River prawns eat the snails that harbor schistosomiasis (also known as snail fever or bilharzia), acting as a natural control on the disease. Schistosomiasis is a disease in humans caused by parasitic flatworms, which causes chronic pain and stunted growth, diarrhea, and bloody urine, and if left untreated, liver and kidney failure, infertility, and bladder cancer. The parasite infects river snails, which pass the parasites to humans when they come into contact with water containing the parasites. New research now shows that dams halt the migrations of predatory river prawns that would otherwise eat the snail hosts of schistosomiasis, resulting in the snails exploding in abundance, and sharp increases in the disease after damming. In all, 277-385 million people are at increased risk of schistosomiasis because they live in regions where river prawns have been extirpated by dams, suggesting that prawn restoration would reduce their disease burden. The new research was coauthored by SAFS prof Chelsea Wood and appears in Philosophical Transactions B.