Swimmer's itch, also called cercarial dermatitis, appears as a skin rash caused by an allergic reaction to certain parasites that infect some birds and mammals. These microscopic parasites are released from infected snails into fresh and salt water (such as lakes, ponds, and oceans). While the parasite's preferred host is the specific bird or mammal, if the parasite comes into contact with a swimmer, it burrows into the skin causing an allergic reaction and rash. Swimmer's itch is found throughout the world and is more frequent during summer months. Most cases of swimmer's itch do not require medical attention.
Image: Left: Cercariae of Austrobilharzia variglandis (left), which can cause cercarial dermatitis. Note the forked "tail" and a pair of "eye spots" near the anterior end (right). Right: A group of geese, a preferred host of the parasite that causes cercarial dermatitis. Credit: DPDx