With warm weather, Swimmers Itch makes annual appearance

SPOONER, Wis. -- As waters warm an annual nuisance known as “swimmer’s itch” is making its seasonal appearance in Wisconsin lakes. Technically known as schistosome dermatitis, swimmer’s itch appears as red itching, bite-like welts within minutes and sometimes hours after leaving the water. The irritation can last from two days to several weeks, depending on the individual’s susceptibility.

There are no permanent effects to people from this pest.

Swimmer’s itch is a widespread occurrence and is found in other states and other parts of the world. According to Frank Koshere, a Department of Natural Resources water resources specialist, 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,” he said, “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 be minor and last much of the season.

The irritation is caused during a life stage of a flatworm parasite (Schistosome) which lives in an adult host such as mice and ducks. The adult worm sheds its eggs via the host’s excretory tract into the water. There 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.
This is when the bug and humans meet.

The cercariae release normally occurs when the water temperatures reach their near-maximum summer temperature -- usually in late June or early July in northern Wisconsin, coinciding with peak water recreational activities. This is the time the organism can accidentally contact bathers and cause swimmer’s itch.

Most cercariae are released during the mid-day hours from noon to 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—and may latch on to human skin. Concentrated near the surface, wind and currents may carry the organism up to four miles.

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 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.

More information and printable signs for cautioning people about swimmer's itch outbreaks are available on the swimmer's itch information page of the DNR Web site.

Prevention measures

Although swimmer’s itch can be a nuisance, people in the U.S. are fortunate that it causes no lasting health problems.

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 sunscreens and lotions reduce the infections but nothing is completely effective. 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 beaches. Attempts to kill or 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 a beach area is sandy, rocky or weedy. Host snails will live on all sites and one species, which commonly harbors swimmer’s itch, prefers sandy-bottom areas. Modern pesticide laws prohibit treatments as they were historically attempted. Chemical treatment for swimmer’s itch is not likely to be permitted in a natural lake where the above adverse effects will occur. Treatments to kill snails are very harsh and kill many non-target plants and animals and may lead to contaminated sediments. Anyone wanting to chemical treat their beach must obtain a permit from the Department of Natural Resources.

Feeding ducks should be discouraged if swimmer’s itch is known to be a problem on the lake, since waterfowl are an important host for the parasite. New occurrences of swimmer’s itch seem to be strongly associated with people feeding and attracting ducks. In recent years, Koshere explained, 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 specific situations. Thus far, the procedure is considered impractical on a lake-wide scale in Wisconsin.

“It’s best to regard swimmer’s itch like mosquitoes, wood ticks and deer flies.” Koshere said. Often these pests are signs of a healthy and diverse environment and they shouldn’t discourage us from enjoying the outdoors.”