Centers for Disease Control/Division of Parasitic Diseases and Malaria
EOL staff
Life cycle of avian schistosomes such as Austrobilharzia variglandis causing cercarial dermatitis ('swimmer's itch') in humansHosts of avian schistosomatid trematode flatworms 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 (1). On exposure to water, the eggs hatch and liberate a ciliated miracidium that infects a suitable snail (gastropod) intermediate host (2). The parasite develops in the intermediate host (3) 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 (4). Humans are inadvertent and inappropriate hosts; cercariae may penetrate the skin but do not develop further (5). 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.From Centers for Disease Control Parasites and Health website
Nassa obsoleta (the little black winkle of the tide flats) devouring a scallop. These little scavengers swarm over the scallop. Occasionally one is active enough to get between the valves, forming a wedge which permits the entrance of others, which quickly consume the scallop. Owing to the alertness of the scallop and its different habitat (Nassa usually being found on the tide flats) little damage is done.
