Division of Parasitic Diseases – Cercarial dermatitis Fact Sheet

Cercarial Dermatitis: Swimmer’s Itch

Cercarial Dermatitis (SIR-care-ee-uhl DER-muh-TIGHT-iss)

What is swimmer’s itch?

Swimmer’s itch, also called cercarial dermatitis, is a skin rash caused by an allergic reaction to infection with certain parasites of birds and mammals. These microscopic parasites are released from infected snails to swim in fresh and salt water, such as lakes, ponds, and oceans used for swimming and wading. Infection is fond throughout the world. Swimmer’s itch generally occurs during summer months.

What are the signs and symptoms of swimmer’s itch?

Within minutes to days after swimming in contaminated water, you may experience tingling, burning, or itching of the skin. Small reddish pimples appear within 12 hours. Pimples may develop into small blisters. Itching may last up to a week or more, but will gradually go away.

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.

There are other causes of rash that may occur after swimming in fresh and salt water.

Do I need to see my health care provider for treatment?

No. Most cases do not require medical attention.

If you have a rash, you may try the following for relief:

corticosteroid cream cool compresses bath with baking soda baking soda paste to the rash anti-itch lotion Calamine* lotion colloidal oatmeal baths, such as Aveeno*

Try not to scratch. Scratching may cause the rash to become infected. If itching is severe, your health care provider may prescribe lotion or creams to lessen your symptoms.

How does water become infested with the parasite?

The adult parasite lives in the bloodstream of infected host animals such as ducks, geese, gulls, swans, as well as in certain aquatic mammals such as muskrat and beaver. The parasites produce eggs that are passed in the feces of the host bird or mammal.

If the parasite-infected feces lands in the water, the water becomes contaminated. Eggs hatch, releasing small, free-swimming larvae, called miricidia. These larvae swim in the water in search of a certain species of aquatic snail. If the larvae find a snail, they burrow into it and undergo a number of developmental changes. New, slightly larger larvae, called cercariae, are released from the snail into the water. This larval form then searches for a host. If the larvae find a suitable host, they burrow into the skin and develop into adult parasites to repeat the cycle. Humans get swimmer’s itch when the cercarial larvae burrow into the skin, thus the name cercarial dermatitis. The larvae cannot develop inside a human and they soon die.

Can swimmer’s itch be spread from person-to-person?


Who is at risk for swimmer’s itch?

Anyone who swims or wades in infested water may be at risk. Larvae are more likely to be swimming along shallow water by the shoreline. Children are most often affected because they swim, wade, and play in the shallow water more than adults. Also, they do not towel dry themselves when leaving the water.

Once an outbreak of swimmer’s itch has occurred in water, will the water always be unsafe?

No. Many factors must be present for swimmer’s itch to become a problem in water. Since these factors change (sometimes within a swim season), swimmer’s itch will not always be a problem. However, there is no way to know how long water may be unsafe. Larvae are generally infective 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. For future snails to become infected, migratory birds or mammals in the area must be infected.

What can be done to reduce the risk of swimmer’s itch?

Avoid swimming in areas where swimmer’s itch is a known problem or where signs have been posted warning of unsafe water. Avoid swimming near or wading in marshy areas where snails are commonly found. Towel dry or shower immediately after leaving the water. Encourage health officials to post signs on shorelines where swimmer’s itch is a current problem. Do not attract birds by feeding them to areas where people are swimming.

Is my swimming pool safe to swim in?

Yes. As long as your swimming pool is well-maintained and chlorinated, there is no risk of swimmer’s itch.

This fact sheet is for information only and is not meant to be used for self-diagnosis or as a substitute for consultation with a health care provider. If you have any questions about the disease described above or think that you may have a parasitic infection, consult a health care provider.

Centers for Disease Control and Prevention