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Swimmer itch (or swimmer's itch) is a patchy, red skin rash that causes itching on the parts of the body that have been in the water. Also known as cercarial dermatitis, it often appears within 48 hours of exposure to the water and may last up to a week. Symptoms normally disappear without treatment. It is also referred to by several other names, including “duckworms,” “duck fleas,” “rice paddy itch” or “clam digger's itch.”

Swimmer itch can result from contact with fresh or salt water. It is caused by the presence of trematode parasites. Trematode parasites are released into the water by both snails and certain birds and mammals, which serve as host animals. Trematode larvae cannot live in the human bloodstream, so the parasites die soon after burrowing into human skin. Nevertheless, they may cause an allergic reaction, which results in swimmer's itch. The condition is not contagious.

Swimmer itch is found throughout the world, and is more common during the summer months because of three reasons:

• Water warms to a temperature conducive for snails to grow and reproduce rapidly.
  
  
• Migrating birds infected with the trematode parasite return from their winter habitats to their summer homes. In addition, other aquatic birds return to full activity.
  
  
• Swimmers and bathers are present in the waters in greater numbers than at any other time of the year.

Children are at especially high risk for swimmer itch. The parasite larvae that spread the disease are most often found near the shoreline in shallow water, where children tend to swim. In addition, children are less likely to towel dry themselves after leaving the water.

In tropical countries, parasites that cause swimmer itch sometimes cause severe disease when found in contaminated water. These parasites are not found in the United States.

Not all rashes that result after swimming are swimmer itch. Many other types of rashes can result from contact with fresh or salt water.