What is Swimmers’ Itch?

Swimmer’s itch (aka Duck Itch) is caused by a parasite carried in the digestive tracts of waterfowl. These parasites, which are not visible to the naked eye, travel from the waterfowl droppings to host snails, which then release the parasite into the water. Normally, these small organisms migrate back into ducks, but if a swimmer gets in their path, they will attach themselves to human skin. When the swimmer comes out of the water and dries off, the parasite will burrow into the skin and will cause the itchy, allergic reaction.

A person who has contracted swimmer’s itch usually is not aware of the infection until he/she emerges from the water. The swimmer may immediately experience an intense itch over all or part of their body, and upon examination, will see red pinpoints where the parasite entered the skin. These tiny red marks will turn into raised bumps, which may come to a head and itch intensely for several days before they disappear. Swimmer’s itch, by itself, is not communicable or dangerous, but it is extremely annoying and may become infected. Some people are more susceptible to it than others, just as some are more sensitive to poison ivy or poison oak. Antihistamines may reduce the itching.

In the past, we had contacted Dr. Harvey Blankespoor of Hope College in Michigan, who is one of the few biologists in the country conducting research and taking the problem seriously. He visited our lake and collected droppings from mallards, mergansers, and cormorants. Consistent with his findings in other lakes in Maine and Michigan, he discovered that only the merganser carries the parasite responsible for swimmer’s itch. Why it is only the mergansers is unclear – it may be because their diet is exclusively fish or because of some factor of their physiology that makes them especially good hosts.

swimmer-itch cycleWhen considering how to eradicate the problem, it is helpful to understand a little bit about the merganser’s habits. They eat fish exclusively, nest in the same site year after year, and are extremely mobile, swimming several miles each day around the lake in search of fish. As they swim, they defecate and infect the snail beds they pass over, thereby spreading the infection throughout the lake. On Little Sebago, we estimate that we only have a few families of mergansers, but because they are so mobile they can do a lot of damage.

So, what can be done? The easiest answer is to eliminate the merganser, but there is no easy and acceptable way to do that. It is always good advice to discourage residents from feeding the ducks, but feeding attracts mostly mallards, which are most likely not infected.

Dr. Blankespoor has developed a control program that is somewhat controversial and very expensive, but has been implemented successfully in Michigan. Simply put, Dr. Blankespoor will travel to a lake and train volunteers to capture the merganser (which are notoriously wily birds) in specially designed nets. The volunteers then inoculate the birds to kill the parasite, clip their wings, and then transport them to a neighboring body of water. This process would need to be repeated each spring, as the mergansers will return to the same nest sites year after year. Dr. Blankespoor suggests three years to fully train a cadre of volunteers and charges in excess of $40,000 per lake to train the volunteers and provide the necessary equipment.

Swimmer’s Itch Tips

  • DO NOT FEED THE WATERFOWL!! No matter how entertaining they may be, feeding them only encourages them to make your swimming area their toilet.
  • Towel off briskly after swimming and wading. This may shake loose the parasites before they can burrow into your skin. Shower after swimming when practical.
  • Avoid swimming on days when the wind is blowing into your shore, which may cause the waves to deposit the parasite in your area.
  • Avoid areas with large numbers of aquatic plants.
  • Although not scientifically proven, some people believe the use of suntan lotion may reduce the risk of swimmer’s itch.

Click here to download an FAQ from the Centers for Disease Control.

CDC Cercarial Dermatitis Frequently Asked Questions (FAQs)

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