Schistosomiasis risk. Women and children washing clothes at a pond in Morogoro, Tanzania. Because the water contains snails which are the intermediate host in human schistosomiasis, they are at risk of becoming infected. Schistosomiasis (bilharzia) is a chronic trematode worm infection widespread in tropical countries. Infection occurs from fresh water containing infective larvae emitted by the snails. The worms mature in the veins of the intestines and bladder, producing large numbers of eggs which provoke inflammation and fibrosis that obstructs blood flow in the liver or urine flow from the ureters. Treatment is with the drug praziquantel.