Schistosomiasis risk. Children standing in a pond containing snails that may carry the schistosomia- sis (bilharzia) parasite's larvae. Schistosomiasis is a tropical disease caused by flukes called schistosomes. When mature, the larvae leave their freshwater snail hosts and enter the water. They can penetrate human skin and reach the bloodstream where they grow into adult worms. The adults produce large numbers of eggs which provoke inflammation and fibrosis that obstructs blood flow in the liver or urine flow from the ureters. The eggs are passed out in the urine or faeces in order to infect freshwater. Treatment is with drugs. Photographed in Tanzania.