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. This rash develops over two or three days, and subsides in about a week. The pustules can become secondarily infected from the inevitable scratching.