Swimmer's Itch in Minnesota Lakes: Causes, Prevention & Treatment

Swimmer's itch is a temporary, itchy rash you can pick up in almost any Minnesota lake. It is caused by a microscopic parasite that is looking for a duck and finds your shin instead. It is not dangerous, it is not contagious, and it is not a sign of dirty water. It is also thoroughly unpleasant, and a few simple habits prevent most cases.

What it actually is

The medical name is cercarial dermatitis. The Minnesota DNR describes it as an allergic reaction to parasites carried by waterfowl, semi-aquatic mammals and snails. The life cycle runs like this:

1. Adult worms live in a bird or mammal host. In Minnesota the DNR names common mergansers, mallards, wood ducks and Canada geese among the hosts.
2. The host's droppings carry the parasite's eggs into the lake.
3. The eggs hatch into a larval stage that seeks out certain snails.
4. Inside the snail the parasite multiplies, and a second free-swimming stage, called cercariae, is released into the water.
5. The cercariae swim off looking for a bird to complete the cycle.

People are a dead end. A cercaria that burrows into human skin dies almost at once, and it cannot develop further. The itch is your immune system reacting to it. Because it is an allergic response, sensitivity varies a great deal. One kid comes out of the lake covered in welts while a sibling who swam beside her has nothing, and people who have had it before tend to react more strongly each time.

What it looks and feels like

The Minnesota Department of Health describes the usual course:

• Tingling, burning or itching, starting anywhere from minutes to days after swimming.
• Small reddish pimples within about 12 hours, sometimes turning into small blisters.
• Itching that can last a week or more before gradually going away.

The rash shows up on skin that was exposed to the water, often the lower legs and wherever the waterline sat while wading. Skin under a snug swimsuit is frequently spared.

If the rash is under the swimsuit, appears days later, or comes with fever or stomach trouble, it is probably something else. Bring those to a clinic.

When and where it happens in Minnesota

There is no official season, and the DNR does not track outbreaks lake by lake. The pattern locals know is that it becomes a problem once near-shore water has warmed, generally from around late June into August, and then tapers off. Take that as a rule of thumb, not a calendar.

The DNR is specific about the conditions that raise risk:

• Warm, shallow water. The larvae concentrate near the surface in the shallows, exactly where children play.
• Mornings. Concentrations are typically higher early in the day.
• Onshore wind. Waves and currents push the larvae toward shore, so the downwind beach can be far worse than the upwind side of the same lake.
• Weedy, marshy shoreline. That is snail habitat.
• Lots of waterfowl. Docks and beaches where ducks and geese loaf, or are fed, keep the cycle supplied.

It occurs statewide. Clear, popular swimming lakes in the Brainerd Lakes area, around Alexandria, in Otter Tail County and across the north get it, as do metro lakes. A lake's reputation can change within one summer, and one bay can be bad while another is fine.

How to avoid it

None of these is a guarantee, but together they cut the odds considerably. All come from DNR and MDH guidance.

Before and during the swim

• Swim in deeper water. Get off the dock or out from a boat instead of wading the shallows. Risk is lower away from shore and later in the day.
• Pick the upwind shore. If the wind has blown onto a beach all morning, try a different one.
• Put a barrier on your skin. The DNR notes that water-repellent products such as waterproof sunscreen, petroleum jelly or skin oils reduce the parasite's ability to get through.
• Cover up. Rash guards and close-fitting swimwear create a physical barrier.
• Heed posted warnings, and ask the resort or the people on the next dock. Locals know when it is going around.

The moment you get out

• Towel off immediately and briskly. This is the single most useful habit. As water droplets evaporate on your skin, larvae in them go looking for somewhere to go. Rubbing them off first denies them the chance. Do not air dry.
• Rinse if you can. MDH recommends showering right after leaving the water.
• Dry kids first. They spend the most time in the shallows and the least time thinking about towels.

If you own lakeshore

• Stop feeding ducks and geese, and discourage them from the dock and swim raft. The DNR is blunt about this: if you are feeding waterfowl from your dock, stop.
• Chemical treatment of a lake to kill snails requires a DNR aquatic plant management permit and affects far more than snails. Contact the DNR before considering it. For most owners, bird discouragement and good swimming habits are the realistic tools.

Treating it

Most cases need no medical care. MDH lists these for relief:

• Corticosteroid cream
• Cool compresses
• Baths with Epsom salts or colloidal oatmeal
• A paste of baking soda and water
• Anti-itch lotion

The DNR adds that oral antihistamines may help and that a pharmacist can point you to over-the-counter options. The main rule is not to scratch. Broken skin can become infected, and that is the only way swimmer's itch turns into a real problem. See a health care provider if itching is severe, if spots look infected, or if you are not sure what the rash is.

Swimmer's itch versus other lake rashes

Problem	Typical clues
Swimmer's itch	Itchy red pimples on exposed skin within hours, clear water, shallow swimming
Blue-green algae exposure	Rash plus possible eye irritation, cough, vomiting or diarrhea after contact with green, scummy water
Bacterial illness	Stomach illness a day or more after swallowing water, often after heavy rain

If the water was green and scummy, read our guide to blue-green algae in Minnesota lakes. That one is worth taking more seriously.

Does it mean you should avoid a lake?

Not really. Swimmer's itch comes and goes with wind, water temperature and bird traffic, and a lake that was bad last week can be fine today. It is a reason to change where and how you swim, not a reason to cancel the trip. Use the swimming lake directory to find lakes with beaches and deeper swimming areas, and see can you swim in Minnesota lakes for the broader water-quality picture.

Bottom line

• Towel dry right away, every time. It costs nothing and it works better than anything else.
• Swim deep, swim later in the day, and avoid the beach the wind is blowing onto.
• If you get it anyway, treat the itch, leave it alone, and expect it to fade in about a week.