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The snail Nassarius obsoletus has some medical significance to humans as the intermediate host of the schistosomatid trematode flatworm Austrobilharzia variglandis, the normal hosts of which are ducks, but which is often implicated in cases of cercarial dermatitis in humans. This snail can be found at marine beaches in temperate climates. Cercarial dermatitis 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 schistosomes. They attempt to, and, sometimes may actually, enter human skin. The penetration causes a dermatitis which is usually accompanied with intense itching, but the cercariae do not mature into adults in the human body. Cases of cercarial dermatitis can occur in both fresh and brackish water environments. 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. In contrast, cercarial dermatitis occurs on the exposed skin outside of close-fitting garments.
Hosts of avian schistosomes can be either year-round resident or migratory birds, including seagulls, shorebirds, ducks, and geese. Adult worms are found in the blood vessels 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, such as Nassarius obsoletus. The parasite develops in the intermediate host to produce free-swimming cercariae that are released under appropriate conditions, penetrate the skin of the avain hosts, 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 trematode flatworms 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. Cercarial dermatitis occurs worldwide, with cases reported from every continent except Antarctica. In the United States, cases are commonly reported from the Great Lakes region.