics before surgical procedures is an example of non-specific chemoprophylaxis to prevent wound infections in the postoperative period. Examples of specific chemoprophylaxis are given below.
The use of chemoprophylaxis to prevent development of infection is illustrated by using chloroquine to prevent malarial parasitaemia caused by Plasmodium vivax, Plasmodium ovale, Plasmodium malariae, and chloroquine-sensitive strains of Plasmodium falciparum. For some chloroquine-resistant strains of Plasmodium falciparum, alternative regimens include mefloquine alone, doxycycline alone, primaquine alone, or an atovaquone/proguanil combination primaquine may be given to reduce the risk of a relapse from intrahepatic forms of Plasmodium vivax and Plasmodium ovale after discontinuation of chemoprophylaxis with any chemosuppressive drugs other than primaquine. Determination of a specific malaria chemoprophylactic regimen is complex. It must take into account the geographic area, the possibility of pregnancy, the weight of an individual (dose size for children is determined by body weight), and the risks of adverse reactions to the chemoprophylactic regimen.
Other examples of prevention of development of infection include the following:
◆ The use of silver nitrate, erythromycin, or tetracycline instilled into the eyes of a newborn to prevent gonococcal ophthalmia by transmission of Neisseria gonorrhoeae from an infected mother during birth.
◆ The use of tetracycline, sulphonamides (including sulfadiazine and trimethoprim-sulfamethoxazole), chloramphenicol, or streptomycin in close contacts of confirmed or suspected cases of plague pneumonia to prevent plague pneumonia by transmission of Yersinia pestis.
◆ The use of benzathine penicillin in those in sexual contact with confirmed cases of early syphilis to prevent syphilis by transmission of Treponema pallidum.
An example of the use of chemoprophylaxis to prevent the progression of an infection to active manifest disease is the use of isoniazid (INH) to prevent the progression of latent infection with Mycobacterium tuberculosis to clinical tuberculosis. People less than 35 years of age who are tuberculosis test positive should receive INH to prevent clinical tuberculosis. The decision to use INH, especially in individuals more than 35 years of age (who are at higher risk of clinical hepatitis from the use of the drug), must be determined based on such information as length of infection, closeness of association with a current case, status of the immune system, presence of acute liver disease, possibilities of pregnancy, and risks of adverse reactions.
Other examples of prevention of progression of an infection to active manifest disease through the use of chemoprophylaxis include the following:
◆ Co-trimoxazole or pentamidine to prevent subclinical latent infection with Pneumocystis jirovecii (formerly carinii) from progressing to clinically manifest Pneumocystis pneumonia in immunosuppressed people such as HIV-infected individuals.
◆ Mebendazole, albendazole, or pyrantel pamoate to prevent infection with Necator americanus, Ancylostoma duodenale, and Ancylostoma ceylanicum from progressing to the clinically manifest anaemia of hookworm disease.
◆ Pyrimethamine combined with sulfadiazine and folinic acid (to avoid possible bone marrow depression) to prevent asymptomatic infants congenitally infected with Toxoplasma gondii from progressing to clinically manifest chorioretinitis and other sequelae of congenital toxoplasmosis.
In some situations, establishing screening programmes to detect and treat asymptomatic infections or unrecognized disease in defined populations is useful. An example is the screening for Chlamydia trachomatis in sexual partners of people infected with Chlamydia trachomatis, women with mucopurulent cervicitis, sexually active women 25 years of age or younger, and women older than 25 years of age with risk factors for chlamydia. A more detailed background on screening as a public health function is given in Chapter 11.4.
An example of the use of chemoprophylaxis to treat an infectious disease to prevent complications of the disease is the use of penicillin (or erythromycin in penicillin-sensitive patients) to treat streptococcal sore throats caused by Streptococcus pyogenes group A to prevent acute rheumatic fever.
Other examples of prevention of complications of an infectious disease through the use of chemoprophylaxis include the following:
◆ Tetracycline for adults, or penicillin for children, for treatment of Lyme disease caused by Borrelia burgdorferi in the erythema chronicum migrans stage to prevent or reduce the severity of late cardiac, arthritic, or neurological complications.
◆ Benzathine penicillin for treatment of syphilis in its primary, secondary, or early latency period to prevent late manifestations of the disease such as cardiovascular syphilis.
◆ Ketoconazole for treatment of blastomycosis caused by Blastomyces dermatitidis in its early stages to prevent progression of chronic pulmonary or disseminated blastomycosis that may lead to death.
Potential problems with the use of chemoprophylaxis may include compromise of the host’s own non-specific defence mechanisms, other replacement infectious agents causing disease by growing in the place of the infectious agent affected by the specific chemoprophylactic regimen (e.g. severe diarrhoea from intestinal infections with Clostridium difficile may occur when the normal bacteria flora in the intestines have been lost due to antibiotics), and emergence of resistant strains of the infectious agent. The development of antibiotic resistance can be reduced by using antibiotics only when needed, selecting the proper antibiotic (or, in some situations, the appropriate multidrug therapy) for the infectious agent, and ensuring compliance with the appropriate regimen for the duration of treatment.
Perhaps the most challenging tool for the control of infectious diseases, and sometimes one of the most powerful and cost-effective, is behaviour change in the host that reduces or eliminates risk of exposure to an agent. Everyone has developed habits of living (lifestyles) that are not easily changed. Some of these behaviours are protective against infectious diseases. Others render the individual at higher risk of infection.
Examples of higher risk of exposure to infectious agents through behaviour, and behaviour changes that can have an impact on the chain of transmission, include the following.
Many infectious agents are transmitted by the direct transmission route through sexual contact, including Chlamydia trachomatis causing chlamydial genital infections, Neisseria gonorrhoeae causing gonorrhoea, Treponema pallidum causing venereal syphilis, Calymmatobacterium granulomatis causing granuloma inguinale, Haemophilus ducreyi causing chancroid, herpes simplex virus causing herpes simplex, Trichomonas vaginalis causing trichomoniasis, human papillomaviruses causing condyloma acuminate, and HIV causing AIDS.
Abstinence behaviour, that is, refraining from sexual activity with other people, eliminates the risk of transmission of these agents through sexual contact. The delaying of age of first sexual activity avoids the risk of transmission of these agents at an early age. Restricting sexual contact to only between two uninfected people who do not have sexual activity with any other people virtually eliminates the risk of transmission of these agents through sexual behaviours. The exceptions are due to other routes of transmission of some of these agents (e.g. HIV acquired through intravenous drug use in one partner being transmitted through sexual contact to the other partner). Limiting the number of sexual partners, and limiting those sexual partners to people who also have few sexual partners, reduces the risk of exposure. However, at the individual level, if one of these sexual partners has an infectious agent transmissible by sexual contact, the risk of transmission may still be high. Finally, condom use during sexual activity in high-risk situations will markedly reduce, but not eliminate, transmission. A more detailed background on sexually transmitted diseases is provided in Chapter 8.12.
Intravenous drug use behaviour
Injection of drugs using non-sterile needles or syringes previously used by other intravenous drug users may transmit infectious agents in blood through the vehicle-borne route of indirect transmission, including HIV causing AIDS; hepatitis B virus causing viral hepatitis B; and Plasmodium vivax, Plasmodium malariae, and Plasmodium ovale causing malaria.
Abstinence behaviour, that is, refraining from intravenous drug use, eliminates the risk of transmission of such agents through contaminated needles and syringes. Using a sterile needle and sterile syringe for intravenous drug use will break the chain of transmission of these infectious agents through this route. Some community public health programmes, in addition to promoting drug abstinence and drug rehabilitation, conduct needle and syringe exchanges and education regarding methods of decontamination to help promote the use of sterile injection equipment among intravenous drug users (see Chapter 9.2).
Eating certain foods may result in exposure to infectious agents through the vehicle-borne route of indirect transmission. These behaviours include consuming raw molluscs by which an infectious agent like the hepatitis A virus can cause viral hepatitis A, eating raw eggs by which an infectious agent like a Salmonella serotype can cause salmonellosis, and consuming raw beef by which an infectious agent like Taenia saginata can cause beef tapeworm infection.
Although food and diet are strongly ingrained behaviours, modification of dietary patterns is possible. Cooking foods like beef, pork, and eggs can markedly reduce risk of transmission of infectious agents. In addition, reducing risks by elimination of infectious agents from the food may be possible (see the section on control methods applied to the environment). Eating shortly after cooking so that foods are not left standing at ambient temperatures for extended times and paying attention to food recalls are also important behaviours. Hand washing before eating also reduces risk of transmission of many infectious agents that are spread through direct or indirect routes of faecal–oral transmission, such as Shigella dysenteriae, Shigella flexneri, Shigella boydii, and Shigella sonnei which may cause shigellosis (bacillary dysentery), which is estimated to cause 600,000 deaths per year with two-thirds of the cases in children under 10 years of age (Heymann 2010).
In certain occupations, many behaviours may result in exposure to infectious agents and should be targets for control programmes in occupational safety and health settings. Specific examples include the following:
◆ Dental workers performing procedures with bare hands may result in exposure to hepatitis B viruses from infected patients.
◆ Health workers improperly handling used needles may result in needle-stick injuries leading to exposure to HIV from infected patients.
◆ Hospital laboratory workers improperly processing specimens containing infectious agents without appropriate glove or eyewear protection may result in exposure to these agents.
◆ Veterinarians who do not properly handle animals may result in brucellosis (undulant fever) due to exposure to Brucella abortus, Brucella melitensis, Brucella suis, or Brucella canis.
Occupational hazards related to non-infectious materials may predispose an individual to increased risk of infectious diseases. For example, working conditions and behaviours in industrial plants and mines that lead to silicosis due to long-term inhalation of free crystalline silica dust will greatly increase the risk of developing tuberculosis.
Working behaviours appropriate for the particular occupational setting may include wearing protective clothing, eyewear, and gloves; hand washing and changing clothes after work; receiving appropriate vaccinations for the working environment; meticulous adherence to needle disposal and equipment sterilization procedures; and using hooded laboratory benches when handling infectious specimens that can become aerosolized.
Other behaviours that may reduce the transmission of infectious agents include the following:
◆ Scheduling outdoor activities at periods of low vector activity, applying insect repellents and sleeping under bednets reduce the indirect transmission of vector-borne agents of infectious diseases like malaria.
◆ Searching oneself for attached ticks every 3 to 4 hours when playing or working in tick-infested areas reduces the indirect transmission of vector-borne agents of infectious diseases like Rocky Mountain spotted fever.
◆ Avoiding sharing of utensils, cups, toothbrushes, or towels reduces the indirect transmission of vehicle-borne agents of infectious diseases like mononucleosis.
◆ Wearing of shoes reduces the direct transmission of infectious agents like those causing hookworm disease.
◆ Frequently bathing and regular washing of clothes in hot soapy water controls body lice.
◆ Breastfeeding reduces diarrhoeal diseases in the infant, although it may transmit HIV from HIV-infected mothers.
◆ Hand washing after defecation or touching potentially contaminated surfaces, people, or animals (including those found in petting zoos) prior to preparing food or touching one’s own eyes, mucous membranes, or mouth reduces the risk of direct or indirect transmission of a wide variety of infectious agents.
◆ Large family sizes and crowding may facilitate airborne transmission of infectious agents in droplet nuclei for infectious diseases like tuberculosis.
Some of these other behaviours, like crowding, are conditioned by circumstances such as poverty that are not easily or directly amenable to programmes promoting behavioural change.
Certain rare circumstances exist where a means of avoiding transmission of an infectious disease to a highly susceptible host is to provide reverse, or protective, isolation. Such isolation attempts to protect infection-prone patients from potentially harmful infectious agents. Reverse isolation procedures range from provision of a private room with the use of masks, gloves and gowns by all people entering the room, to elaborate facilities with laminar airflow rooms and sterilization of all food. Protective isolation is usually conducted for a limited time until the normal immune system recovers, a regimen of passive immunization is begun, or a bone marrow transplant is successful.
Examples of people who may need periods of reverse isolation include those who have such diseases as X-linked agammaglobulinaemia, DiGeorge’s syndrome, and severe combined immunodeficiency; or those who have received therapies, such as some forms of cancer chemotherapy, that have severely compromised the person’s immune system in combating many infectious diseases.
One tool of control that can be applied to the host is the use of barriers between the host and the infectious agent. The effectiveness of such barriers, however, may be dependent on the behaviour of the host to use them consistently and education to use them correctly. Examples of barriers include the following:
◆ Screens, bednets (including bednets impregnated with pyrethroid insecticides such as permethrin), long-sleeved shirts and trousers (with the cuffs tucked into boots as a mechanical barrier), and repellents (such as N,N-diethyl-meta-toluamide known as DEET) to prevent transmission of malaria through the bite of infected female Anopheles mosquitoes or West Nile virus through Culix mosquitoes.
◆ Condoms to prevent transmission of HIV and other sexually transmitted infectious agents through sexual intercourse.
◆ Masks (air-purifying respirators) to prevent transmission of tuberculosis through airborne droplet nuclei from patients with sputum-positive pulmonary tuberculosis.
General improvement in host resistance
Improving host resistance though general improvement of the immune system is a non-specific approach, but may be important in certain settings. Kwashiorkor, marasmus, and other forms of malnutrition debilitate the host’s immune system and may make an individual more susceptible to infectious diseases. Moreover, people who are malnourished and succumb to an infectious disease are at higher risk of the disease being of greater severity and leading to other complications.
Malnutrition also encompasses micronutrient deficiencies. Vitamin A deficiency, for example, has been linked to higher rates of mortality associated with measles disease. Correcting vitamin A deficiency, through programmes of supplementation, fortification and dietary modification in high-risk populations, can reduce mortality rates due to measles.
A complex interaction exists between infectious diseases, such as diarrhoeal diseases, and malnutrition. A downward spiral of infection may lead to malnutrition that, in turn, leads to more infections, and so on. If unchecked, especially in developing countries, this downward spiral can ultimately result in death.
The special situation of international travel combines many control measures applied to the host already mentioned. The increase in the numbers of travellers, the speed of travel, and the ability to reach areas previously infrequently visited have reduced the effectiveness of surveillance for infectious diseases at ports of arrival and increased infectious disease risks. Advice for prevention against infectious diseases must be both general and specific.
General advice includes such issues as avoidance of eating and drinking potentially contaminated food or drink (including ice) and swimming or bathing in polluted water. Specific advice provided by health professionals should be provided based on information about the area to be visited and may include such measures as active immunization against yellow fever, active or passive immunization against hepatitis A, chemoprophylaxis against malaria, repellents against potentially infected mosquitoes, and not walking barefoot in areas of risk for infection with hookworms Strongyloides stercoralis and Strongyloides fuelleborni. A more detailed background on international travel and health is provided in the annually updated WHO publication on International Travel and Health (WHO 2012b).
Control measures applied to vectors
Vector-borne transmission is the only or main route of transmission for many infectious diseases. There exist more than 100 arthropod-borne viruses that may produce clinically manifest diseases in humans. Control of vector-borne diseases includes measures to: (1) change behaviour and create barriers to the susceptible host, (2) reduce or break the chain of transmission of the infectious agent from an infected host to the vector, and (3) directly control the vector population itself. Chemical, environmental, and biological controls are the primary means of directly controlling the vector population.
Chemicals used in the control of vectors that act as digestive poisons, contact poisons, or fumigants include minerals, natural plant products (botanicals), chlorinated hydrocarbons, organophosphates, carbamates, and fumigants. Chemical control measures include the following public health interventions:
◆ Spraying chemical insecticides such as organochlorine insecticides (e.g. dichlorodiphenyltrichlorothane, or DDT, and dieldrin), organophosphorus insecticides (e.g. malathion and fenitrothion), and carbonate insecticides (e.g. propoxur and carbaryl) to prevent malaria through control of mosquitoes.
◆ Spraying chemical biodegradable insecticides such as temephos (Abate®) to prevent onchocerciasis through control of Simulium fly vectors.
◆ Using traps impregnated with decamethrin to prevent African trypanosomiasis (sleeping sickness) through reduction of the population of infective species of Glossina (tsetse fly) vectors.
◆ Treating snail breeding places with chemical molluscicides to prevent schistosomiasis due to the free-swimming cercariae (larval forms) of Schistosoma mansoni, Schistosoma haematobium, and Schistosoma japonicum that develop in snails.
◆ Treating step-wells and ponds with chemical insecticides such as temephos (Abate®) to prevent dracunculiasis due to infected cyclops (a crustacean copepod).
◆ Suppressing rat populations by poisoning, preceded or accompanied by measures to control fleas, as an additional measure to supplement environmental sanitation to control rodent populations to prevent human plague.
The use of spraying for control of mosquitoes is complicated due to concerns about environmental contamination by chemicals such as DDT and dieldrin which have led to them being banned in many countries. In addition, the emergence of mosquito vectors resistant to the insecticides diminishes their effectiveness in many areas. New methods of application, such as ultra-low-volume spraying of malathion, reduce the amounts of insecticide used.
Environmental control of vectors includes the following public health interventions:
◆ Eliminating breeding sites of mosquito larvae by filling and draining areas of stagnant water and removing objects around houses that may collect water.
◆ Destroying the habitats of the tsetse fly vector.
◆ Placing roach and fly traps.
◆ Constructing rat-proof houses.
◆ Eliminating rodent habitats.
It is also important to note that certain development projects may have an impact on the environment that facilitates the growth of vector or intermediate-host populations and results in increased infectious diseases. Construction of artificial waterways may serve as breeding sites for Simulium fly vectors that can transmit Onchocerca volvulus resulting in onchocerciasis. Irrigation schemes can foster the growth of snail intermediate hosts required for the transmission of species of Schistosoma resulting in schistosomiasis. Carefully conducted environmental and health impact studies that consider the impact of a construction project on the vector and intermediate host populations, and ways to modify the project to reduce such populations, are important environmental control measures.
Biological control of vectors includes the following public health interventions:
◆ Introduction of predators and parasites: the introductions of Gambusia affinis, a small fish that feeds on mosquito larvae, and of Coelomomyces, a fungal parasite, are examples of control measures that are effective against Aëdes mosquitoes.
◆ Insect growth regulators: the use of such regulators may result in death or sterility of vectors by interfering with normal insect development. An example is the use of methoprene (Altosid®) to control flood water mosquitoes.
◆ Genetic modification: although still at an experimental phase, researchers have developed transgenic, or genetically modified, mosquitoes that are malaria resistant with higher survival rates that could eventually replace mosquitoes that carry malaria parasites.
Control measures applied to infected humans
Control measures may be applied to infected people at the individual level, in the institutional or hospital setting, and at the community level.
Hospital infection control
The hospital setting is a unique situation that requires special efforts to prevent and control nosocomial infections, or healthcare-associated infections (HAIs), which are infections that originate or occur in a hospital or other healthcare setting. HAIs are a major problem worldwide. In the United States alone, some 2 million HAIs occur annually resulting in an estimated 90,000 deaths (McKibben et al. 2005). It has been estimated that HAIs result in some US$35.7–45 billion in annual direct medical hospital patient costs based on using the Consumer Price Index for inpatient hospital services (Scott 2009).
Infection control programmes for hospitals should ideally include the following elements:
◆ An infection control committee responsible for overall coordination of infection control activities.
◆ One or more infection control practitioners responsible for nosocomial disease surveillance, analysis of data, consultation, and training of hospital staff.
◆ A hospital epidemiologist to supervise the infection control practitioners, oversee data collection and analysis, and implement any needed emergency infection control measures.
◆ An engineer to direct engineering and preventive maintenance operations, especially ventilation equipment.
◆ A sanitarian to develop procedures for proper disposal of liquid and solid wastes; and sanitation of water, ice, and food.
◆ Effective guidelines for patient care practices.
◆ Surveillance of patient care practices, patient infections, and environmental contamination by infectious agents.
◆ Coordination with other departments (microbiology laboratory, central services, housekeeping, food service, and laundry).
◆ Vector control.
◆ Thorough investigation of problems.
Examples of specific control measures that may be applied to infected humans at the individual, institutional and community levels are detailed as follows.
Treatment of people with infectious diseases or subclinical infections may be a control tool for some infectious diseases. Such treatment may or may not have an impact on disease progression in the patient. It should be noted that rapid case detection and prompt application of appropriate chemotherapeutic agents are needed to limit infectivity.
Some important examples of control by chemotherapy include the following:
◆ Treatment of patients with sputum-positive pulmonary tuberculosis with appropriate multidrug therapy will usually result in sputum conversion rendering them non-infectious to others within a few weeks. Recommended treatment regimens include isoniazid (INH) combined with one or more of the following antibiotics: rifampin, streptomycin, ethambutol, and pyrazinamide. The WHO has recommended that adherence to a complete course of multidrug therapy be directly observed by another responsible person as part of the DOTS (directly observed treatment, short-course) global strategy for the control of tuberculosis.
◆ Patients with leprosy treated with appropriate multidrug therapy are considered no longer infectious within 3 months of regular and continued treatment. Recommended treatment regimens for multibacillary leprosy include the following antibiotics: rifampicin, dapsone, and clofazimine.
◆ Treatment of patients with streptococcal sore throats with penicillin (or erythromycin for penicillin-sensitive patients) will usually render them no longer infectious after 24–48 hours.
◆ Patients with pertussis treated with antibiotics such as erythromycin or trimethoprim-sulfamethoxazole, although they may not affect the patient’s symptoms, will usually result in the patient no longer being infectious after 5–7 days.
A person or animal that harbors a specific infectious agent without discernible clinical disease, and which serves as a potential source of infection. The carrier state may exist in an individual with an infection that is unapparent throughout its course (such an individual is commonly known as healthy or asymptomatic carrier), or during the incubation period, convalescence, and post-convalescence of a person with a clinically recognizable disease (commonly known as incubatory or convalescent carrier). Under either circumstance the carrier state may be of short or long duration (temporary or transient carrier, or chronic carrier). (Heymann 2010)
A chronic carrier of diphtheria, for example, may shed the infectious agent Corynebacterium diphtheriae for 6 months or more, but appropriate antibiotic therapy will usually promptly stop the carrier state. Another example is that of untreated patients with typhoid fever due to Salmonella typhi. Between 2 to 5 per cent of such patients will become permanent carriers. Treatment with appropriate antibiotics may be effective in ending the carrier state.
Antibiotic treatment may not always eliminate a carrier state for some infectious agents. For example, the treatment of people with salmonellosis with an antibiotic may not terminate the period of communicability and can even result in emergence of antibiotic-resistant strains. However, antibiotic therapy may be still warranted under certain circumstances.
In some situations, establishing screening programmes in defined target populations for identification of asymptomatic or unrecognized infections that could be transmitted to others may be appropriate. Such screening should include the necessary follow-up with appropriate chemotherapy and counselling. An example would be screening close contacts of diphtheria patients with nose and throat cultures for the presence of Corynebacterium diphtheriae. Identified carriers with positive cultures should be treated with appropriate antibiotic therapy.
Isolation is the ‘separation, for a period at least equal to the period of communicability, of infected persons or animals from others, in such places and under such conditions as to prevent or limit the direct or indirect transmission of the infectious agent from those infected to those who are susceptible to infection or who may spread the agent to others’ (Heymann 2010).
The US CDC and the Hospital Infection Control Practices Advisory Committee (HICPAC) have provided guidelines for isolation precautions in hospital settings (Siegel et al. 2007). There are two levels of isolation precautions, namely: (1) a standard precautions level designed for the care of all hospitalized patients, and (2) a transmission-based precautions level designed for the care of hospitalized patients that are suspected or confirmed to be infected by agents spread by contact, droplet, or airborne routes of transmission. These are summarized from guidelines as follows.
Standard precautions are universally applied precautions designed to reduce the risk of transmission by infectious agents from blood; body fluids, secretions, and excretions; non-intact skin; and mucous membranes. The essential elements of standard precautions include handwashing; personal protective equipment (use of gloves, appropriate application of mask and eye protection or a face shield, and utilization of gowns); proper handling of patient-care equipment; adequate environmental control measures for routine care, cleaning, and disinfection of frequently touched surfaces; appropriate handling, transporting, and processing of used linen; proper handling and disposal of needles, scalpels, and other sharp instruments; adequate protection when undertaking patient resuscitation; respiratory hygiene/cough etiquette; and placement of patients who contaminate the environment in private rooms.
Airborne precautions are used, in addition to standard precautions, in settings where patients are suspected or confirmed to be infected by agents transmitted by airborne droplet nuclei. The essential elements of airborne precautions include preferably placing patients in an airborne infection isolation room (AIIR) that has monitored negative air pressure (if necessary, it is possible to use cohorting of patients with the same active infections); use of mask respirators (N-95 air-purifying respirators); and limiting patient movement and transport from the room (placing a surgical mask on the patient if they are being moved for an essential purpose). An example of an infectious disease for which patients are recommended to be placed under airborne precautions is a patient in hospital with measles through the fourth day of rash. Although isolation of patients with measles not in hospital is not practical in the general population, schoolchildren should remain out of school until at least the fourth day of rash.
Droplet precautions are used, in addition to standard precautions, in settings where patients are suspected or confirmed to be infected by agents transmitted by droplets. The essential elements of droplet precautions include placement of patients in a private room (if necessary, it is possible to use cohorting of patients with the same active infections or maintaining a spatial separation of at least 3 feet between the infected patient and other patients and visitors); use of a mask when working within 3 feet of the patient; and limiting patient movement and transport from the room (placing a surgical mask on the patient if they are being moved for an essential purpose). Examples of infectious diseases for which patients are recommended to be placed under droplet precautions include pharyngeal diphtheria caused by Corynebacterium diphtheriae and pneumonic plague caused by Yersinia pestis.
Contact precautions are used, in addition to standard precautions, in settings where patients are suspected or confirmed to be infected or colonized by agents transmitted by direct or indirect contact. The essential elements of contact precautions include placement of the patient in a private room (if necessary, it is possible to use cohorting of patients with the same active infections); use of gloves when entering the patient’s room and removing them before leaving the room; wearing a gown when entering the patient’s room and removing the gown before leaving the room; limiting patient movement and transport to essential purposes only; and, when possible, dedicate the use of patient-care equipment to a single patient (if necessary, it is possible to use such equipment on a cohort of patients with the same active or colonized infections). Examples of infectious diseases for which patients are recommended to be placed under contact isolation precautions include cutaneous diphtheria caused by Corynebacterium diphtheriae, rubella, and disseminated herpes simplex caused by herpes simplex virus.
Quarantine of potentially infected persons
Quarantine is the ‘Restriction of activities for well persons or animals who have been exposed (or are considered to be at high risk of exposure) to a case of communicable disease during its period of communicability (i.e., contacts) to prevent disease transmission during the incubation period if infection should occur’ (Heymann 2010).
Two categories of quarantine are as follows (Heymann 2010):
Absolute or complete quarantine: the limitation of freedom of movement of those exposed to a communicable disease for a period of time not longer than the longest usual incubation period of that disease, in such a manner as to prevent effective contact with those not so exposed.
Modified quarantine: a selective, partial limitation of freedom of movement of contacts, commonly on the basis of known or presumed differences in susceptibility and related to the assessed risk of disease transmission. It may be designed to accommodate particular situations. Examples are exclusion of children from school, exemption of immune persons from provisions applicable to susceptible persons, or restriction of military populations to the post or to quarters. Modified quarantine includes: personal surveillance, the practice of close medical or other supervision of contacts to permit prompt recognition of infection or illness but without restricting their movements; and segregation, the separation of some part of a group of persons or domestic animals from the others for special consideration, control or observation; removal of susceptible children to homes of immune persons; or establishment of a sanitary boundary to protect uninfected from infected portions of a population. (Heymann 2010)
Examples of diseases where quarantine may be considered include the following.
◆ Pneumonic plague: people who have been in the same household or in face-to-face contact with patients with pneumonic plague and who do not accept chemoprophylaxis should be placed under absolute quarantine with strict isolation, including careful surveillance, for 7 days.
◆ Measles: although absolute quarantine is impractical, a modified quarantine is recommended in settings where young children are living in dormitories, wards, or institutions. When measles occurs in such institutional settings, strict segregation of infants is recommended.
◆ Lassa fever: close personal surveillance of all close contacts is recommended. Such people include those who live or are in close contact with Lassa fever patients as well as laboratory personnel testing specimens from such patients.
Restriction of activities
Controlling infectious disease transmission by restriction of the activities of people in the community who are potentially infectious to others may be appropriate in certain circumstances. Examples include the following:
◆ Individuals with a diarrhoeal disease should be excluded from handling food and caring for patients in hospital, children, and elderly people.
◆ Known carriers of Salmonella typhi should be excluded from food handling and care of patients.
◆ People with staphylococcal disease should avoid contact with debilitated people and infants.
◆ People with rubella should be excluded from school or work for 7 days after the onset of rash and from contact with pregnant women.
Behaviour change in an infected person to protect others may be difficult to accomplish and often requires continuing education and counselling. However, this should be considered in preventing the transmission of infectious agents in the following situations.
Examples of infectious agents transmitted through sexual activities are discussed in the earlier section on control measures applied to the host and in more detail in Chapter 8.12. Individuals who suspect that they may have a sexually transmitted disease should be encouraged to have health-seeking behaviours. People with a sexually transmissible infectious agent should be treated and asked to cooperate with health officials to trace their sexual contacts. Patients with diseases such as lymphogranuloma venereum and syphilis, for example, should refrain from sexual contact until all lesions are healed. HIV-infected individuals should be counselled to treat genital ulcer disease promptly since such disease may increase transmissibility of HIV. Also, HIV-infected people should avoid sexual intercourse with HIV-negative individuals or, if having sexual intercourse with HIV-negative individuals, use methods such as condoms to reduce the risk of transmission. For a more detailed overview of HIV and AIDS see Chapter 8.13.
Intravenous drug use behaviour
In addition to counselling to abstain from intravenous drug use and establishing drug rehabilitation programmes to help individuals who wish to abstain, promoting behaviour change in the use of injection equipment is important. Discouraging the sharing of injection equipment and education on methods for the decontamination of needles and syringes for intravenous drug use reduce risks of transmission of infectious agents through contaminated injection equipment.
Food preparation behaviour
Individuals who should be restricted from handling food (e.g. carriers of Salmonella typhi) should be counselled regarding their condition and potential to infect others if they handle food. Food handlers who have an infectious disease that is potentially transmissible through the vehicle-borne means of food should be discouraged from handling food for others. The importance of hand washing, especially after defecation and before handling food, should be stressed.
Other behaviours that may reduce risk of transmission of infectious agents to other people include the following:
◆ Cough and sneeze behaviour: patients with infectious diseases directly transmitted by droplet spread or airborne transmitted by droplet nuclei (e.g. patients with sputum-positive tuberculosis) should cover their mouth and nose when coughing or sneezing.
◆ Avoidance of contaminated drinking water: people suffering from dracunculiasis should avoid entering a source of drinking water if they have an active ulcer or blister.
◆ Avoidance of donating organs or bodily fluid by certain people: individuals who are infected with HIV or who have sexual and other behaviours that have placed them at increased risk for HIV infection should not donate blood, plasma, tissues, cells, semen for artificial insemination, or organs for transplantation.
Control measures applied to animals
A zoonosis is any infectious agent or infectious disease that may be transmitted under natural conditions from vertebrate animals, both wild and domestic, to humans. A detailed approach to zoonoses is given in the comprehensive work CRC Handbook Series in Zoonoses (Beran and Steele 1994). In the control of zoonoses many approaches are used that are applied to animals, including the following.
Active immunization, or vaccination, of selected animals may protect susceptible animal hosts from certain infectious diseases. This protection of animals, in turn, prevents susceptible humans from exposure to the infectious agent of those diseases from animals. An example of an infectious disease in animals in which some control can be achieved through immunization in selected animal populations is rabies. The reservoir of the rabies virus is varied and includes dogs, foxes, wolves, skunks, raccoons, and bats. Preventive measures include efforts to vaccinate all dogs.
Other examples of immunization of animals under certain conditions include: (1) immunization of young goats and sheep using a live attenuated strain of Brucella melitensis and calves using a strain of Brucella abortus in areas of high endemicity for brucellosis; and (2) immunization of animals at risk for acquiring infection with Bacillus anthracis that could be transmitted to man causing anthrax.
Restriction or reduction
Restriction is the limiting of the movement of animals and includes isolation and quarantine. Reduction is the killing, known as culling, of selected animals. Selective use of restriction of animals or reduction of animal populations that are infected, or potentially infected, with a zoonotic infectious agent are methods used to decrease or eliminate the opportunity of exposure of susceptible humans, or other animals, to such animals.
The example of rabies can also be used to illustrate the use of restriction or reduction of an animal population to help control an infectious disease. Heymann (2010) recommends the following measures:
1. Register, license, and vaccinate all owned dogs, and other pets when feasible, in enzootic countries; control ownerless animals and strays. Educate pet owners and the public on the importance of local community responsibilities (e.g. pets should be leashed in congested areas when not confined on the owner’s premises; strange-acting or sick animals of any species,—domestic or wild—should be avoided and not handled; animals that have bitten a person or another animal should be reported to relevant authorities, such as the police/local health departments; if possible, such animals should be confined and observed as a preventive measure; and wildlife should be appreciated in nature and not be kept as pets). Where animal population reduction is impractical, animal contraception and repetitive vaccination campaigns may prove effective.
2. Detain and observe for 10 days any healthy-appearing dog or cat known to have bitten a person (stray or ownerless dogs and cats may be euthanized and examined for rabies by fluorescent microscopy); dogs and cats showing suspicious clinical signs of rabies should be euthanized and tested for rabies.
3. Euthanize unvaccinated domestic animals bitten by known rabid animals; if detention is elected, hold the animal in a secure facility for at least 6 months under veterinary supervision, and vaccinate against rabies 30 days before release. If previously vaccinated, booster immediately with rabies vaccine, and detain for at least 45 days.
4. Cooperate with wildlife conservation authorities in programs to reduce the carrying capacity of wildlife hosts of sylvatic rabies, and to reduce exposures to domestic animals and human populations—such as in circumscribed enzootic areas near campsites, and in areas of dense human habitation. (Heymann 2010)
Epidemic (epizootic) measures
In urban areas of industrialized countries, strict enforcement of regulations requiring collection, detention and euthanasia of ownerless and stray dogs, and of non-immunized dogs found off owners’ premises; control of the dog population by castration, spaying or drugs have been effective in breaking transmission cycles. (Heymann 2000)
Other examples of restricting or reducing of animal populations include the following:
◆ Rat-proofing dwellings and reduction of the rat population to prevent rat bites that may transmit the infectious agents Streptobacillus moniliformis and Spirillum minus causing the rat-bite fevers of streptobacillosis and spirillosis, respectively.
◆ Rat suppression by poisoning (after achieving flea control) in rodent populations with a high potential for epizootic plague.
◆ Elimination of animals infected with Brucella abortus, Brucella melitensis, Brucella suis, and Brucella canis by segregation or slaughter to prevent brucellosis.
◆ Slaughtering dairy cattle that test positive for infection with Mycobacterium bovis, the infectious agent of bovine tuberculosis.
Chemoprophylaxis and chemotherapy
Chemoprophylaxis of an animal is using chemical substances (e.g. antibiotics) that prevent infection or its progression to clinically manifest infectious disease in the animal. Chemotherapy of an animal is using these chemical substances to treat an infectious disease in an animal. Both chemoprophylaxis and chemotherapy are control measures that may be used to reduce or prevent the opportunity of an infectious agent from being transmitted from an animal to susceptible humans. However, caution should be applied in the routine use of chemoprophylaxis in cattle, feed lots, and poultry farms that can promote the emerging of drug resistance and its associated problems in humans as well.
Psittacosis is an example of a zoonosis controlled by chemoprophylaxis or chemotherapy in selected animal populations. The infectious agent, Chlamydia psittaci, may be directly transmitted to humans from infected birds when the dried droppings, secretions, or dust from the feathers of such infected birds are inhaled. Imported psittacine species of birds should be placed under quarantine and receive an appropriate antibiotic chemotherapeutic regimen such as chlortetracycline administered in their feed for 30 days.
Another example is chemoprophylaxis in selected dogs at high risk of infection with Echinococcus granulosus. This infectious agent can be transmitted to humans through hand-to-mouth transmission of the tapeworm eggs from dog faeces causing echinococcosis due to Echinoccus granulosus, or cystic hydatid disease. Such high-risk dogs should periodically receive antihelminth treatment with a chemotherapeutic agent such as praziquantel (Biltricide®).
Control measures applied to the environment
Control measures applied to the environment are designed to interrupt the routes of transmission by which an infectious agent may be spread through the environment. Just as the routes of transmission are varied, so, too, are the control methods that can be applied. Control measures that affect transmission that can be applied to the host, agents, vectors, infected humans, and other animals are reviewed elsewhere in this chapter. Environmental factors may also have a direct impact on the host, agent, or vector. For example, low humidity may predispose to certain infections due to a greater permeability of mucous membranes in the host; cold, dry climates inhibit development of the infective larvae agent of hookworm disease; and higher altitudes and colder climates limit the mosquito vector.
The recognition of the relationship between disease and filth led to a sanitary revolution in industrialized countries that markedly reduced infectious diseases even before the arrival of the antibiotic era. Improved methods for storing and preserving food, better housing, and smaller families with a resultant decrease in the risk of infections at an early age all contributed to reductions in infant and child mortality rates.
This section focuses on general environmental control measures not mentioned elsewhere. Some of these methods, such as provision of safe water, have the potential to prevent several different infectious diseases and significantly reduce rates of disease in the community.
Provision of safe water
It has been estimated that some 783 million still lack safe drinking water in 2010 (WHO and United Nations Children’s Fund 2012). Contaminated drinking water, sometimes the result of poorly designed or maintained systems of sewerage, may lead to the water-borne indirect transmission of such infectious agents as Giardia lamblia causing giardiasis, pathogenic serotypes of Salmonella causing salmonellosis, and Cryptosporidium species causing cryptosporidiosis.
Purification of water can occur though natural methods or human intervention. Examples of natural methods that contribute to water purification include the processes of evaporation and condensation, filtration through the earth, plant growth, aeration, and reduction and oxidation of organic material by bacteria. Purification of water for public consumption is conventionally done through such processes as coagulation of colloids by aluminium salts or with other techniques; filtration through such materials as coal, sand, or diatomaceous earth; and disinfection with such chemicals as chlorine derivatives. In special situations, boiling and distillation can be used for purification (Solomon et al. 2009).
Proper disposal of faeces
It has been estimated that some 2.5 billion people in the developing world do not have an adequate system for proper disposal of faeces with some 1.1 billion people practising open defecation (WHO and United Nations Children’s Fund 2012). Infectious agents in faeces that may result in infectious diseases include poliovirus causing poliomyelitis; Shigella dysenteriae, Shigella flexneri, Shigella boydii, and Shigella sonnei causing shigellosis; and Entamoeba histolytica causing amoebiasis.
Infectious agents in faeces may be transmitted by the direct transmission route (including the faecal–oral mode), the vehicle-borne route (including water as noted in the previous section), and the vector-borne route (including the simple mechanism of flies carrying infected faeces on their feet). Public health environmental control measures to interrupt these routes of transmission by ensuring the proper disposal of faeces include the following:
◆ Appropriate on-site disposal through such means as properly constructed sanitary privies in rural areas with no sewerage systems.
◆ On-site disposal of domestic wastewater (such as use of septic tanks or cesspools).
◆ Sewerage systems with treatment of wastewater. Such treatment may include preliminary treatment, sedimentation, chemical coagulation and flocculation, biological treatment (such as activated sludge units and trickling filters), stabilization ponds, sludge management, and disinfection (usually with chlorine) of effluents discharged into drinking, bathing or shellfish-growing waters.
The importance of personal health-promoting behaviours of using toilets, keeping toilets clean, and hand washing after defecation are a part of control efforts aimed at the proper disposal of faeces.
Food-borne infectious diseases remain a problem in both industrialized and developing countries. In the United States in 2011 alone, it is estimated that some 48 million people became ill, 128,000 were hospitalized, and 3000 died of foodborne-related diseases (CDC 2012a). Significant food-borne outbreaks and sporadic cases continue to occur due to such factors as the following:
◆ Contamination of meat, poultry, and eggs with infectious agents, including pathogenic serotypes of Salmonella, Yersinia pseudotuberculosis and Yersinia entercolitica, and Listeria monocytogenes.
◆ Contamination of vegetables, especially lettuce and leafy green vegetables, with the infectious enterohaemorrhagic strain of Escherichia coli O157:H7. Other outbreaks in fruits, juices, and vegetables have been due to contamination with hepatitis A and pathogenic serotypes of salmonella.
◆ Larger and more centralized production and processing facilities, coupled with increasingly extensive distribution networks, which may result in transmission of infectious agents to many people if a commercial product becomes contaminated.
Industrialized countries have significantly reduced the transmission of some infectious agents through major public health programmes in food sanitation, including: (1) inspecting eating and drinking establishments, (2) inspecting meat and poultry, (3) improving shellfish sanitation, and (4) promoting adequate cooking, canning, and refrigeration methods. Some cities and counties have instituted restaurant grading systems based on inspection reports, including required public display of the restaurant’s grade as a guide for consumers.
Examples of vehicle-borne indirect transmission of infectious agents through food that can be controlled though a comprehensive public health food sanitation programme include the following:
◆ Pathogenic serotypes of Salmonella transmitted by ingesting food made from infected animals or contaminated by the infectious agent in faeces that may cause salmonellosis. Methods of control—preventive measures: ‘a) handwashing before, during and after food preparation; b) refrigerating prepared foods in small containers; c) thoroughly cooking all foodstuffs derived from animal sources, particularly poultry, pork, egg products and meat dishes; d) avoiding recontamination within the kitchen after cooking is completed; and e) maintaining a sanitary kitchen and protecting prepared foods against rodent and insect contamination’ (Heymann 2010).
◆ Staphylococcus aureus causing staphylococcal food intoxication by ingesting food containing the staphylococcal enterotoxin. Methods of control—preventive measures: ‘Reduce food-handling time (from initial preparation to service) to a minimum, no more than 4 hours at ambient temperature. If foods are to be stored for more than 2 hours, keep those that are perishable hot (above 60°C/140°F) or cold (below 5°C/41°F), in shallow containers, and covered.’ ‘Temporarily exclude people with boils, abscesses and other purulent lesions of hands, face or nose from food handling.’ (Heymann 2010).
◆ Trichinella spiralis transmitted by ingesting raw or improperly cooked meat or meat products, mainly pork, containing infectious encysted larvae that may cause trichinosis. Methods of control—preventive measures:
1. Educate the public on the need to cook all fresh pork and pork products and meat from wild animals at a temperature and for a time sufficient to allow all parts to reach at least 71°C (160°F), or until meat changes from pink to grey, which allows a sufficient margin of safety. This should be done unless it has been established that these meat products have been processed either by heating, curing, freezing or irradiation adequate to kill trichinae.
2. Grind pork in a separate grinder, or clean the grinder thoroughly before and after processing other meats.
3. Adopt regulations to encourage commercial irradiation processing of pork products. Testing carcasses for infection with a digestion technique is useful, as is immunodiagnosis of pigs with an approved ELISA test.
4. Adopt and enforce regulations that allow only certified trichinea-free pork to be used in raw pork products that have a cooked appearance, or in products that are traditionally not heated sufficiently in final preparation to kill trichinae.
5. Adopt laws and regulations to require and enforce the cooking of garbage and offal before feeding to swine. (Heymann 2010).
Milk may be a vehicle for indirect transmission of such infectious agents as: Mycobacterium bovis causing tuberculosis, Corynebacterium diphtheriae causing diphtheria, Listeria monocytogenes causing listeriosis, and Campylobacter jejuni and Campylobacter coli causing Campylobacter enteritis.
Public health control measures to break the chain of transmission of infectious agents through milk include:
◆ Mechanization and sanitization of milking processes.
◆ Refrigeration of milk to inhibit bacterial growth.
◆ Pasteurization of milk through high-temperature short-time, batch, ultra-pasteurization, or ultra-high-temperature methods that help to kill any bacteria from the cow or others that may have come into direct contact with milk during the milking and handling process.
◆ Monitoring milk quality by testing for bacteria using a standard bacterial plate count, by testing for density of coliform organisms, and by use of the phosphatase test to assay for pasteurization.
◆ Periodically testing cows for tuberculosis and brucellosis.
The use of raw milk for human consumption may result in outbreaks. A study showed that some 60 per cent of dairy-related outbreaks reported to the CDC between 1993 and 2006 were associated with raw milk products (CDC 2012b). It should be noted that post-pasteurization contamination of milk may also result in outbreaks of milk-borne diseases.
Design of facilities and equipment
The design and proper maintenance of buildings, rooms, and equipment can help break the chain of transmission of infectious agents. Laminar airflow hoods in laboratory workbenches, ventilation systems in hospitals, and disposable intravenous equipment are examples of systems designed to reduce risk of transmission. Routine maintenance needed to retain the original design standards for control of transmission of infectious agents include: (1) replacement of air filters, (2) cleaning of cooling towers, (3) monitoring of positive pressure rooms and airlocks, and (4) replacement of in-dwelling peripheral venous catheters.
Examples of infectious agents whose transmission can be reduced through proper design and maintenance include the following:
◆ Legionella species, the infectious agents responsible for legionellosis, are usually transmitted through airborne transmission via aerosol production. Transmission of the agent from cooling towers can be reduced by periodically cleaning off any scale or sediment, routinely using biocides to kill slime-forming organisms, and draining such towers when not in use.
◆ Staphylococcus aureus, the infectious agent responsible for staphylococcal disease in medical and surgical wards of hospitals, can be controlled by enforcing strict aseptic technique, including procedures to change intravenous infusion sites at least every 48 hours and replace indwelling peripheral venous catheters every 72 hours.
◆ Bacillus anthracis, the infectious agent responsible for anthrax, can be transmitted, among other ways, through inhalation of anthrax spores. Proper design of industrial plants that handle raw animal fibres include providing facilities for adequate ventilation and control of dust, washing and changing clothes after work, and eating away from the places of work.
In addition to the environmental methods of control of transmission of infectious agents already mentioned, the following are other methods, some specific and some general, that should also be noted.
◆ Improvement of housing conditions to reduce crowding (as measured by the number of people per room and not total population density). Reduction in crowding is a general measure that can reduce the transmission of infectious agents, especially direct transmission from direct contact or direct projection (droplet spray).
◆ Improvement in working conditions can affect the risk of infectious disease. For example, control of particulate matter by proper ventilation in occupations such as textile mill workers, metal grinders, and pottery factory workers can reduce inflammation of the lungs and thus decrease the risk of developing tuberculosis. Excessive physical exertion and the stress of exhausting work can also increase the risk of tuberculosis.
◆ Improved irrigation and agricultural practices and removing vegetation or draining and filling of snail-breeding sites can reduce or eliminate the freshwater snail hosts of such infectious agents as Schistosoma mansoni, Schistosoma haematobium and Schistosoma japonicum that cause schistosomiasis in humans.
◆ Adequate screening of blood, serum, plasma, tissues, or organs can break the chain of vehicle-borne transmission from such biological products. Examples include screening for hepatitis B surface antigen and HIV antibodies in donated blood to prevent transmission of hepatitis B and HIV, respectively.
◆ Installation of screened living and sleeping quarters and use of bednets, including bednets impregnated with a synthetic pyrethroid such as permethrin, can reduce exposure to mosquitoes infected with the infectious agents of malaria.
Control measures applied to the agent
Control of some infectious diseases can be achieved through means that remove the infectious agents from the environment or inactivate the agents. Physical measures (such as heat, cold, ultraviolet light, and ionizing radiation) and chemical measures (such as liquid disinfectants and antiseptics, gases, and chlorination) can be used. Examples of control measures applied to infectious agents include the following:
◆ Cleaning is the removing of infectious agents from surfaces through such physical actions as vacuum cleaning or washing and scrubbing using soap or detergent and hot water. Cleaning also helps remove organic materials that might support the growth or survival of infectious agents.
◆ Cooling may inhibit bacterial multiplication and some infectious agents, such as Trichinella cysts and Taenia solium larvae (cysticerci), can be killed by freezing temperatures.
◆ Pasteurization is heating to a temperature of 75°C/167°F for 30 minutes to kill pathogenic vegetative bacteria. It does not inactivate bacterial spores. Pasteurization is a commonly used process to help ensure safety of milk and to prolong its storage quality.
◆ Disinfection is the reduction or killing of vegetative harmful bacterial infectious agents outside the body or on objects. Disinfection may not inactivate all bacterial spores and viruses. Disinfectants are used to eliminate pathogenic bacteria from the skin surface and from contaminated inanimate surfaces and include: (1) alcohols, (2) halogens such as iodine and chlorine, (3) surface active compounds such as the quaternary ammonium compound benzalkonium chloride, (4) phenolics, and (5) alkylating agents such as glutaraldehyde and formaldehyde. Antiseptics are a class of disinfectant that can be applied on body surfaces; they have a lower toxicity than environmental disinfectants and are usually less effective in killing microorganisms.
◆ Sterilization is the complete removal or killing of all infectious agents in, or on, an object. Sterilization of equipment for surgery and wound dressings; parenteral administration of drugs, vaccines or nutrients; catheterization; and dental work are all important means of controlling infectious diseases by killing infectious agents. Sterilization can be accomplished through use of fire, steam (such as in an autoclave), heated air, certain gases (such as ethylene oxide), ultraviolet light, ionizing radiation, liquid chemicals, and filtration. The method of sterilization chosen depends on the type of equipment to be sterilized.
The use of sterilized disposable equipment, such as disposable needles, syringes, and catheters, has the potential to reduce the risk of transmission of infectious agents. However, it must be assured that such equipment is disposed of properly and is not reused. For example, disposable syringes cannot be properly resterilized because the plastic from which they are made cannot withstand the heat necessary for sterilization. Technologies, such as the single-use disposable needle and syringe developed for immunization programmes, help assure such disposable equipment is not reused.
Control and prevention programmes
The preceding sections have considered the issues and given examples of control measures for infectious diseases at individual, institutional, and community levels and the tools for control directed at the host, routes of transmission, and the agent. Control and prevention programmes using these tools must be developed according to a number of factors including: (1) the risk of disease; (2) the magnitude of disease burden (as measured by mortality, degree of disability, morbidity, and economic costs); (3) the feasibility of control strategies; (4) the cost of control measures; (5) the effectiveness of such measures (on current levels of disease and impact on future cases or outbreaks); (6) the adverse effects or complications of the control measures; and (7) the availability of resources. Public health planning for the control of infectious diseases must consider these issues to design optimal, evidence-based control and prevention programmes.
The tools of disease surveillance for recognition and evaluation of the patterns of disease can provide information on the risk and magnitude of disease burden to individuals, people in institutions, subgroups of populations, and the community at large. Establishment and maintenance of the infrastructure for surveillance, including a system for the reporting of notifiable infectious diseases and unusual events, must be a high priority.
Feasibility of possible control and prevention strategies must be assessed through operational research, pilot projects, or from field experience. The fact that a particular measure can help control a disease does not mean it can be applied on a sufficient scale to have the desired impact. The cost of control activities (in both human and material resources) can be assessed through costing studies that can also provide the data needed to conduct more rigorous cost–benefit and cost-effectiveness analyses. A costly measure, even if it provides a high degree of control for an infectious disease, may not be affordable to the society or reasonable to apply in the light of other less expensive alternative strategies. For example, use of sterile, disposable products may be more costly than reusable, sterilizable products. Effectiveness of control measures may be assessed through epidemiological studies to find out their impact on reduction in the incidence or prevalence of disease.
The availability of resources for prevention and control programmes forces public health planners to set priorities by taking into account all these factors and then designing programmes that have maximum impact within available resources. Planners have a responsibility to mobilize additional necessary resources by raising public awareness and generating political will. Effective communication of disease burden and the results achievable through well-managed and effective control programmes can be a powerful tool for advocacy. Ideally, communities or their representatives should actively participate in the planning, execution, and evaluation of public health programmes.
Prevention effectiveness is ‘the measure of the impact on health (including effectiveness, safety and cost) of prevention policies, programmes, and practices. The assessment of prevention effectiveness is the ongoing process of applying evaluation tools to prevention practices’ (CDC 1995). Recognizing that systems for assessing the effectiveness of prevention strategies (including prevention strategies for infectious diseases) are weak or non-existent in both developing and industrialized countries alike, the CDC has suggested the following objectives for prevention effectiveness activities: ‘evaluate the impact of prevention, use results of evaluation research to establish programme priorities, and establish or apply standardized methods to compare the benefits and effectiveness of alternative prevention strategies’ (CDC 1995).
The current situation of international migration of many people worldwide presents an additional complexity to the design of programmes for the control of infectious diseases. Pertinent issues include refugee camps, legal status of migrants in recipient countries, and temporary return migration. Public health officials must consider the most effective mix of combined control measures applied to the host, agent, and routes of transmission when designing suitable control and prevention programmes (Gellert 1993).
International commerce and transportation are important areas of concern for public health infectious disease control programmes, especially as the speed of travel has increased. The tools of control include such measures as:
◆ Spraying insecticides effective against mosquito vectors of malaria in aircraft before departure, in transit, or on arrival.
◆ Rat-proofing or periodic fumigation to control rats on ships, docks, and warehouses to prevent plague.
The challenge facing infectious disease control programmes is to design an optimal set of interventions at local, institutional, community, national, and international levels supported and accepted by the political leadership and the people to whom these measures are applied.
A unique end point in the control of infectious diseases is that of eradication. Eradication is the cessation of all transmission of an infectious agent by extermination of that agent. To date, only one human infectious disease, smallpox, and one animal infectious disease, rinderpest, have been eradicated. The WHO World Health Assembly in May 1980 confirmed its global eradication some 3 years after the last naturally acquired case of smallpox in October 1977 (Fenner et al. 1988). The magnitude of this accomplishment is appreciated when one realizes that, in the early 1950s, it was estimated that 50 million cases of smallpox still occurred each year in the world, some 150 years after Edward Jenner had performed the first vaccination and wrote: ‘it now becomes too manifest to admit of controversy, that the annihilation of the Small Pox, the most dreadful scourge of the human species, must be the final result of this practice’ (Fenner et al. 1988). Rinderpest (known as cattle plague or steppe murrain) was an infectious viral disease of cattle and some other even-toed ungulates, that was associated with a high mortality rate. In 2011, after widespread vaccination campaigns, the United Nations declared the disease eradicated.
The goal of global eradication has been set by the World Health Assembly for two other infectious diseases, poliomyelitis caused by wild poliovirus and dracunculiasis (Guinea worm infection), the latter caused by the infectious agent Dracunculus medinensis. A high level of sustained political will, aggressively applied disease surveillance, and effective control measures are the required elements to achieve eradication of the infectious agents for these diseases.
Impressive progress has been made towards the global eradication of poliomyelitis since the 1988 World Health Assembly set the goal for its eradication. Polio cases have decreased by over 99 per cent since that date, from some 350,000 cases in over 125 countries to only 650 reported cases in 2011. In 2012, only parts of three countries in the world (Nigeria, Pakistan, and Afghanistan) remain endemic for the disease (WHO 2012c). Poliomyelitis control measures that will lead to eradication include the following:
◆ Achieving and maintaining high levels of routine coverage of infants with at least three doses of oral polio vaccine.
◆ Mass application of oral polio vaccine in countries where poliomyelitis is endemic through national immunization days, usually by providing oral polio vaccine to every child less than 5 years of age twice each year, separated by 4–6 weeks, and conducted during the low season of poliovirus transmission.
◆ ‘Mopping-up’ operations after the use of national immunization days has reduced transmission of disease to defined focal geographic areas, usually by providing oral polio vaccine house-to-house to all children less than 5 years of age on two occasions separated by 4–6 weeks.
◆ Aggressive action-oriented surveillance for acute flaccid paralysis. Such surveillance includes: (1) case investigation, (2) a laboratory network for isolation and characterization of polioviruses in suspect cases of poliomyelitis and people in close contact with them, and (3) limited outbreak response immunization providing one house-to-house round of oral polio vaccine to children less than 5 years of age living in the same village or neighbourhood of the patient.
Significant strides in the eradication of dracunculiasis have also been made. Over the last decade the total number of dracunculiasis cases has declined by more than 95 per cent. In 2010, fewer than 1800 cases were reported and, by 2011, the disease was limited to only certain areas of sub-Saharan Africa. Dracunculiasis control measures that are leading to its ultimate eradication include the following:
◆ Establishing a national programme office, conducting baseline surveys, and preparing and refining a national plan of action.
◆ Educating the population in endemic areas that the source of Guinea worm comes from their drinking water.
◆ Ensuring that people with blisters or emerging worms do not enter sources of drinking water through behaviour changes and by converting step-wells into draw-wells.
◆ Promoting the boiling or filtering of water through a fine mesh cloth to remove copepods. Treating drinking water with chlorine or iodine will also kill the copepods and infective larvae.
◆ Providing non-infected water through construction of wells or rainwater catchments.
◆ In selected endemic villages, controlling copepod populations with temephos (Abate®) insecticide placed in reservoirs, tanks, ponds, and step-wells.
◆ Implementing an intensified surveillance and aggressive case-containment strategy as programmes get close to achieving eradication.
The eradication of a disease requires a unique set of conditions, including the following: (1) a defined, accessible reservoir of the infectious agent; (2) affordable and effective control measures that can interrupt the chain of infection directed at the host, agent, or route of transmission; and (3) a surveillance mechanism adequate to monitor and ultimately certify the disappearance of the infectious agent.
It is likely that measles may be targeted for global eradication in the future. Some countries and geographic regions have already targeted measles for elimination—a term sometimes used to describe the eradication of a disease from a large geographic area. Other diseases that may potentially be targeted for eradication in the future include: mumps, rubella, hepatitis B, leprosy, and diphtheria.
Emerging infectious diseases
New, emerging and re-emerging infectious diseases have become a focus for the attention of public health prevention and control programmes in both industrialized and developing countries. Such infectious diseases have thwarted any expectation that infectious diseases will soon be eliminated as public health problems and resulted in a widening spectrum of diseases, many of which were once thought to be almost conquered. Krause has reflected on this as follows:
Bacteria reproduce every 30 minutes. For them, a millennium is compressed into a fortnight. They are fleet afoot, and the pace of our research must keep up with them, or they will overtake us. (Krause 1998)
◆ Human demographic change by which people begin to live in previously uninhabited remote areas of the world and are exposed to new environmental sources of infectious agents, insects, and animals.
◆ Breakdowns of sanitary and other public health measures in overcrowded cities and in situations of civil unrest and war.
◆ Economic development and changes in the use of land, including deforestation, reforestation, and urbanization.
◆ Climate change, including global warming, which may extend the favourable habitats for vectors such as mosquitoes.
◆ Other human behaviours, such as increased use of child-care facilities, sexual and drug use behaviours, and patterns of outdoor recreation.
◆ International travel and commerce that quickly transport people and goods vast distances.
◆ Changes in food processing and handling, including foods prepared from many different individual animals and transported great distances.
◆ Evolution of pathogenic infectious agents by which they may infect new hosts, produce toxins, or adapt by responding to changes in the host immunity.
◆ Development of resistance of infectious agents such as Mycobacterium tuberculosis and Neisseria gonorrhoeae to chemoprophylactic or chemotherapeutic medicines.
◆ Resistance of the vectors of vector-borne infectious diseases to pesticides.
◆ Immunosuppression of people due to medical treatments or new diseases that result in infectious diseases caused by agents not usually pathogenic in healthy hosts.
◆ Deterioration in surveillance systems for infectious diseases, including laboratory support, to detect new or emerging disease problems at an early stage.
◆ Toxic shock syndrome, due to the infectious toxin-producing strains of Staphylococcus aureus, illustrates how a new technology yielding a new product, super-absorbent tampons, can create the circumstances favouring the emergence of a new infectious disease threat.
◆ Lyme disease, due to the infectious spirochete Borrelia burgdorferi, illustrates how changes in the ecology, including reforestation, increasing deer populations, and suburban migration of the population, can result in the emergence of a new microbial threat that has now become one of the most prevalent vector-borne disease in the United States.
◆ Shigellosis, giardiasis, and hepatitis A are examples of emerging diseases that have become threats to staff and children in child-care centres as the use of such centres has increased due to changes in the work patterns of societies.
◆ Opportunistic infections, such as pneumocystis pneumonia caused by Pneumocystis jirovecii (formerly carinii), chronic cryptosporidiosis caused by Cryptosporidium species, and disseminated cytomegalovirus infections, illustrate emerging disease threats to the increasing number of people who are immunosuppressed because of cancer chemotherapy, organ transplantation, or HIV infection.
◆ Foodborne infections such as diarrhoea caused by the enterohaemorrhagic strain O157:H7 of Escherichia coli and waterborne infections such as gastrointestinal disease due to Cryptosporidium species are examples of emerging disease threats that have arisen due to such factors as changes in diet, food processing, globalization of the food supply, and contamination of municipal water supplies.
◆ Hantavirus pulmonary syndrome first detected in the United States in 1993 and caused by a previously unrecognized hantavirus illustrates how exposure to certain kinds of infected rodents can result in an emerging infectious disease.
◆ Nipah virus disease first detected in Malaysia in 1999 and caused by a previously unrecognized Hendra-like virus demonstrates how close contact with fruit bats and pigs can result in an emerging infectious disease.
◆ Severe acute respiratory syndrome (SARS), a viral respiratory illness caused by the SARS-associated coronavirus (SARS-CoV) that was first reported in Asia in February 2003 and spread to more than 20 countries in North America, South America, Europe, and Asia affecting over 8000 people and killing over 700 before the SARS global outbreak was contained in 2004, shows the need for continued vigilance in surveillance and outbreak response capacity.
◆ The emergence of the novel influenza A virus in birds, highly pathogenic avian influenza (H5N1), that has occasionally infected humans is an example of an antigenic shift in an influenza virus that, should it develop into a form easily transmissible from human to human, could potentially lead to a pandemic.
Antimicrobial drug resistance as a major factor in the emergence and re-emergence of infectious diseases deserves special attention. Although significant reductions in infectious disease mortality have occurred since the introduction of antimicrobials for general use in the 1940s, antimicrobial drug resistance has emerged because of their widespread use in humans.
Drugs that once seemed invincible are losing their effectiveness for a wide range of community-acquired infections, including tuberculosis, gonorrhoea, pneumococcal infections (a leading cause of otitis media, pneumonia, and meningitis), and for hospital-acquired enterococcal and staphylococcal infections. Resistance to antiviral (e.g. amantadine-resistant influenza virus and aciclovir-resistant herpes simplex), antifungal (e.g. azole-resistant Candida species), and antiprotozoal (e.g. metronidazole-resistant Trichomonas vaginalis) drugs is also emerging. Drug-resistant malaria has spread to nearly all areas of the world where malaria occurs. Concern has also arisen over strains of HIV resistant to antiviral drugs. Increased microbial resistance has resulted in prolonged hospitalizations and higher death rates from infections; has required much more expensive, and often more toxic, drugs or drug combinations (even for common infections); and has resulted in higher health care costs (CDC 1994).
Antimicrobial drug resistance has also emerged because of the use of antimicrobials in domesticated animals. For example, the use of fluoroquinolones in poultry has created a reservoir of quinolone-resistant Campylobacter jejuni that has now been isolated in humans.
An aggressive public health response to these new, emerging and re-emerging infectious disease threats must be made to characterize them better and to mount an effective response for their control. For example, the 1999 outbreak of West Nile fever in New York City and surrounding areas that, within a 4-year period, spread throughout the United States demonstrates how a viral encephalitis, initially classified as Saint Louis encephalitis and later confirmed to be due to West Nile virus, can reach far beyond its normal setting.
The WHO has outlined the following high-priority areas (WHO 1995):
◆ Strengthen global surveillance of infectious diseases.
◆ Establish national and international infrastructures to recognize, report, and respond to new disease threats.
◆ Further develop applied research on diagnosis, epidemiology, and control of emerging infectious diseases.
◆ Strengthen the international capacity for infectious disease prevention and control.
Emerging infectious diseases are addressed in detail in Chapter 8.17.
Bioterrorism: the deliberate use of biological agents to cause harm
Another unfortunate source of an infectious disease threat is the spectre of biological warfare or bioterrorism, especially in an age where terrorist acts are frequent events (Christopher et al. 1997). The 2002 World Health Assembly resolution urges member states ‘to treat any deliberate use, including local, of biological and chemical agents and radionuclear attack to cause harm also as a global public health threat, and to respond to such a threat in other countries by sharing expertise, supplies and resources in order rapidly to contain the event and mitigate its effects’ (WHO 2002).
The WHO recommends the following (WHO 2004):
◆ Public health authorities, in close cooperation with other government bodies, should draw up contingency plans for dealing with a deliberate release of biological or chemical agents intended to harm civilian populations. These plans should be consistent or integral with existing plans for outbreaks of disease, natural disasters, large-scale industrial or transportation accidents, and terrorist incidents.
◆ Preparedness for deliberate releases of biological or chemical agents should be based on standard risk-analysis principles, starting with risk and threat assessment in order to determine the relative priority that should be accorded to such releases in comparison with other dangers to public health in the country concerned. Considerations for deliberate releases should be incorporated into existing public health infrastructures, rather than developing separate infrastructures.
◆ Preparedness for deliberate releases of biological or chemical agents can be markedly increased in most countries by strengthening the public health infrastructure, and particularly public health surveillance and response, and measures should be taken to this end.
◆ Managing the consequences of a deliberate release of biological or chemical agents may demand more resources than are available, and international assistance would then be essential.
Many countries are developing rapid response capability to deal with such contingencies, especially in the light of the 2001 bioterrorist attack using anthrax in the United States.
Only through worldwide concerted action will the effort to control infectious disease be effective. We are now in an era where, as Nobel Laureate Dr Joshua Lederberg has stated, ‘The microbe that felled one child in a distant continent yesterday can reach yours today and seed a global pandemic tomorrow’ (quoted in CDC 1994). As Hans Zinsser stated over 60 years ago:
Infectious disease is one of the few genuine adventures left in the world. The dragons are all dead and the lance grows rusty in the chimney corner…About the only sporting proposition that remains unimpaired by the relentless domestication of a once free-living human species is the war against those ferocious little fellow creatures, which lurk in the dark corners and stalk us in the bodies of rats, mice and all kinds of domestic animals; which fly and crawl with the insects, and waylay us in our food and drink and even in our love. (Hans Zinsser 1934, quoted in Murphy 1994)
Text extracts from Heymann, D.L. (ed.), Control of Communicable Diseases Manual, Nineteenth Edition, American Public Health Association, Washington, USA, Copyright © American Public Health Association 2008, with permission from American Public Health Association.
Text extracts from World Health Organization (WHO), Public Health Response to Biological and Chemical Weapons: WHO Guidance, Second edition of Health aspects of chemical and biological weapons: report of a WHO Group of Consultants, WHO, Geneva, Switzerland, Copyright © 2004, reproduced with permission from the World Health Organization, available from http://whqlibdoc.who.int/publications/2004/9241546158.pdf.
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