Schistosomiasis

Schistosomiasis is an infection with a relatively low mortality rate but a high morbidity rate; it is endemic in 74 developing countries, with more than 80% of infected people living in sub-Sa-haran Africa. Infection is caused by trematode flatworms (flukes) of the genus Schistosoma: in freshwater, intermediate snail hosts release infective forms of the parasite. There are five species of schistosomes able to infect humans: Schistosoma haematobium (the urinary form) and S. japonicum, S. mekongi, S. mansoniand S. intercalatum (the "intestinal" forms).

If people are in contact with water where infected snails live, they become infected when larval forms of the parasites penetrate their skin. Later, adult male and female schistosomes pair and live together in human blood vessels. The females release eggs, some of which are passed out in the urine (in S. haematobium infection) or stools (S. mansoni and S. japonicum), but some eggs are trapped in body tissues. Immune reactions to eggs lodged in tissues are the cause of disease. Systemic complications are bladder cancer, progressive enlargement of the liver and spleen, intestinal damage due to fibrotic lesions around eggs lodged in these tissues, and hypertension of the abdominal blood vessels. Most cases of cerebral schistosomiasis are observed with S. japonicum, constituting 2-4% of all S. japonicum infections. However, CNS schistosomiasis also can occur with other species and involves seizures, headache, back pain, bladder dysfunction, paresthesias and lower limb weakness. Death is most often caused by bladder cancer associated with urinary schistosomiasis and by bleeding from varicose veins in the oesophagus associated with intestinal schistosomiasis. Children are especially vulnerable to infection, which develops into chronic disease if not treated. Diagnosis is made by using urine filtration and faecal smear techniques, antigen detection in endemic areas and antibody tests in non-endemic areas.

The disease is controlled through an integrated approach: drug treatment with praziquantel or oxamniquine (effective only against S. mansoni), provision of an adequate safe water supply, sanitation and health education (39).

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