Swimmer's itch is a 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. This is 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.
The organism that causes swimmer's itch has a complicated life history. It starts out in the intestinal lining of waterfowl, mostly ducks. The life stage that causes swimmer's itch is called a "cercaria" which is an immature stage of a blood fluke common in waterfowl. The cercaria only lives for a day or so and typically inhabits the upper few inches of lake water. This increases its chances of coming into contact with a duck (its definitive host). Once it's in the duck, it easily moves around the lake, and ultimately along the shoreline.
Swimmer’s itch is not spread from person to person.
There are actions you can take to help reduce the odds of getting swimmer's itch.
If these strategies don't work for you there is the option of getting a permit from your DNR regional fisheries office to use copper sulfate to kill the snails which are an intermediate host of the parasite. Infected snails release the life stage (cercaria) that actually causes swimmer's itch. 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. The relief provided may be temporary and more than one treatment may be required. Usually only a small percent of the snails in a lake carry the infection, and they may not be the ones around your dock.