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Swimmer's itch (or technically schistosome cercarial dermatitis) is a transient, itchy rash on the uncovered skin of fresh water bathers (rivers and lakes). It is caused by the skin penetration of cercariae, a larval form of animal flat worms (schistosomes).
The flatworms are parasitic to certain freshwater snails. The microscopic larvae, after being released from snails, swim in the water seeking a warm-blooded host, for example a duck. The larvae may accidently encounter and penetrate the skin of a human, but do not develop further or cause systemic disease. While human schistosomes can cause disease in tropical countries, these larvae die and simply cause a very itchy rash.
In the United States, the states surrounding the Great Lakes have the highest incidence of this condition. Outbreaks are determined by snail maturation. Shedding of the larvae occurs on bright, warm days in early or midsummer, with the worst infestation occurring near the shore.
A person must first be allergically sensitized to the larvae to develop the reaction. This means that on the first encounter, there will not be a rash, and little in the way of itching. Sensitization occurs in one to two weeks.
The next exposure produces the typical symptoms. The first symptom is intense itching, as the larvae penetrate the skin. This subsides after about an hour. Then within a few hours tiny pus-filled bumps (pustules) surrounded by redness appear. This rash develops over two or three days, and subsides in about a week. The pustules can become secondarily infected from the inevitable scratching.
Treatment consists basically of relieving symptoms as the eruption fades of its own accord. Oral antihistamines (Benadryl®), cool compresses, and calamine lotion are helpful. Intense inflammation may require topical steroids. Towel drying right after leaving the water is an effective preventative measure, since most larvae penetrate the skin as water is evaporating.