What is swimmer's itch?

Swimmer's itch, also called cercarial dermatitis, appears as a skin rash caused by an allergic reaction to specific parasites found in certain birds and mammals . These microscopic parasites are released from infected snails into fresh and salt water (such as lakes, ponds, and oceans). Rather than finding a host bird or mammal, the parasite instead burrows into the nearby swimmer's skin, causing an allergic reaction and rash. Swimmer's itch is found throughout the world and is more frequent during summer months.

How does water become infested with the parasite?

The adult parasite lives in the blood of infected animals such as ducks, geese, gulls, swans, and certain aquatic mammals such as muskrats and beavers. The parasites produce eggs that are passed in the feces of infected birds or mammals.

If the eggs land in the water, the water becomes contaminated. Eggs hatch, releasing small, free-swimming larvae. These larvae swim in the water in search of a certain species of aquatic snail.

If the larvae find one of these snails, they infect the snail and undergo further development. Infected snails release a different type of larvae ( cercariae , hence the name cercarial dermatitis) into the water. This larval form then swims about searching for a suitable host (bird, muskrat) so they can start the lifecycle over again . Although humans are not suitable hosts, the larvae burrow into the swimmer's skin, and may cause an allergic reaction and rash. Because these larvae cannot develop inside a human, they soon die.