[This article is adapted from the new Extension “Lake Home and Cabin Kit”]

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 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 percent 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.

Where Does it 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.

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 their skin begin to evaporate, the tiny cercariae burrow into the skin in an effort to survive. The swimmer may feel tingling 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. People cannot become a host for the parasite, either through skin penetration or by swallowing lake water.

Is there any treatment?

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.

People often want to control the snail hosts or the free-swimming cercariae, but neither option is practical because the cercariae can swim or be carried long distances. To control severe infestations of snails, the application of copper sulfate in the lake is a possibility, but it requires application over a large area and copper sulfate can also kill smaill fish. Waters treated with copper sulfate should not be used for 48 hours after application. ANY chemical treatment in the water requires a permit from the Department of Natural Resources, Section of Fisheries. Contact your regional DNR fisheries office for assistance and permit information.

How Can I Avoid Swimmer’s Itch?

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 percent successful in preventing an outbreak, you can minimize the extent of irritation and itching.

• 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. Encourage children to dry off thoroughly each time they leave the water.
• Shower with soap and fresh water or change into dry clothes as soon as possible after swimming.
• Don’t wade or play in shallow water, especially in weedy areas. Swimming off of a raft or pontoon minimizes your exposure.
• Clean beaches of weeds or other debris that have washed up on shore. They can harbor the snails.
• Don’t swim when there has been an onshore breeze that may have carried parasites to your beach.
• Don’t feed geese and ducks or allow them to congregate near your beach. Waterfowl are an important adult host for the parasites.

Posted in Lakes |