The Dermatologic Perils of Swimming: Swimmer’s Itch

A 6-year-old girl (A) and an 11-year-old boy (B) each presentedfor evaluation of an erythematous, pruritic, papularrash that developed after swimming in a Wisconsin lake.Each child was otherwise completely healthy.Both children have cercarial dermatitis, commonlyknown as swimmer's itch. This is a hypersensitivity reactionthat occurs after exposure to schistosome larvae infreshwater lakes. Schistosome eggs (of trematodes, generaSchistosoma and Trichobilharzia) are passed intofreshwater lakes via bird feces. The eggs hatch and developinto the larval stage called miracidia, which infecttheir intermediate host, freshwater mollusks. The miracidiatransform into a second larval form called cercaria,which leave the mollusks and infect birds either by beingingested or penetrating their skin. Cercaria can also penetratehuman skin and may infect persons swimming ininfested water. The cercaria are unable to complete theirlife cycle in humans and die in the epidermis.Although most common in the Great Lakes regionof the United States, outbreaks occur during the summer months throughout the world. As the skin dries and thecercaria penetrate the epidermis, patients often describethe initial itching sensation. Small macules may be visibletransiently at the site of penetration. Several days to 2weeks after initial exposure, a mild to moderately pruritic,erythematous, papular, sometimes pustular eruption developson any skin that had been exposed to infested water.In sensitized persons, the eruption develops within hoursof exposure. The onset and intensity of the eruption correspondto the degree of sensitization. The rash peaks within48 to 72 hours and usually resolves within 1 week.Treatment is supportive and consists of oral and topicalantihistamines and topical corticosteroids. Anthelmintictherapy is not indicated for this self-limited condition.Preventive measures include avoiding exposure tofresh water that contains mollusks and toweling off immediatelyafter swimming (since penetration into the skin occursduring evaporation).