On a hot and humid summer day, the waters of our local lakes look cool and inviting. But a quick dip to beat the heat may come at a price: many of Minnesota’s watering holes are hotbeds for a pesky parasite that leaves swimmers anything but refreshed.
Swimmer’s itch, also known as “duck itch” or “lake itch,” is the common term for cercarial dermatitis, is a skin rash caused by an allergic reaction to parasites stemming from aquatic birds and mammals.
Swimmer’s itch life cycle
The parasite’s trek begins with infected animals—usually ducks, geese and swans as well as muskrats and beavers. The parasite produces eggs that are shed with the animals’ droppings into the lake water. Those hatching eggs transform into larvae who seek out aquatic snails for the next phase of their life cycle. The infected snails now release the parasite into the water in the form of free-swimming cercariae (from the Greek kerkos, “tail”), who are looking for waterfowl once again. The cercariae burrow into the skin of their host, not caring whether it is duck skin or human skin, in order to enter the animal’s bloodstream and begin the life cycle anew. Luckily for us, the larvae cannot develop inside humans and they soon die. But not before leaving behind red patches in the form of an itchy rash.
The swimmer’s itch–causing cercariae are transparent and only about 0.7 mm (1/32 inch) long, so they are essentially undetectable with the naked eye.
Treating water with copper sulfate
City and county officials may take steps to treat lakes if they’ve received complaints of swimmer’s itch. When added to the water, copper sulfate kills snails, thus reducing that needed link in the parasite’s life cycle. The use of copper sulfate is by no means a “silver bullet” or cure for swimmer’s itch for many reasons. First, the compound will only kill the snails present at the time of the application; any snails that enter the area afterward will not be affected and can easily become infected by the cercariae alive and swimming in the water. Second, there is some evidence that snails may be capable of developing resistance to copper sulfate, which would limit the effectiveness of the treatment. And since copper is a toxic heavy metal, long-term, heavy applications of copper sulfate can pose a significant threat to the health of aquatic environments.
Keep it deep and windy, if you can
Two tactics you can take to avoid swimmer’s itch are to swim in deeper areas, not the shallows near shore (where cercariae tend to congregate) and swim where there is an offshore breeze; cercariae are weak swimmers and have trouble attaching to hosts while being blown about by the wind.
Of course, if you’re hanging out with toddlers and babies by the beach, in waist-deep or shallower water, you certainly don’t have the luxury of escaping to deep and windy water.
Interestingly, the more days a person is in an infected body of water, the higher the chance of getting the skin irritation. And the greater the number of exposures to contaminated water, the more intense and immediate symptoms of swimmer's itch will be. Why? Like our reaction to poison ivy or poison oak, swimmer’s itch is a response of our immune system to an irritant. There is a built-in threshold for such irritants, and once a person’s limit to that irritation has been reached, an episode is certain—unless you’re one of the lucky 20-25 percent of the population who has no reaction to cercariae burrowing into the skin.
But why chance it? There are steps you can take to keep yourself and your family itch-free and in the waters this summers.
The DNR recommends taking these precautions to help reduce the odds of getting swimmer's itch:
• Keep waterfowl away from your dock and shoreline. If you are feeding waterfowl (ducks and geese) from your dock, stop!
• Apply a water repellant substance such as petroleum jelly, waterproof sunscreen or other skin oils to reduce the ability of the cercariae to penetrate the skin.
• Dry off with a towel as soon as you get out of the water. When you get out of the lake, don't let the water evaporate off your skin. The organism in the droplets of water on your skin will look for somewhere to go as the droplet of water evaporates.
Take these precautions whether or not the lake you are entering has been treated recently with copper sulfate; the application of that compound is not a panacea.