Cercarial dermatitis (“swimmer’s itch”, “clam-digger’s itch”, “duck itch”) is caused by the cercariae of certain species of schistosomes whose normal hosts are birds and mammals other than humans. These cercariae seem to have a chemotrophic reaction to secretions from the skin and are not as host-specific as other types of human-infecting schistosomes. Skin penetration by these zoonotic cercariae causes dermatitis, but the cercariae do not mature into adults in the human body.
Several genera/species are known to cause cercarial dermatitis; the most commonly implicated genus globally is the waterfowl schistosome Trichobilharzia spp. (T. ocellata, T. brevis, T. stagnicolae, T. physellae, T. regenti, and others). Other avian schistosomes that cause cercarial dermatitis include Ornithobilharzia spp., Austrobilharzia spp. (A. ), Bilharziella polonica, and Gigantobilharzia huronensis. Cases involving mammalian schistosomes Heterobilharzia americana, Bivitellobiharzia spp., Schistosomatium spp., and some aberrant zoonotic Schistosoma spp. (S. spindale, S. (=Orientobilharzia) turkestanicum) occur occasionally. These schistosomes all use different snail intermediate hosts, commonly those from the families Nassariidae, Lymnaeidae, and Physidae.
Cercarial dermatitis should not be confused with seabather’s eruption, which is caused by the larval stage of cnidarians (e.g., jellyfish). The areas of skin affected by seabather’s eruption is generally under the garments worn by bathers and swimmers where the organisms are trapped after the person leaves the water. Cercarial dermatitis occurs on the exposed skin outside of close-fitting garments.
Adult worms are found in the blood vessels of definitive hosts and produce eggs that are passed in the feces On exposure to water, the eggs hatch and liberate a ciliated miracidium that infects a suitable snail (gastropod) intermediate host . The parasite develops in the intermediate host to produce free-swimming cercariae that are released under appropriate conditions and penetrate the skin of the birds and migrate to the blood vessels to complete the cycle . Humans are inadvertent and inappropriate hosts; cercariae may penetrate the skin but do not develop further . A number of species of trematodes with dermatitis-producing cercariae have been described from both freshwater and saltwater environments, and exposure to either type of cercaria will sensitize persons to both.
Life cycle image and information courtesy of DPDx.