Swimmer's itch

What is Swimmer's Itch?

Swimmer's itch is a temporary skin rash that is caused by an allergic reaction to microscopic parasites that are carried by waterfowl, semi-aquatic mammals, and snails. As a part of their life cycle, these parasites are released by infected snails into the water, where they may come in contact with people and burrow into their skin.

The good news is the organism that causes swimmer's itch cannot complete its life history in the human body. Your body's immune system detects it as a foreign protein, then attacks and kills it shortly after it penetrates your skin. The itching and welts are not caused by the organism living under your skin, but by an allergic reaction.

Not everyone is sensitive to swimmer's itch. Some people show no symptoms of swimmer's itch even though others swimming at the same time and place break out severely. Much like poison ivy, however, your sensitivity to swimmer's itch will increase with each exposure. There are several over the counter remedies your pharmacist can recommend to help relieve the discomfort, but see your physician for a definitive diagnosis.

Where does it come from?

The organism that causes swimmer's itch has a complicated life history.

  1. It starts out as a worm (parasite) in the intestinal lining of aquatic birds and mammals (host).
  2. The worms lay eggs inside the the host animal.
  3. The eggs are eliminated by the host and drop to the bottom of the lake.
  4. Then the eggs hatch into miracidia that swim around in the water until they find a snail.
  5. Once they find a snail, they live inside them and develop into cercariae.
  6. The snails then release the cercariae back into the water where they look for another host - aquatic birds, mammals or humans.
  7. When the cercariae find a potential host, they burrow into the host's skin.
  8. If the host is suitable, the life cycle starts all over again.
  9. Humans are not a suitable host and the cercariae die after penetrating the skin.

Swimmer’s itch is not spread from person to person.

Reducing the odds of getting swimmer's itch

The cercariae only live for a day or so and typically swim around in the upper few inches of lake water. This increases their chances of coming into contact with a duck. Once the cercariae is in the duck, it easily moves around the lake as a parasite, and ultimately along the shoreline.

There are actions you can take to help reduce your 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. If ducks like to rest on your dock, do what you can to discourage them. You can try putting an owl wind sock or statue on your dock and move it around occasionally so the ducks don't become accustomed to it.
  • Stay out of the water by the shore. The swimmer's itch organism may originate somewhere else in the lake and is being brought to your shoreline by wave action or currents. You may want to try swimming from a raft or boat farther out from shore where you are less likely to come into contact with the cercaria. Of course, this strategy may not be practical if you don't swim or have young children who want to play in the water near shore.
  • Apply a water repellant substance such as petroleum jelly, waterproof sunscreen or other skin oils to reduce the ability of the Cercariae from penetrating 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.

If all else fails...

Copper sulfate may be used to kill the snails which are an intermediate host of the parasite.

  • You MUST get a permit from your DNR regional fisheries office to use copper sulfate.
  • The copper sulfate will only kill the snails present at the time of application, any snails which enter the area afterwards will not be effected.
  • Copper sulfate may only be temporary and will require more than one treatment.
  • Usually only a small percent of the snails in a lake carry the infection, and they may not be the ones around your dock.

More information

Centers for Disease Control and Prevention (CDC)