Schistosomiasis is caused by digenetic blood trematodes.
The three main species infecting humans are Schistosoma haematobium, S. japonicum, and S. mansoni. Two other species, more localized geographically, are S. mekongi and S. intercalatum. In addition, other species of schistosomes, which parasitize birds and mammals, can cause cercarial dermatitis in humans. Adult worms in humans reside in the mesenteric venules in various locations, which at times seem to be specific for each species. For instance, S. mansoni occurs more often in the superior mesenteric veins and S. japonicum more frequently in the inferior mesenteric veins.