Swimmer's itch is a troublesome itching rash that can affect swimmers after they emerge from the water. The rash is caused by penetration of a tiny larval parasitic flatworm that normally resides in waterfowl and a certain species of aquatic snail. The burrowing parasite quickly dies, but the body remains at the site and can be the stimulus of acute inflammatory reaction. Like any allergic reaction, the degree of discomfort and bodily reaction differs with the sensitivity of the individual and degree of infestation. In some people, considerable pain, fever and severe itching may result along with noticeable swelling of affected areas.
Some bathers appear to be immune.
Welts normally disappear within a week but red coloration may persist some time longer. The inflamed areas resemble chigger bites, but differ in distribution on the body. Chiggers tend to attack the legs, groin, waistline, and underarm areas, whereas swimmers' itch can be distributed over any exposed area of the body. Swimmers' itch does not produce watery blisters like poison ivy, does not spread from the point of penetration of the parasite, and is not contagious like bacterial infections. Outbreaks of swimmers' itch are related to the distribution of infected waterfowl and vary from year to year in any water body, but are generally most common in July when snail infections mature and parasite production increases.