Most cases are caused by parasites that use waterfowl as the vertebrate host. These avian schistosomes cannot complete their life cycles in mammals, but can accidentally infect humans, giving rise to mildly itchy spots on the skin. Within hours, these spots become raised papules that are more intensely itchy. The papules are caused by localized inflammatory immune reactions, each corresponding to the penetration site of a single parasite, which dies in the skin within hours.