Swimmers Alert

What Is Swimmer’s Itch?

Swimmer’s Itch, technically known as schistosome dermatitis, is a common malady around Minnesota’s lakes during midsummer. It appears on your skin as red, itchy, bite-like welts within several hours of leaving the water. The irritation may last from a few days to several weeks, depending on an individual’s sensitivity. About 30-40% of people who come in contact with the parasite are sensitive and experience irritation. There are no reported long-term effects from swimmer’s itch and the parasite that causes it will not survive in humans.

How Can I Avoid It?

You can reduce the likelihood of suffering swimmer’s itch by following these simple guidelines. Although even careful adherence to the recommendations may not be 100% successful in preventing an outbreak, you can minimize the extent of irritation and itching.

  1. Dry off as soon as you leave the water. Rub your skin briskly to remove water drops before they begin to evaporate. Be sure to dry underneath waistbands and around leg openings of swimming suits.
  2. Don’t sit around in your wet swimsuit.
  3. Shower with soap and fresh water as soon as possible after swimming.
  4. Don’t wade or play in shallow water, especially in weedy areas. Swimming from a raft or pontoon minimizes your exposure.
  5. Clean beaches of weeds or other debris that have washed up on shore. They can
    harbor the snails that are host to the swimmer’s itch parasite.
  6. Don’t swim when there has been an onshore breeze that may have carried parasites to your beach.
  7. Don’t feed geese and ducks or allow them to congregate near your beach. Waterfowl are an important adult host for the parasites and swimmer’s itch outbreaks seem to be associated with people feeding ducks.

It can be especially difficult to keep children free of swimmer’s itch, because they frequently play in the shallows and often wear their swimming suits for hours as they play in and out of the water. If you clear debris, watch the wind direction, don’t feed waterfowl, and most importantly, teach them to dry o ff thoroughly each time they come out of the water, you’ll minimize your children’s risk of getting swimmer’s itch.

Where does swimmers itch come from?

Swimmer’s itch comes from a microscopic flatworm parasite (schisosome cercariae) that lives as an adult in aquatic birds or mammals, usually waterfowl. The adult worm sheds its eggs into the feces of the host, and the eggs are released into the water where they hatch into free-swimming “miracidiae.” The miracidiae swim
in search of an intermediate host, one of four species of snail that inhabit shallow waters in Minnesota. The host snails live in all sorts of areas including weedy, rocky, and sandy bottoms. After 3-4 weeks in the snail, a second free-swimming stage, called a “cercaria,” emerges in search of a primary host (another bird or mammal) to complete its life cycle. The cercariae are about 2 mm long and barely visible to the naked eye.

The release of cercariae typically occurs in late June or early July, when
lakes are nearly at their warmest summer temperatures. If the spring
has been very warm, problems with swimmer’s itch may begin earlier in
the summer. Most cercariae are released around midday, and will
swim to the surface to increase their chances of finding a host.
Wind and currents have been shown to carry cercariae as much as four miles from the area they were released.

In some areas snail populations may be as high as 400 per square meter, and one infected snail may release up to 4,000 cercariae per day. Even if not all the snails are infected, that can mean millions of cercariae on a typical beach each midsummer day.

When a swimmer leaves the water and the water drops on his/her skin begin to evaporate, the tiny cercariae burrow into the skin in an effort to survive. Sometimes the swimmer feels a tingling sensation on exposed parts of the body. Where water is held near the skin (at waistbands and leg openings) the cercariae have more time to burrow in. The cercariae are killed by the body’s natural defense mechanisms, but they cause a welt, or red itchy spot like a mosquito bite. This parasite poses no serious threat to humans. People cannot become a host for the parasite, either through skin penetration or by swallowing lake water.

Can Swimmers Itch Be Treated?

some sunscreens and lotions may reduce the infections, although nothing is known to be completely effective. If you get swimmers’ itch, lotions or ointments may relieve the itching. In severe cases, you may need antihistamines or steroid creams that can be prescribed by a physician.

You can avoid infection by vigorously toweling dry immediately after swimming, before water droplets dry on your skin.


University of Minnesota Extension Partial Credit

Contact Us

(218) 335-7400 DRM

Name Title Phone
Brown, Levi Environmental Director 335-7417
Harper, Jeff Water Resources Program Manager 335-7415

Centers for Disease Control and Prevention