Swimmer’s itch, also known as lake itch, duck itch, cercarial dermatitis, and Schistosome cercarial dermatitis,:432 is a short-term, immune reaction occurring in the skin of humans that have been infected by water-borne schistosomatidae. Symptoms, which include itchy, raised papules, commonly occur within hours of infection and do not generally last more than a week.
A number of different flatworm parasites in the family Schistosomatidae are what cause swimmer’s itch. These parasites use both freshwater snails and vertebrates as hosts in their parasitic life cycles. Mostly waterfowl are used as the vertebrate host. During one of their life stages, the larvae of the parasite, cercaria, leave the water snails and swim freely in the freshwater, attempting to encounter water birds. These larvae can accidentally come into contact with the skin of a swimmer. The cercaria penetrates the skin and dies in the skin immediately. The cercaria larvae cannot infect humans, but they cause an inflammatory immune reaction. This reaction causes initially mildly itchy spots on the skin. Within hours, these spots become raised papules which are intensely itchy. Each papule corresponds to the penetration site of a single parasite.