Tip of the Mitt Watershed Council
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What you should know about
Swimmer’s Itch

What is it?

Swimmer’s Itch is a skin irritation that is caused by a larval form of certain flatworms from the Schistosomidae family. It is a fairly common occurrence in many of the lakes in our area.

Schistosome flatworms (shown on right) are parasites with a complex life cycle (usually involving certain species of snails and waterfowl). Even though the Schistosome species found in Northern Michigan are not parasites of humans, their larvae do burrow into human skin seeking to complete their life cycle. The larvae are only 1/32 of an inch long and generally invisible to the naked eye. Since humans are not the proper host, the larvae soon die. The itching sensation is caused by an allergic reaction many people develop to the dead larvae under the skin.

Many species of parasitic flatworms are naturally occurring in almost all lakes. However, not all larval species cause Swimmer’s Itch. The life cycle and host requirements of those species responsible for Swimmer’s Itch differ widely, and the ecology of most is poorly understood. Swimmer’s Itch has probably been around as long as human beings. It is known to occur in at least 30 states as well as Canada, Europe, Africa, and Asia. In the United States, the problem appears to be concentrated in the Midwest region.
Life Cycle of Swimmer's Itch

Where does it come from?

The life cycle of the flatworm involves two very specific hosts. Each flatworm has just one species of snail and one kind of waterfowl as hosts in its life cycle. Both must be present in the same lake for the life cycle to be completed.
swimmers itch, Schistosome

What are the symptoms?

Not all people are sensitive to Swimmer’s Itch. Some who are exposed to the larvae never develop the itch. Those who are sensitive may feel a dull prickly sensation as the larvae burrow into the skin. This may occur either while swimming or immediately after leaving the water. At each point of entry a small red spot may appear and begin to itch.

Symptoms include intermittent periods of itching that will continue for several days. Many suffering from cercarial dermatitis (Swimmer’s Itch) experience the most severe itching early in the morning. After approximately 24 hours, the reddened areas reach their largest size. The itchy, reddened, and raised areas are often confused with bites from chiggers or mosquitoes and the symptoms may be misdiagnosed as those resulting from poison ivy or stinging nettles. Chigger bites are usually located at points where clothing contacts the skin such as wrists, waist, ankles, etc. Itching is limited to points of cercarial entry and will not spread and will never develop into water blisters.

Swimmer's itch, although extremely annoying and uncomfortable, is not a communicable or fatal condition. Over-the-counter drugs are available to reduce the symptoms of swimmer's itch. Antihistamines can be used to help relieve the itching while topical steroid creams may help to reduce the swelling. Before taking any of these drugs, however, consult your physician or dermatologist for advice.

Controlling Swimmer’s Itch

Since the life cycle of the flatworm depends on the presence of both of its two intermediate hosts, the elimination of either will block reproduction. The traditional method of controlling Swimmer’s Itch has been to attempt to kill the host snails with copper sulfate. A permit from Michigan Department of Natural Resources (MDNR) in Lansing is required prior to any aquatic use of the compound.

Copper sulfate is a nonspecific poison which means that it is toxic not only to snails, but also to many non-target aquatic plants and animals. Copper, a toxic heavy metal, accumulates in lake sediments and can bioaccumulate in the living tissues of aquatic animals. Long term heavy applications of copper sulfate can pose a significant threat to the health of aquatic environments.

Copper sulfate treatments rarely kill enough of the target snails to eliminate Swimmer’s Itch. There is also some evidence that snails may be capable of developing resistance to copper sulfate. Despite the application of tens of thousands of pounds of copper sulfate in some lakes during the past 50 years, the occurrence and severity of Swimmer’s Itch has not noticeably diminished.

NOTE: The Tip of the Mitt Watershed Council does not endorse or encourage the use of copper sulfate to prevent or control Swimmer’s Itch.

Experimental Alternatives to Copper Sulfate

One highly promising approach to controlling Swimmer’s Itch has been developed by Dr. Harvey Blankespoor of Hope College in Holland, Michigan. His method involves determining which waterfowl are involved in the flatworm life cycle and treating them with a drug to kill the parasite. This does not harm the birds, but does interrupt the life cycle of the flatworm, eliminating the need for adding a persistent toxic chemical to the lake environment.

Initial research/control programs utilizing this method have been carried out on several lakes. The incidents of Swimmer’s Itch was greatly reduced compared to pre-treatment levels. For more information about this program visit http://www.swimmersitch.org.

Another control method being used in our service area is Merganser Control Programs. As part of the Walloon Lake Association's Swimmer's Itch Initiative, the association hired a wildlife management company to hunt Mergansers during the 2009 fall hunting season. They removed 60 Mergansers that were primarily resident birds to Walloon Lake. Wildlife biologists were also hired to harass Mergansers in the spring of 2010, before they nested on Walloon Lake. This helped reduce the number of Mergansers on the lake, this year and in subsequent years, thus reducing the level of the parasite that causes Swimmer's Itch.

What can our lake association do about swimmer's itch?

It can do several things including the following: educate members about Swimmer's Itch, assess the problem of Swimmer's Itch on its lake, make recommendations for relieving the itching, and begin a control program if Swimmer's Itch is a regular problem.

Tip of the Mitt Watershed Council Recommends…

There are several means by which you can significantly reduce your chances of contracting the Swimmer’s Itch parasite
  • Since itch-causing larvae usually live in the shallows near shore, it is best to avoid this area as much as possible. This is especially important when the wind is blowing toward the shore.

  • Towel off thoroughly as soon as you leave the water, and at frequent intervals. The fragile cercaria of some species can sometimes be rubbed off before they fully penetrate the skin.

  • Do not feed waterfowl! Feeding waterfowl may aggravate the problem by concentrating potential hosts in a limited area.

  • Maintain a healthy greenbelt along your shoreline property with a variety of native plants (including trees, shrubs, and herbaceous plants) to prevent waterfowl from congregating on your property. Shading of near-shore areas as a result of a shoreline greenbelt will also help reduce the amount of bottom-dwelling algae growth, which is a primary food source for the type of snails that are commonly hosts in the schistosome cycle.

  • If you get Swimmer’s Itch, ask your doctor or pharmacist for the best treatment available to help reduce the itching sensation.

Click here for our What You Should Know About Swimmer's Itch informational flyer.

Tip of the Mitt Watershed Council • 426 Bay Street, Petoskey, MI 49770
PH: (231) 347-1181 • Fax: (231) 347-5928 • www.watershedcouncil.org
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