Swimmer's Itch

Swimmer's itch occurs in some Wisconsin Lakes each year. Swimmer's itch is caused by the larvae (immature stage) of certain flatworms that can be picked up while swimming. Technically known as schistosome dermatitis, swimmer’s itch appears as red itching, bite-like welts within several hours of leaving the water. It is neither dangerous nor contagious, but is very uncomfortable.

When the larva penetrates the skin, it causes a small red welt. The degree of discomfort and bodily reaction varies with the person's sensitivity and the degree of infestation. In some people, the reaction may be hardly noticeable. Others have considerable pain, fever, severe itching, and swelling. The swelling usually subsides within a week, but the redness can last longer.

Swimmer's itch organisms are most commonly noticed in early summer, when the water is its warmest. The season is relatively short—usually four to six weeks, depending on the weather.

It’s best to regard swimmer’s itch in the same manner as mosquitos, woodticks and deer flies; there really is nothing that can be done to eliminate them, and our best action is to learn how to reduce exposure. Often these creatures we consider pests are signs of a healthy and diverse outdoors environment. Overall they shouldn’t discourage us from enjoying the many outdoor activities we can experience when we venture into their outdoor habitat.

Preventing Swimmer's Itch

Some people have noted that waterproof sunscreens and lotions reduce the infections.

If you decide to go in the water when and where swimmer's itch larvae are present, stay clear of plants growing in the lake. Swimming rather than playing or wading in shallow water will reduce exposure. Swim offshore if possible. If swimmer’s itch is known to be present, avoid swimming when winds are likely to be carrying the organisms into the beach.

The most important thing to do to prevent the itch is to rub down very briskly right after leaving the water. This can crush the organisms before they can penetrate the skin. Showering shortly after leaving the water also should help.

Easing Swimmer's Itch

After the swimmer's itch organisms have penetrated the skin, there is little that can be done to treat it. You may get some relief by using soothing lotions such as calamine or lotions containing antihistamines and/or local anesthetics. In severe cases, see a physician.

More About the Cause of Swimmer's Itch

Swimmer’s itch is widespread in Wisconsin and has been reported in many other states and also in Europe and elsewhere in the world. There seem to be no special characteristics of lakes having the problem. Some of the finest recreational waters in the state experience swimmer’s itch annually, whereas other lakes may have an occasional outbreak or none at all. An outbreak may be severe, but last for only a few days, or minor and last much of the season.

The flatworm parasite (schistosome) lives as an adult in suitable mammals and birds, such as mice and ducks. The adult worm sheds its eggs via the host’s excretory tract into the water. Here they hatch into a free-swimming stage called a miracidium. The miracidium swims in search of a proper second host animal, a particular type of snail. If a proper snail is found, the miracidium will penetrate into the snail’s tissue and develop further. After a three- or four-week development period, another free-swimming stage called a cercaria emerges from the snail in search of the proper primary bird or mammal host.

Life cycle of a flatworm: (A) blood fluke carried by bird. (B) egg. (C) miracidium. (D) snail host. (E) cercaria seeking host.
DNR graphic

The cercariae release normally occurs when the water temperatures reach their near-maximum summer temperature. This usually occurs in late June or early July in northern Wisconsin, coinciding with peak water recreational activities. At this time the organism can accidentally contact bathers and cause swimmer’s itch. In years of warm spring weather, swimmer’s itch has occurred as early as May in northwestern Wisconsin.

A swimmer’s itch problem may develop with a few as 2 percent of the snails infected. However, snail populations may be as high as 400 per square meter. One infected snail may release up to 4,000 cercariae per day. At the 2 percent infection rate, this would mean up to 32,000 cercariae would be produced per square meter per day. On a typical 100’ x 100’ beach area, this translates into a potential 30 million cercariae released each day.

Most cercariae are released between noon and 2 p.m. With little free-swimming abilities, the cercariae will swim to the surface to optimize their chance of contacting a suitable animal host. Concentrated near the surface, wind and currents may carry the cercariae up to four miles from the release area.

The cercariae may not penetrate the skin until after the bather leaves the water, at which time the person may feel a slight tingling sensation. The cercariae are soon killed by the body’s natural defense mechanism, but will continue to cause irritation. Studies have shown that 30 to 40 percent of people contacting the parasites are sensitive and experience irritation. Small children playing in shallow water are most susceptible because of the alternate wetting and drying with the arms, legs and waist area most prone to infection.

There is no effective way for people to eliminate swimmer’s itch on their beach. Any attempts to control swimmer’s itch by treatment to kill either the cercariae or their snail hosts are ineffective because cercariae are capable of swimming or drifting long distances from non-treated areas. It makes no difference if your beach area is sandy, rocky or weedy. Host snails will live on all sites and one species which commonly harbors swimmer’s itch actually prefers sandy-bottom areas.

Feeding of ducks should be discouraged if swimmer’s itch is known to be a problem on the lake, since waterfowl are an important adult host to the parasite. New occurrences of swimmer’s itch seem to be strongly associated with people feeding and attracting ducks. In recent years, there have been experimental attempts at treating the host birds with veterinary medicines. The theory is to rid the birds of the adult parasite before they can infect the snail population with miracidia. Depending on the different kinds and numbers of adult hosts, success at this method will be limited to very specific situations. Thus far, the procedure is considered impractical on a lakewide scale in Wisconsin.

Modern pesticide laws prohibit treatments as they were historically attempted. Treatments to kill snails are very harsh and kill many non-target plants and animals and may also lead to contaminated sediments. Some high-use public beaches on specific sites where incoming drift of cercariae is unlikely, have been issued permits for a highly reduced treatment, but the result is very temporary and questionable. Anyone proposing any kind of pesticide or chemical treatment for any purpose must obtain a permit from the Department of Natural Resources.

More about Swimmer's Itch

Swimmer's Itch warning sign [PDF]
What You Should Know about Swimmer's Itch in Michigan [Exit DNR]
Wisconsin Dept. of Health Services - Disease Fact Sheet Swimmer's Itch [Exit DNR]