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Readers Want to Know: Swimmer's Itch


I was fixing my dock in a Lake Superior bay. Later the same day I discovered about 30 seriously itchy red welts resembling black fly bites on the parts of my legs that were submerged. Do you know what attacked me?


You were attacked by cercariae, the larvae of flatworm parasites in the Schistosomatidae family. These particular flatworms rely on freshwater snails and vertebrates, usually birds, to complete their life cycle. When a cercaria leaves its snail host in a quest to relocate into the body of a water bird, it can accidently burrow into human skin. Big mistake. Under human skin, cercariae die immediately.

Usually within 30 minutes after a cercaria gets under your skin, a red spot appears. The spot will expand over the next day into what is called a papule. Itchy papules can bother you for a week and are limited to areas of the body that get exposed to infected water. Toweling off immediately after swimming or wading may reduce the possibility of swimmers itch; it really depends on the species of parasite. You can also swim farther from shore, away from where snails accumulate and wind blows cercariae. Swimmer’s itch is most common in summer, when water temperatures permit snails to reproduce rapidly, aquatic birds are raising families, and swimmers are frequent.

Don't let the threat of an itch stop you from swimming, though!

If you get swimmer's itch, medically known as cercarial dermatitis, try not to scratch so hard that you create abrasions and skin infections. Wash the affected areas with isopropyl alcohol and coat them with calamine lotion. Antihistamine or mild cortisone cream could help the itch, as might these home remedies:

  • Bathing with baking soda or colloidal oatmeal,

  • Applying baking soda paste or cold compresses,

  • Washing with salt water, acetic acid (vinegar).

Swimmer's itch is uncommon in the shallows of Lake Superior but
occasionally people report that they have contracted it there. The Minnesota Department of Health, Northeast District Office tracks reports of gastrointestinal
illness due to water contact but does not specifically track reports of swimmer’s itch along Minnesota’s coast. For more information on Swimmer’s itch visit:

By Sea Grant Staff
June 2011

Return to June 2011 Seiche

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