Infection Infected people excrete the eggs in faeces or urine, the eggs hatch and infect certain fresh-water snails where they develop into the next larval stage. This stage (the cercarie) escapes from the snails swimming freely in the water looking for a new host where they penetrate the skin and cause the infection.
Symptoms Initially the infection may cause "swimmers itch" with an itching rash where the cercaria have penetrated the skin. In rare cases an acute allergic reaction with general malaise and fever are seen weeks after infection (Katayama fever).
The initial infection is usually without symptoms but may without treatment cause chronic liver and kidney disease, impaired bladder function, and cancer of the bladder and rectum.
Diagnosis Detection of specific antibodies in a blood sample and finding eggs in urine and faeces.
Treatment Several antiparasitic drugs are effective.
Prevention Avoid swimming in fresh water in areas where Schistosomiasis is found (see map below). Schistosomiasis is particularly prevalent in tropical and subtropical Africa in lakes like Kariba, Tanganyika and Victoria, but most rivers and fresh water lakes must be considered infected unless authoritative local sources know this not to be the case.