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Swimmer itch (or swimmer's itch) is a patchy, red rash that develops when the skin comes into contact with fresh or salt water contaminated with parasites. It often appears within 48 hours of exposure to the water and may last up to a week. It is also referred to by several other names, including “duckworms,” “duck fleas,” “rice paddy itch” or “clam digger's itch.”

Although there are several parasites that can cause reactions after swimming, swimmer's itch usually refers to infestation with trematode parasites. These parasites are found in infected snails, which shed larvae that burrow into certain animals, such as waterfowl, muskrats and beavers, and live in their bloodstream. The host animals shed a different form of larvae in their feces. If these larvae land in water, they swim until they find a suitable snail and infect that snail, then the life cycle starts over again. Humans are not suitable host animals, so the larvae die after they burrow into human skin. This causes the allergic reaction that is known as swimmer's itch.

Swimmer itch typically produces a mild reaction in which the skin becomes red and raised. Itchy areas, sometimes resembling small pimples or blisters, appear. The skin may also tingle or burn. In most cases, only skin not covered by swimsuits, wet suits or waders is affected.

Symptoms associated with swimmer itch normally disappear without treatment. In some cases, patients may use creams and ointments to treat symptoms associated with this condition.