Swimmer's itch, also known as cercarial dermatitis or clam digger's itch, is an allergic dermatitis to the abortive penetration into the skin by cercariae of mammalian and avian schistosomes (Trichobilharzia and Bilharziella) after exposure to water containing the intermediate snail hosts. The schistosomes may cause an immunological reaction in the form of cercarial dermatitis in individuals who have been sensitized to the schistosomes.

There is a worldwide distribution, with human schistosomes more common in tropical climates (most commonly Schistosoma mansoni and Schistosoma hematobium).

Swimmer's itch typically presents as a pruritic rash on skin 24 hours after exposure to water containing cercaria in endemic areas. The cercariae enter the stratum corneum and die within hours. The rash is associated with moderate pruritus that is self-limiting but may persist for several weeks. Some patients may develop a delayed rash several days after initial exposure.

Cercarial dermatitis may become more severe with repeated exposure. Patients may develop edema, papulovesicles, and intense pruritus.

Rare presentations have been reported on the upper extremities after cleaning fish tanks.

There are no age or sex predilections.