Researcher Studies Swimmer's Itch Incidence And Risk Factors

When Verbrugge, an avid swimmer, built a home with her husband on a northern Michigan lake, her scientist's curiosity was piqued by the irritation her skin developed every time she indulged her hobby.

"I started to swim and itched intensely afterward," said Verbrugge, a research professor and senior distinguished research scientist at the University of Michigan Institute of Gerontology. "I'm a curious person and started asking questions about what was in the lake."

Out of that was born an article in the May issue of the American Journal of Public Health. Verbrugge examined the incidence and risk factors of swimmer's itch, the lay term for cercarial dermatitis, caused by parasites in the water.

"Exposure to shallow water and areas with onshore winds are key risks for swimmer's itch," Verbrugge concluded. The more days a person was in the lake, the higher his or her chances for having an episode of the painful skin irritation.

Like human reaction to poison ivy or poison oak, swimmer's itch is a reaction of the skin to an irritant. Larvae of the parasite burrow into the skin of swimmers, causing red, itchy bumps.

When Verbrugge began exploring the epidemiology of swimmer's itch, she found that most existing research looked at ducks and snails, because the parasite that causes the itch has a two-host life cycle, first involving snails, and later ducks. Verbrugge felt it was important to study human effects if it was to be taken seriously as a public health concern.

She designed a study asking people to keep diaries recording the frequency and duration of time in the water and the occurrence of swimmer's itch. Other studies have asked swimmers to report outbreaks, but Verbrugge pointed out that kind of study doesn't get at the likelihood of skin irritation, since researchers wouldn't know if the person went in the water one time or 20 times before being affected.

Verbrugge conducted her research at the U-M Biological Station on the shores of Douglas Lake near Pellston. Ultimately, Verbrugge found that the more days people spent in the water, the more likely they were to develop swimmer's itch. More minutes in the water on a given day, as well as morning swimming, also seemed to increase incidence, but the study was not large enough to confirm these statistically.

Also, time spent in water waist deep or shallower increased likelihood of irritation, as did time spent in areas of the lake with onshore, or receiving winds—winds that blow in toward shore. Onshore winds keep locally produced cercariae in place, and also bring in cercariae produced in other areas, so they become more concentrated.

"The study confirms some of people's hunches, but now it's science," Verbrugge said. And that makes it easier for concerned residents and public health officials to know what to do.

Verbrugge said she's sought out lake associations for every major inland lake in Michigan, and all have concerns about swimmer's itch. Some have tried to control ducks or snails to fight the cycle of the parasite, but with little luck.

Instead, Verbrugge said it is probably up to individual swimmers to take precautions. She hopes additional epidemiological studies will learn what preventive actions are successful.

Verbrugge wears a full Lycra wetsuit to keep the cercariae out of her skin. She never walks through the shallow water near shore, instead walking out to the end of a long dock to get into the lake.

She also showers immediately after coming in from a swim, using a stiff brush to loosen any cercariae that might have found their way through.

That's been enough to allow her to enjoy swimming again and to spend more time on her main areas of research: issues in disability, such as looking at how helpful equipment is in assisting those with disabilities, comparing arthritis disability with heart disease disability, and looking at the effects of disability on middle-aged and older people's activities.

Related links:

More on Verbrugge --http://www.iog.umich.edu/faculty/verbrugge.htm

A CDC fact sheet on swimmer's itch -- http://www.cdc.gov/ncidod/dpd/parasites/schistosomiasis/factsht_cardmermatitis.htm

A Hope College FAQ on swimmer's itch -- http://www.hope.edu/swimmersitch/

U-M Biological Station -- http://www.umich.edu/%7Eumbs/