Swimmer's Itch: A Common Summertime Pest (res3)
by Frank Koshere
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. 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 but last much of the season.
The irritation is caused during a life stage of a parasite flatworm (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 cercaria 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 swimmers itch problem may develop with as 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 cercaria per day. At the 2% infection rate, this would mean up to 32,000 cercaria would be produced per square meter per day. On a typical 100' x 100' beach area, this translates into a potential 30 million cercaria released each day.
Most cercarias are released during the midday hours from noon to 2:00 pm. With little free-swimming abilities, the ceccaria 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 cercaria up to four miles from the release area.
The cercaria normally don't penetrate the skin until after the bather leaves the water, at which time the person may feel a slight tingling sensation. The cercaria fleas 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.
Prevention 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 cercaria onto the beach. Brisk and vigorous toweling immediately after leaving the water will crush the cercaria before they can penetrate the skin. It has also been noted that some sunscreens and lotions reduce the infections, however, nothing has been found to be 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 the problems on their beach. Any attempts at treatment to kill either the cercaria flea or their snail hosts are ineffective because the cercaria 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 that 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 lake wide scale in Wisconsin.
Some high-use public beaches on specific sites where drift is unlikely have been issued swimmer's itch control permits, but the treatments are restricted because of the lethal effects on non-target plants and animals, and very limited effectiveness. Modern pesticide laws prohibit treatments as they were historically attempted. Anyone attempting chemical treatment of any kind must have a permit from the Department of Natural Resources before they are allowed to place any kind of pesticide in the water.
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 mosquitoes, wood ticks, and deer flies; the reality is that nothing can be done about them, but overall they shouldn't discourage us from enjoying the outdoors.
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