BACKGROUND Causative agents. Human schistosomiasis is caused by the digenetic trematodes Schistosoma haematobium, Schistosoma intercalatum, Schistosoma japonicum, Schistosoma mansoni and Schistosoma mekongi. Infection occurs when larval forms of the parasites, known as cercariae, are released from aquatic snails (such as those of the genera Bulinus, Oncomelania, Biomphalaria and Neotricula) and penetrate the skin during water contact. Once inside a human host, cercariae transform into schistosomula and are transported to the portal circulation of the liver, where they mature and mate. Adult worms of S. intercalatum, S. japonicum, S. mansoni and S. mekongi migrate to the mesenteric vessels, and those of S. haematobium move to the veins that drain the urinary system. Parasite eggs are deposited in several tissues, primarily the liver, the bladder and the urinary tract. The presence of eggs in tissues elicits granulomatous reactions, causing disease. Chronic infection results in periportal fibrosis of the liver, calcification of the bladder and other sequelae. Genital schistosomiasis might increase the risk of HIV infection. Eggs are released into the excreta and, on reaching water, hatch into miracidia (another larval form of the parasite), which infect the intermediate snail hosts.