You can’t see them when you step out of the water after a swim, but boy, can you feel them later.

Microscopic parasites lurking in lake water are out in full force now, capable of causing a skin rash that’s itchy, scratchy and just plain ugly.

Swimmer’s itch — also known as “duck itch” or “lake itch” — is plaguing lake-goers earlier than usual this year thanks to an early spring. The warmer water temperatures jump-started the growing season for lake vegetation — food sources for the parasites and their hosts, namely ducks, geese and snails.

“With the ice going off the lakes earlier this year, everything seems to be ahead of schedule,” said Brian Vlach, water resources supervisor for the Three Rivers Park District.

The district recently reopened a swimming beach at French Regional Park after treating the water for the second time this season because of reports of swimmer’s itch.

Other public swimming areas affected by the early outbreak include Coon Lake in Anoka County and Lakeside Beach in Forest Lake, the Minnesota Department of Natural Resources reported.

The critter responsible for causing the annoying temporary rash is a flatworm with a complex life cycle.

The worm begins in the intestinal lining of water animals such as ducks, geese, beaver and muskrat, according to the Minnesota DNR. The worms lay eggs inside the host animal and the animal excretes the eggs into the lake water.

The eggs then hatch and the parasites swim around in search of a snail, a second host. They live inside the snail until it releases them back into the water, where they seek yet another host.

This is when they often come across a human and burrow into the skin. People aren’t suitable hosts, and the parasite soon dies. But the itching is already underway, spurred by the body’s immune system.

Not everyone who comes in contact with the parasite reacts. Some people show no signs of swimmer’s itch.

But for those unlucky souls who do, the symptoms can be quite annoying — though ultimately harmless. Red welts are a telltale sign. So is the can’t-stop-scratching feeling that can last for days or even weeks.

“These parasites grab onto people,” explained Shane McBride, a DNR aquatic plant management specialist. “Your body encapsulates it, grabs hold of it, and it itches like no tomorrow.”


Reports of swimmer’s itch often increase with hot, sticky days.

“You get these 90-degree days, and everybody wants to be in the water,” McBride said. “You get more people in the water, so there is more exposure.”

Once enough complaints surface, city and county officials move to treat the lakes by using copper sulfate to target the snails and the larvae form of the parasite. But the DNR allows only two copper sulfate treatments per season per beach.

The French Regional Park staff spent one of its allotted treatments in May, but the swimmer’s itch problem returned. A decision was then made to treat the swimming area a second time just before the July 4th holiday — even amid the knowledge that there will be no more treatments if the problem recurs later this summer.

“We only have two bullets to use, and we’ve already used one,” Vlach said just before administering the second round of treatment. He said if it doesn’t take this time, it’s possible the beach would have to be closed for the season.

“The approach we usually take is to tell people to swim at your own risk,” he said. “But if we know the presence of swimmer’s itch is there, we put up signs saying it’s closing. Unfortunately, it’s the individual swimmers that are the guinea pigs.”