Environmental Health
Surface Water (Lakes, Rivers & Streams)
Swimming in Thurston County
  Swimmer's Itch  

What is Swimmer's Itch?

Photo of swimmer's itch rashEach summer, a skin rash, commonly called Swimmers Itch (also called cercarial dermatitis), is reported by swimmers in many of the lakes found throughout Washington State, particularly in the western and northern regions. It is caused by an allergic reaction to a parasite (cercariae) that has burrowed into the swimmer’s skin.

Where do the parasites come from?

The adult parasite exists in the blood of infected water fowl, such as ducks, geese, swans and gulls and in aquatic animals such as beaver and muskrat. The eggs produced by the adult parasite develop in the intestinal tract of its host and are excreted in the feces of the bird or animal into the water. These eggs can hatch in the water, releasing small free-swimming larvae which swim in search of its second host, a certain species of aquatic snail. The larvae infect the snail, multiply and develop into a different type of larvae. This larval form then swims about searching for a suitable host (bird or aquatic animal) to continue the life cycle. There is no way to know how long the water may be unsafe. Larvae generally survive for 24 hours once they are released from the snail. However, an infected snail will continue to produce cercariae throughout the remainder of its life.

Who is at risk of Swimmer's Itch?

Anyone who swims or wades in infested waters may be at risk. The larvae are more likely to be present in shallow water by the shoreline, therefore children are most often affected because they tend to swim, play, and wade in shallow water more than adults.


What are the signs and symptoms of Swimmer's Itch?

While humans are not natural hosts, we can become involved accidentally. A small amount of water remains on the skin when a swimmer leaves the water. As the water evaporates or runs off, the larvae that are present quickly burrow into the skin. Human skin and tissue is not a suitable environment for these parasites, therefore the larvae soon die. The presence of the foreign protein material under the skin causes an allergic type of reaction, resulting in the rash and itching. Symptoms may occur within minutes to days after swimming in contaminated water. Symptoms include:

  • Tingling, burning, or intense itching of the skin
  • Small reddish pimples appear within 12 hours which may become small blisters or pustules

The itching is intense and causes considerable scratching. Scratching the infected areas may result in secondary bacterial infection. After a week or so, the itching subsides and the lesions heal, however in some instances, recovery takes as long as 30 days. Because swimmer’s itch is caused by an allergic reaction to infection, the more often you swim or wade in contaminated water, the more likely you are to develop more serious symptoms. The greater the number of exposures to contaminated water, the more intense and immediate symptoms of swimmer’s itch will be.

What is the treatment for Swimmer's Itch?

The following may relieve the itching:

  • Corticosteroid cream
  • Apply cool compresses to the affected areas
  • Bathe in Epson salts or baking soda
  • Soak in colloidal oatmeal baths or use lotions such as Aveeno
  • Apply baking soda paste to the rash (stir water into baking soda until it reaches a paste-like consistency)
  • Use an anti-itch lotion, such as calamine lotion
  • Consult your doctor for additional information on treatment.

Is there a way to prevent Swimmer's Itch

The following easy steps can be taken to prevent Swimmer’s Itch

  • Avoid swimming in areas where there is evidence of ducks and geese.
  • Do not swim or wade in marshy areas where snails are commonly found.
  • Shower, towel off, or wipe water off the body with the palms of the hands immediately following swimming.
  • Apply a good waterproof sunscreen prior to entering the water.

Resources & Other Helpful Information


  • Report algae blooms or swimming-related illness
  • General Questions
This page last updated: 04/05/18