The Itch: Ducks and snails and parasite tails

If you’ve been swimming in a lake, particularly a warm, shallow area, and are now covered in itchy spots, there’s a chance it could be Swimmer’s Itch. According to Hope College’s SwimmersItch.org:

“Swimmer’s itch is a dermatitis that develops on parts of the body that have been exposed to lake water containing certain larval forms of flatworms. Reddened spots, called papules, form on the body within hours after exposure and may itch intensely for several days before subsiding. After approximately one week, the symptoms usually disappear.”

The group of flatworms that cause swimmer’s itch are called schistosomes, and they rely on two types of hosts for their lifecycle: a bird host (or sometimes a rodent host,) and a snail host. The presence of these flatworms does not indicate a dirty or poluted lake. They can be present wherever the right species of waterfowl and snails live.

They become a problem for humans when they mistakenly attempt to enter a person’s skin. Upon entering the parasites quickly die, but once dead they often provoke a local allergic reaction around the site of entry.

Now what?
Then it is up to the body to rid itself of the dead parasite, and the only treatments in the meantime are typical anti-itch creams, steroid creams and antihistamines.

Of course, preventing a case of swimmer’s itch is the best option. This advice is offered:

• Avoid swimming for long periods in shallow water
• Avoid swimming in areas where swimmer's itch is a problem and where there is an onshore wind
• Towel off immediately after leaving the water may help reduce swimmer's itch caused by cercariae, as a few species enter only as the water dries on the skin.
• Post appropriate signs on beaches where swimmer's itch is an annual problem
• Do not encourage birds to stay in your area by feeding them
• Avoid placing rip-rap on your shore. This provides an excellent surface for certain species of snails to attach their eggs. The higher the number of snails, the greater the chance for swimmer's itch.

Where swimmer's itch is a perennial problem, measures can be taken to reduce the parasite population in a body of water through water treatments to reduce the snail population, though it is unclear whether there are environmental drawbacks to this approach.

Alternatively, Dr. Harvey Blankespoor and his colleagues of SwimmersItch.org have developed a method of safely relocating merganser duck populations from inland lakes to Lake Michigan, thus breaking the life cycle of the parasite, though he regrets that licensing for this method is currently being blocked by the state's Depatment of Natural Resources.

Think you have swimmer's itch?
The creators of SwimmersItch.org encourage anyone who has a case of swimmer’s itch to report it to their site, saying, “This information is essential in our efforts to secure funding that will help find a solution to the swimmer's itch problem.”


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Swimmer's itch appears as raised red bumps which can be spread apart, as here, or close together, appearing more like a rash.

The common merganser duck is the primary water fowl responsible for carrying the flatworms that cause swimmer's itch.