Cryptosporidium and cryptosporidiosis

Cryptosporidia are small coccidian parasites that infect the mucosal epithelia of a variety of vertebrate hosts, including humans, affecting the health, survival, and economic development of millions of people and animals worldwide. Human infection is mainly caused by two species: (1) Cryptosporidium parvum—also prevalent in young livestock; can be transmitted from animals to humans (zoonotic transmission, particularly important in children), from person to person (‘urban’ cycle, due to faecal–oral spread), through contamination of public drinking-water supplies (which can produce massive outbreaks) or food (prepared by a sick food handler), and nosocomially. (2) C. hominis—essentially a human parasite; may produce large waterborne outbreaks.

Clinical features—infection involves either children or adults, but is a major cause of diarrhoea in children under 5 years old in both developed and developing countries. Patients may be asymptomatic or experience acute or chronic diarrhoea, depending on their age and immune status: (1) immunocompetent humans—infection usually results in acute self-limiting diarrhoea; (2) patients immunocompromised by drugs or AIDS, and those with concurrent infections such as measles or chickenpox—clinical symptoms are more severe and persistent and may become chronic, leading to electrolyte imbalance, wasting and even death. Since 2004, Cryptosporidium has been included in the WHO ‘Neglected Diseases Initiative’, in recognition of the importance of this infection in developing countries.

Diagnosis and treatment—diagnosis is usually made by detection of oocysts in stool, often by use of direct fluorescent-antibody tests. Detection of soluble Cryptosporidium antigens in faecal samples by enzyme-linked immunosorbent assay (ELISA) is useful for the screening of large numbers of specimens. Molecular methods allow reliable identification of species and genotypes, and are therefore of paramount importance for environmental or epidemiological research purposes. Patients who are immunocompetent are usually managed symptomatically: there is no very effective anticryptosporidial treatment, but those with persistent disease can be given nitazoxanide. Management of patients who are immunocompromised is difficult: aside from supportive care, highly active antiretroviral therapy (HAART) is effective, both by immune reconstitution (in patients with HIV/AIDS) and by direct inhibition of parasite proteases.

Prevention—primary control is by limiting the opportunity for faecal–oral transmission, both direct and indirect, with maintenance of drinking-water quality and general hygiene (especially in hospitals, wards, etc.) essential for the prevention of the infection. Secondary control, when water supplies are contaminated, can be achieved by boiling water or filtering it (using an appropriate device) before drinking.