Scientists look for cause of spike in swimmer’s itch plaguing Minnesota lakes

Marjorie Mathison Hance's five grandchildren love spending time in the water at her place on Pelican Lake in Otter Tail County.

But she said one summer day in 2020, they emerged from the water, their skin blistering with hundreds of itchy, red welts. It took six weeks for one of the children’s sores to clear up.

"They were crying and really agitated, because it's a very painful and very aggressively itchy swimmer's itch,” Hance said. Her grandchildren no longer want to go in the water, she said.

When Hance talked to other Pelican Lake residents, they reported similar experiences. Swimmer’s itch had a profound impact on people's ability to enjoy the lake, she said.

"Kids are scared of it. Adults are scared of it,” she said. “It's really had a very dramatic effect."

Swimmer's itch is caused by microscopic parasites in the water. When those parasites come into contact with people, they can burrow into their skin, causing an allergic reaction, also called cercarial dermatitis.

It’s not a new phenomenon in Minnesota lakes, and the rash usually goes away within a week. But in the past few years, reports of severe cases of swimmer’s itch have been on the rise. Scientists are working to figure out what factors are driving the increase, and whether anything can be done to prevent it.

People often think of swimmer’s itch as a nuisance rash, comparable to a mosquito bite, said Curt Blankespoor, a biology professor at Jackson College in Michigan, who studies swimmer’s itch. But there are more than 20 different parasite species that cause the condition, and some cause much worse reactions, he said.

“It's not uncommon for kids and young adults who get severely infected with swimmer’s itch to have to go to the ER, because they just can't deal with the itching and the pain,” Blankespoor said.

Before microscopic parasites infect swimmers, they pass through two different kinds of hosts. The parasites infect ducks, geese and some mammals. Their eggs get into the water through the animals' waste. Those eggs hatch, and the larvae then infect certain types of aquatic snails.

The snails, in turn, release a different type of larvae called cercaria — the one known to penetrate humans’ skin.

Blankespoor thinks the rise of severe cases in Pelican and some other lakes can be traced to the common merganser, a species of duck that prefers to raise its young next to clear, sandy lakes. 

“Unfortunately, folks that vacation like clear lakes and sandy bottoms,” he said. “That's where you get the overlap between mergansers and humans. You're going to get lots of swimmer’s itch in those places.”

This summer, the Pelican Lake Property Owners Association asked Blankespoor to assess the lake. He found high levels of swimmer’s itch in snails in Pelican and other nearby lakes. 

Blankespoor proposed capturing mergansers and relocating them to the Great Lakes area, where there isn’t the right type of snail to host the parasites. But he said the U.S. Fish and Wildlife Service wouldn’t approve the permit, because the agency was concerned that moving the birds could spread avian influenza.

The Fish and Wildlife Service didn’t make anyone available for an interview. In an emailed statement, a spokesperson said it’s “continuing to work with partners to evaluate options” to address swimmer’s itch concerns, including those that directly involve common mergansers or other migratory birds.

The Minnesota Department of Natural Resources also declined to approve a permit to capture and euthanize mergansers on Pelican Lake and Bay Lake in Crow Wing County.

“Scientific consensus does not support the removal of common mergansers or other waterfowl to mitigate swimmer’s itch, and there could be significant ecological harm from doing so,” DNR spokesperson David Tauchen stated in an email. The agency “cannot set the precedent of wide-scale removal of wildlife from natural habitats,” he stated.

Tauchen said the DNR recognizes that people experience swimmer’s itch in varying degrees depending on their sensitivity, and it can be very uncomfortable for some. But he noted that the Minnesota Department of Health does not consider it a public health threat, because it doesn’t spread from person to person.

Blankespoor said he’s now hoping to catch mergansers from Pelican Lake and treat them for the parasite, then release them back into the wild. That plan is awaiting the DNR’s approval.

In the meantime, scientists are looking for other possible causes contributing to the rise of swimmer’s itch reports.

This past summer, the University of Minnesota’s Aquatic Invasive Species Research Center launched a study to look for any link between invasive species in Minnesota lakes and the itchy condition.

“We're hearing anecdotally that it's getting a lot worse, and that people are seeing it the entire summer,” said Amanda Schuermann, a research scholar at the center. “We know invasives are becoming more and more of a problem. So let's see if there's a connection.”

This weekend, Schuermann is wrapping up visits to a dozen lakes across Minnesota, including Leech Lake, Lake Minnetonka and Detroit Lake in Becker County. 

On Monday, she visited Pleasant Lake in the wealthy, woodsy suburb of North Oaks, northeast of St. Paul. Wearing chest-high waders, Schuermann sloshed through the water at a popular public beach, searching for any snails lying on the sandy bottom or clinging to the dock, debris or floating weeds.

She found plenty of zebra mussels, which were first discovered in Pleasant Lake in 2007, and some larger Chinese mystery snails. Native to Asia, the snails were brought to the U.S. by the aquarium trade, then released into the wild. They were first found in Minnesota in the early 2000s.

It’s possible zebra mussels or another invasive species are altering the lake, creating a better environment for native snails or waterfowl that carry the parasite to thrive, Schuermann said.

“We also believe that maybe mystery snails are acting as a host in some small way,” she said. “So we're collecting those to see if they are containing the parasite that causes swimmer's itch.”

Schuermann dropped the snails into containers to take back to the lab, where she will monitor them to see if they release the parasite. Then she dragged a fine-mesh net through the water, looking for the microscopic larvae. 

Schuermann also takes measurements of the lake’s temperature and the amount of light penetrating the water. It’s possible that lakes that are getting clearer because of filter-feeding zebra mussels are contributing to the parasites’ success, she said.

“But if we find that that's not the case, it could be a different pathway,” Schuermann said. “Their presence is very determined by their environmental conditions. So it could be changing water temperatures, changing air temperatures, that drives it.”

The research center plans to sample several dozen lakes across the state next summer, aiming to provide some answers to the itchy plague that’s keeping swimmers from enjoying Minnesota waters.

Susan Miller, a North Oaks resident who lives near Pleasant Lake, hopes the research study brings some explanation. She said the lake is one of the community’s premier summer recreation spots.

“People are just really angry that they're not able to take their kids down to the beach for a quick dip without coming home with red welts all over themselves,” Miller said. “So there's just a lot of community interest in getting to the bottom of why this might have changed.”

Even with more information about the cause of the itch, there’s no easy solution.

Copper sulfate has been used to treat swimmer’s itch in some Minnesota lakes, because it kills snails. But its effectiveness in controlling the parasite is questionable. And its long-term effects on the lake’s ecology are unclear, Blankespoor said.

There are a few things people can do to reduce their likelihood of getting swimmer’s itch. Blankespoor recommends swimming later in the day, in deep areas and when there’s an offshore wind to reduce exposure to the parasites.

In the past, common advice was to apply a waterproof cream or lotion on your skin before swimming, and shower or towel off quickly after getting out of the water. 

However, the type of parasite carried by mergansers can infect humans while they’re in the lake, so those remedies aren’t as effective, Blankespoor said. 

“You're not going to prevent parasites that have already penetrated your skin since you've been in the water,” he said.

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