Schistosomiasis research. A schistosomiasis researcher sampling flora and fauna around the edge of a lake in Tanzania. Snails found during the survey are taken back to the laboratory for identification. Certain freshwater snails are the intermediate hosts in human schistosomiasis. Schistosomiasis (bilharzia) is a chronic trematode worm infection widespread in tropical countries. Infection occurs from fresh water containing worm 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 which may obstruct blood flow or urine flow.