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Epidemiology of sexually transmitted infections 

Epidemiology of sexually transmitted infections
Chapter:
Epidemiology of sexually transmitted infections
Author(s):

David Mabey

DOI:
10.1093/med/9780199204854.003.0801

May 30, 2013: This chapter has been re-evaluated and remains up-to-date. No changes have been necessary.

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Essentials

Although accurate incidence figures are not available in most countries, sexually transmitted infections (STIs) (excluding HIV) are estimated to cause more than 5% of the global burden of disease. The burden falls especially heavily on women and infants, with more than half a million perinatal deaths attributable to syphilis annually. Mobile populations, those with many sexual partners, and those whose partners have many partners are at increased risk, and the prevalence of treatable STIs is many times higher in poor populations, who often lack access to effective treatment. Other STIs, especially those that cause genital ulceration, increase the risk of HIV transmission.

Incidence

In Western countries, the reported incidence of many STIs fell during the 1980s and 1990s, probably as a result of changes in sexual behaviour resulting from the HIV epidemic, but has increased subsequently. The reported incidence of Chlamydia trachomatis infection has increased in the general population, especially in teenagers and young adults, and the incidence of syphilis has increased in core groups, including homosexual men.

Strategies to control STIs

These include health education and the promotion of condoms; the provision of accessible, acceptable, and affordable clinical services to provide effective treatment and hence prevent complications and further transmission; and partner notification to reach infected people who may not present to a health facility. Since many STIs are asymptomatic, screening programmes may also play an important role. Screening of pregnant women for syphilis is recommended policy in most countries, and has been shown to be cost-effective even where the prevalence is low. Screening programmes for C. trachomatis infection have recently been implemented in some Western countries, but their impact is uncertain.

Introduction

Few countries outside Western Europe and North America have accurate reporting systems for sexually transmitted infections (STIs). As a result, in most of the world’s population, the incidence of these infections is unknown. Knowledge of their epidemiology is based on the results of improvised prevalence surveys undertaken in convenient populations (e.g. STI or antenatal clinic attenders), but these are often unrepresentative of the population as a whole.

In an attempt to calculate the worldwide incidence of the curable STIs—syphilis, gonorrhoea, trichomoniasis, and chlamydial infection—the World Health Organization (WHO) estimated the prevalence of each infection by region, on the basis of published surveys, and divided this figure by the estimated duration of the infection. They concluded that, each year, 333 million cases of curable STIs occurred worldwide (Fig. 8.1.1). The most common is trichomoniasis (170 million cases), followed by chlamydial infection (89 million), gonorrhoea (62 million), and syphilis (12 million). In view of the uncertainty surrounding the prevalence estimates, the duration of untreated STIs and the mean duration before effective treatment is received, these figures cannot be considered definitive. In 1993, the World Bank estimated that 5.4% of the worldwide burden of disease resulted from STIs excluding HIV.

Fig. 8.1.1 The global distribution of five curable STIs (chancroid, syphilis, gonorrhoea, chlamydial infection, and trichomoniasis).

Fig. 8.1.1
The global distribution of five curable STIs (chancroid, syphilis, gonorrhoea, chlamydial infection, and trichomoniasis).

Transmission of STIs

The rate at which an STI spreads in a population depends on the average number of new cases of infection generated by an infected individual—the basic reproductive number (R0). This in turn depends on the mean rate of sexual partner change (c), the average duration of the infection (D), and its infectiousness (i.e. the likelihood of it being transmitted per sexual act, β‎). This relationship has been described by the simple formula R0 = β‎cD.

When R0 falls below 1 in a given population, the infection will eventually disappear. However, even when R0 is less than 1 in the general population, infections may be maintained in core groups with a high rate of change of sexual partners, and may continue to occur in the general population as a result of sexual contact with members of high-risk groups.

The duration of a curable infection depends on the time that elapses before effective treatment is given. A disease such as chancroid, which almost always causes unpleasant symptoms, is likely to be treated rapidly in populations with access to effective treatment. For this reason, it has almost disappeared in most industrialized countries, but remains endemic in core groups in many developing countries. In contrast, chlamydial infection, which is often asymptomatic in both sexes, is likely to be of longer duration and thus to persist even in affluent populations, unless a comprehensive screening programme is established.

Risk factors for STIs

By definition, STIs are usually transmitted by sexual intercourse, although mother-to-child transmission is also of great public health importance in the case of syphilis and gonorrhoea. Those at highest risk are therefore those with many sexual partners, or those whose partners have many partners—in other words, those who belong to high-risk sexual networks. These include sex workers and their clients, mobile populations such as migrant labourers, truck drivers, fishermen, and soldiers. The youngest sexually active age groups, especially young women, are at particularly high risk.

STIs are more common in poor populations. The incidence of gonorrhoea in inner-city ethnic minorities in the United States of America is at least 30-fold greater than that in middle-class white Americans, and similar to that in many developing countries. Gonorrhoea and syphilis have almost disappeared from affluent countries such as Sweden and Canada, whereas the economic hardship caused by the collapse of the Communist system has led to a dramatic increase in the incidence in reported cases of syphilis in the former Soviet Union.

Poor people are at increased risk of STIs for several reasons. They may have to travel long distances away from their families in search of work. Many poor rural villagers have migrated into cities in developing countries in the last few decades, and many more have been displaced by wars and famines. Poverty and lack of education drive many women into sex work. Health education messages warning of the dangers of HIV/AIDS may be lost on those whose most pressing need is the cost of their next meal. But perhaps most importantly, poor people often lack access to effective treatment for curable STIs.

In China, paradoxically, rapid economic development has coincided with a dramatic increase in the incidence of reported STIs. Syphilis had been virtually eliminated from China in the 1950s, following a massive public health campaign including compulsory screening and treatment for those at risk; but since 1990, its reported incidence has increased more than 20-fold. This reflects the fact that free medical care is no longer available in China, making screening and treatment for syphilis unaffordable for many.

STIs in developed countries

In the United Kingdom, a free and confidential service for people with STIs was established in 1916. Details of patients seen at genitourinary medicine (GUM) clinics are reported to the Health Protection Agency and, since few patients are treated for STIs outside these clinics, the data are believed to be fairly complete and comprehensive. Since the epidemiology of STIs is similar in most countries in Western Europe, the figures for the United Kingdom will be cited as an example (see http://www.hpa.org.uk).

Gonorrhoea

The number of reported cases of both gonorrhoea and syphilis declined steadily from a peak in the early 1980s to the late 1990s, presumably as a result of changes in sexual behaviour following the HIV epidemic (Fig. 8.1.2). The number of cases of gonorrhoea increased in the late 1990s, but has declined in both men and women since 2002 (Fig. 8.1.3). The overall reported incidence in 2005 was 39 per 100 000 total population. The incidence in London was more than twice the national average, and, as in the United States of America, was highest in ethnic minorities of African and Caribbean origin.

Fig. 8.1.3 The annual incidence of reported gonorrhoea in England and Wales by sex and age group, 1996–2005. Health Protection Agency

Fig. 8.1.3
The annual incidence of reported gonorrhoea in England and Wales by sex and age group, 1996–2005. Health Protection Agency

Syphilis

The number of cases of primary and secondary syphilis increased more than 10-fold in the United Kingdom between 2000 and 2005 (Figs. 8.1.4 and 8.1.5). A high proportion of cases were in homosexual men, in whom HIV was a common coinfection. In 2005, the overall incidence was only 5.7 per 100 000 total population, but it was 19 per 100 000 in men aged 25 to 34 years.

Fig. 8.1.4 The annual incidence of reported early syphilis in England and Wales, 1931–2005.
Health Protection Agency

Fig. 8.1.4
The annual incidence of reported early syphilis in England and Wales, 1931–2005.

Health Protection Agency

Fig. 8.1.5 The annual incidence of reported syphilis in England and Wales by sex and age group, 1996–2005.
Health Protection Agency

Fig. 8.1.5
The annual incidence of reported syphilis in England and Wales by sex and age group, 1996–2005.

Health Protection Agency

In Eastern Europe, an epidemic of syphilis in the newly independent states of the former Soviet Union was reported in the 1990s. In 1999, the incidence of reported syphilis in these countries ranged from 55 to 180 per 100 000, with increases particularly evident in older adolescents. There was a 20-fold increase in the reported incidence of syphilis in Russia between 1992 and 1996.

Chlamydial infection

The number of reported chlamydial infections trebled in the United Kingdom between 1996 and 2005 (Fig. 8.1.6). Similar increases were seen in other Western countries over this period, including Sweden and Canada, despite active screening programmes for chlamydia. It is not clear to what extent this increase is due to an increase in the number of people tested, or to the use of the more sensitive nucleic acid amplification tests, which became widely used in the late 1990s. Paradoxically, the incidence of complications of chlamydial infection, such as pelvic inflammatory disease and ectopic pregnancy, declined in many developed countries over the same period. The overall incidence of reported chlamydial infection in the United Kingdom in 2005 was 223 per 100 000 total population, with the highest rate (1300/100 000) in young women aged 16 to 19 years.

Fig. 8.1.6 The annual incidence of reported chlamydial infection in England and Wales by sex and age group, 1996–2005.
Health Protection Agency

Fig. 8.1.6
The annual incidence of reported chlamydial infection in England and Wales by sex and age group, 1996–2005.

Health Protection Agency

As part of the national survey of sexual attitudes and lifestyles, there was a population-based prevalence survey for Chlamydia trachomatis infection in men and women aged 16 to 44 in the United Kingdom. Infection was found in 2.2% of men, and 1.5% of women, with the highest prevalences in men aged 25 to 34 (3.1%) and women aged 16 to 24 years (3.0%).

Lymphogranuloma venereum (LGV), caused by the more invasive L1, L2, and L3 strains of C. trachomatis, is a rare disease in industrialized countries, and is generally considered a ‘tropical’ STI. In 2003, an outbreak of LGV proctitis due to the L2 serovar was reported among homosexual men in the Netherlands, and in the subsequent 3 years, several hundred cases were reported in homosexual men in Europe and North America, the majority of them HIV positive.

Genital herpes

The incidence of reported genital herpes in women aged 16 to 24 was more than 150 per 100 000 in the United Kingdom in 2005, and in men aged 20 to 24 years it was about half this figure. Classically, genital herpes is due to herpes simplex virus type 2 (HSV2), while herpes simplex type 1 (HSV1) causes oral lesions and is a common childhood infection. Once acquired, these infections persist lifelong, causing recurrent vesicular and ulcerative lesions. In the United Kingdom, the proportion of genital ulcers due to HSV1 is increasing, presumably because of changing sexual practices. More than 50% of episodes in women, and more than 25% in men, are now due to HSV1. The seroprevalence of HSV2 infection is 3% in male blood donors, and 12% in female donors, rising to 16% in pregnant women aged more than 29. In GUM clinic attenders, the seroprevalence exceeds 20% in both sexes.

Human papillomavirus (HPV)

Certain types of HPV (predominantly 6 and 11) cause genital warts, while others (predominantly 16 and 18) cause cervical carcinoma (see Chapter 7.5.19). Genital warts are the most frequently reported viral STI in GUM clinics in the United Kingdom (more than 90 000 cases in 2005), cases increasing by 76% between 1996 and 2005. The incidence in both men and women in the 20- to 24-year-old age group was approximately 700 per 100 000 total population in 2005.

STIs in developing countries

Few reliable data are available on the incidence of STIs in developing countries. Based on numbers of cases seen at health facilities, it is estimated that the incidence of gonorrhoea is at least 50 times higher in sub-Saharan Africa than in the United Kingdom. Several large population-based surveys have confirmed that the prevalence of STIs is high in sub-Saharan Africa, even in rural populations. For example, 5 to 10% of adults were found to be infected with syphilis, 20 to 30% of women, and 10% of men with Trichomonas vaginalis, and up to 50% of women were found to have bacterial vaginosis. Syphilis is estimated to cause almost 500 000 stillbirths or neonatal deaths per year in Africa alone.

A population-based study in rural Tanzania found that 50% of women and 25% of men were infected with HSV2 by the age of 20 years. Seropositivity was rare before the age of 16 in both sexes, confirming that HSV2 is mainly transmitted sexually in this population. The proportion of genital ulcers caused by HSV2 has increased in Africa as a result of the HIV epidemic, as recurrences become more frequent and prolonged in the immunocompromised. At the same time, chancroid has apparently become less common in high-risk populations in Africa, perhaps as a result of behavioural change resulting from the HIV epidemic. Cervical carcinoma is the most common malignancy in women in much of the developing world, reflecting the high incidence of sexually transmitted HPV infection.

Interactions between HIV and other STIs

Diseases such as chancroid, syphilis, and herpes, which cause genital ulceration, facilitate sexual transmission of HIV by increasing infectivity and susceptibility. A prospective study of STI clinic attenders in Nairobi, Kenya showed that the likelihood of a man who had acquired a genital ulcer from an HIV-positive sex worker also acquiring HIV was about 1 in 6 after a single sexual exposure. This suggests that the presence of a genital ulcer increases the risk of transmission 50- to 100-fold. STIs such as gonorrhoea that cause genital discharge increase shedding of HIV in both seminal and cervicovaginal secretions.

A community-randomized trial in Mwanza, Tanzania found that improved STI services in rural health centres and dispensaries, using the syndromic approach, reduced the incidence of HIV infection by 40% over a 2-year period. In Uganda, a community-randomized study found that periodic mass treatment for STIs had no impact on the incidence of HIV. In this trial, the HIV epidemic was more advanced, and a high proportion of genital ulcers were caused by HSV2, which was not treated. HIV and HSV2 appear to facilitate transmission of one another, leading to a vicious circle (Fig. 8.1.7). Control of HSV2, perhaps by vaccination, could greatly reduce transmission of HIV in the developing world.

Fig. 8.1.7 Interactions between HIV infections and genital herpes.

Fig. 8.1.7
Interactions between HIV infections and genital herpes.

Control of STIs

Strategies for the control of STIs aim to reduce β‎ (transmissibility), c (rate of partner change), or D, the duration of infection.

Primary prevention

Transmissibility can be reduced by the use of condoms. Health promotion and health education aim to encourage the use of condoms, and to persuade people to have fewer sexual partners. This is sometimes referred to as primary prevention, since these measures can prevent people from ever becoming infected. There have been few formal trials of health education in the primary prevention of STIs; but the example of health education in schools suggests that, although education often improves knowledge, it seldom influences behaviour. In Thailand, legislation to close down brothels where condom use was not mandatory, was successful in reducing the incidence and prevalence of HIV infection in the general population.

Secondary prevention: case management

The duration of treatable STIs can be reduced by the provision of accessible, acceptable, and affordable clinical services, combined with partner notification. Prompt treatment of STIs should be seen as a ‘public good’, equivalent to the treatment of pulmonary tuberculosis, since it prevents transmission to others, as well as benefiting the person treated.

The aims of patient care are:

  • to detect or rule out infection

  • to give treatment if necessary

  • to educate and counsel on treatment compliance, STD/HIV prevention, and condom use

  • to ensure that sexual partner(s) are evaluated and managed (contact tracing)

  • to test for other STIs, including HIV

In most developing countries, case management of STIs must be syndromic, because laboratory diagnosis is not available outside a few specialist centres. Syndromic management of genital ulcers and genital discharge in men is straightforward and cost-effective, but syndromic management of vaginal discharge in women is not, because symptoms are poor predictors of the presence of an STI. A cheap, simple, dipstick-type test for gonorrhoea and chlamydial infection in women would be valuable in the control of these infections.

To provide an adequate clinical service, the following components are needed:

  • Training should be given to health workers, for instance in the use of flowcharts to simplify the management of STD patients, or to strengthen their health education and counselling skills.

  • Laboratory services need to be expanded, depending on the level of health care provided. A reference laboratory should be developed in each country to provide quality control and to support operational research.

  • Research should be undertaken to include studies on the aetiology of common syndromes, and assessment of antimicrobial sensitivity.

  • Information systems or surveillance are needed to gather epidemiological data, to assess trends, and to provide data for programme planning and monitoring. Various surveillance methods can be used—clinician notification, laboratory notification, sentinel site surveillance (either of syndromes or of aetiological diagnoses), prevalence studies in specific population groups, and aetiological surveys in patients.

Screening programmes

Many people with STIs have no symptoms, and so do not seek medical care. While effective programmes for partner notification may identify some of these, screening programmes have been advocated to identify and treat these people. Because of the severe adverse effects of syphilis on the fetus, screening of pregnant women for syphilis is recommended policy in most countries, and remains a highly cost-effective intervention even when the prevalence of syphilis in pregnant women is less than 0.1%. If universally implemented, it could prevent more than 500 000 perinatal deaths worldwide. The increased resources now available for the prevention of mother-to-child transmission of HIV in many developing countries offer an important opportunity to increase the coverage of antenatal syphilis screening.

Screening of other groups is more controversial. In some countries where sex work is legal or tolerated, screening programmes for sex workers are routinely implemented. A study in the United States of America found that population-based screening for chlamydial infection reduced the incidence of pelvic inflammatory disease. A national screening programme for chlamydial infection in young people seeking health care for any reason has recently been implemented in England, but there is no convincing evidence that such programmes are effective.

Conclusion

A successful STI control programme, by reducing both the incidence and prevalence of STIs, will reduce the morbidity, suffering, and economic cost associated with these diseases. By eliminating STIs as a facilitating factor in HIV transmission, and by contributing to behavioural changes towards safer sex, it will play an important role in the prevention and control of HIV/AIDS. In the longer term, control of STIs will depend on improved living conditions for poor people, particularly women, in both developed and developing worlds.

Further reading

Chen Z-Q, et al. (2007). Syphilis in China: results of a national surveillance programme. Lancet, 369, 132–8.Find this resource:

Fenton KA, et al. (2001). Sexual behaviour in Britain: reported sexually transmitted infections and prevalent Chlamydia trachomatis infection. Lancet, 358, 1851–4.Find this resource:

Fleming DT, Wasserheit JN (1999). From epidemiological synergy to public health policy and practice: the contribution of other sexually transmitted diseases to sexual transmission of HIV infection. Sex Transm Infect, 75, 3–17.Find this resource:

Gerbase AC, Rowley JT, Mertens TE (1998). Global epidemiology of sexually transmitted diseases. Lancet, 351 Suppl. III, 2–4.Find this resource:

Grosskurth H, et al. (1995). Impact of improved treatment of STD on HIV infection in rural Tanzania: randomised controlled trial. Lancet, 346, 530–6.Find this resource:

Low N (2007). Chlamydia screening programmes: when will we ever learn? BMJ, 334, 725–8.Find this resource:

Obasi A, et al. (1999). Antibodies to herpes simplex virus type 2 as a marker of sexual risk behaviour in rural Tanzania. J Infect Dis, 179, 16–24.Find this resource:

Over M, Piot P (1993). HIV infection and sexually transmitted diseases. In: Jamison DT, et al. (eds.) Disease control priorities in developing countries. Oxford University Press, Oxford.Find this resource:

    Schmid G (2004). Economic and programmatic aspects of congenital syphilis prevention. Bull World Health Organ, 82, 402–9.Find this resource:

    Peeling RW, et al. (2004). Avoiding HIV and dying of syphilis. Lancet, 364, 1561–3.Find this resource:

    UK Collaborative Group for HIV and STI Surveillance (2006). A complex picture: HIV and other sexually transmitted infections in the UK: 2006. Centre for Infections, Health Protection Agency, London.Find this resource:

      Wawer MJ, et al. (1999). Control of sexually transmitted diseases for AIDS prevention in Uganda: a randomised community trial. Lancet, 353, 525–35.Find this resource: