SWIMMER’S ITCH: A COMMON SUMMERTIME PEST

SPOONER, WI - An annual nuisance known as swimmer’s itch is again making its appearance in Wisconsin lakes. Technically known as schistosome dermatitis, swimmer’s itch appears as red itching, bite-like welts within several hours of leaving the water. The irritation can last from two days to several weeks, depending on the individual’s susceptibility. Preventive measures can be taken, and there are no permanent effects from swimmer’s itch.

Swimmer’s itch is a widespread occurrence 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 can be minor and last much of the season.

The irritation is caused during a life stage of a flatworm parasite (Schistosome) which 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 swim 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 undergo further development. 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.

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. It is at this time that 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% 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% 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 during the mid-day hours from noon to 2:00 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 30-40% of individuals 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.

Preventive measures can be taken to either reduce exposure or attempt to prevent the penetration. Swimming rather than playing or wading in shallow water will reduce exposure. If swimmer’s itch is known to be present, avoid swimming when winds are likely to be carrying cercariae into the beach. Swim offshore if possible. Brisk and vigorous toweling immediately after leaving the water can crush the cercariae before they can penetrate the skin. Some recreationists have noted that some sunscreens and lotions reduce the infections, and a professional aquatic biologist reports applying a waterproof sunscreen before swimming has personally been successful to prevent swimmer's itch. Once the irritation has developed, various soothing lotions or ointments may be applied to relieve the itching. For severe cases, prescription antihistamines and topical steroid creams may be prescribed by a physician.

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.

Although swimmer’s itch can be a major nuisance, we are fortunate in the U.S. that it causes no lasting health problems. In some parts of Africa and China, a similar organism actually infects humans as part of its life cycle and causes a debilitating disease known as schistosomiasis.

Here in Wisconsin, 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.