Contributor: Gordon K. Klintworth
Schistosomiasis (bilharziasis) is an infection due to a trematode. It is the second most prevalent major vector-borne disease affecting ~10% of the world population (200 million people). The most prevalent is malaria. The population at risk for schistosomiasis is 600 million and this disease causes more morbidity and a greater mortality than other helminth diseases and is clearly the most important worm infestation of humans. Three species of schistosome (Schistosoma mansoni, Schistosoma japonicum, Schistosoma haematobium) account for most human infection. Infection starts by a penetration of the skin by cercariae and this produces a pruritic rash. Schistosoma mansoni and Schistosoma japonicum inhabit the portal and hepatic venous system; Schistosoma haematobium resides in the pelvic and urinary veins. The major sites of egg deposition are in these regions. The manifestations of schistosomiasis are largely due to immunologic and inflammatory reactions to the eggs that are deposited in the tissues. The eggs become surrounded by a granuloma or cellular infiltrate of eosinophils and neutrophils. While alive within veins adult schistosomes fail to elicit an inflammatory response. Schistosoma mansoni and Schistosoma japonicum cause liver disease that begins as periportal granulomatous inflammation. In severe cases portal hypertension follows an obstruction to the portal blood flow. Schistosoma haematobium causes urogenital schistosomiasis and squamous cell carcinoma [carcinoma - squamous cell] of the bladder [carcinoma - bladder] may develop. The eyelids may be involved in the generalized cutaneous edema of the acute phase of schistosomiasis. Rarely nodules form in the conjunctiva and in an exceptional case ova of  Schistosoma japonicum have been found in the lacrimal gland. The diagnosis of schistosomiasis is made by finding the relevant eggs in the stools or urine or by finding the ova in tissue.