Swimmer’s itch is a short-term, immune reaction occurring in the skin of humans that have been infected by water-borne trematode parasites. Symptoms, which include itchy, raised papules, commonly occur within hours of infection and do not generally last more than a week.

Since it was first described in Michigan in 1928,^[2] swimmer’s itch has been reported from around the world.

The genera most commonly associated with swimmer’s itch in humans are Trichobilharzia and Gigantobilharzia. Trematodes in these groups normally complete their life cycles in birds. However, swimmer’s itch can also be caused by schistosome parasites of non-avian vertebrates, such as Schistosomatium douthitti, which infects snails and rodents. Other taxa reported to cause the reaction include Bilharziella polonica and Schistosoma bovis.

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.

Orally adiministered hydroxyzine, an antihistamine, is sometimes prescribed to treat swimmer’s itch and similar dermal allergic reactions.