21 year old woman (camp counselor) developed multiple, red, pruritic bumps on her arms and legs after swimming in a lake. The rash only affected the area not covered by her swimming suit. Other campers and counselors developed the same rash after swimming in the lake, too.

What is your diagnosis?

           Cercarial dermatitis, commonly known as swimmer’s itch, is a distinctive papular eruption caused by penetration of the skin by cercariae of nonhuman schistosomes.  This cutaneous schistosomiasis is usually limited to the exposed areas of the body.   It is associated with freshwater lakes, although “clam digger’s itch” has been reported from the saltwater tributaries of Long Island Sound, New York.  The cercariae responsible infect birds, rodents, or ungulates, and belong to the species Schistosoma, Ornithobilharzia, Gigantobilharzia, Austrobilharzia, Trichobiliharzia, and Orientobilharzia.  Cercarial dermatitis is a potential hazard worldwide, wherever people share an aquatic environment with vertebrates and mollusks harboring schistosomes. 

            These blood flukes require an intermediate snail host and a definitive vertebrate host to complete the life cycle.  Inasmuch as man is an accidental host, development cannot proceed, and clinical manifestations of cercarial penetration resolve spontaneously within a week.  A prickling sensation lasting minutes to an hour results from exposure to cercariae-infested water.  Pruritic erythematous macules, papules, and occasionally, papulovesicles and wheals can be seen soon thereafter.  Postinflammatory hyperpigmentation is a common sequel.  Edema, lymphangitis, and regional adenopathy develop in some individuals, as may eosinophilia and systemic symptoms of generalized urticaria, nausea, and vomiting.  Repeated exposures produce more severe insect bite reaction, with perivascular lymphocytes, histiocytes, and eosinophils. 

            Treatment is oriented toward the relief of symptoms.  Oral antihistamines, topical steroids, and topical antipruritic agents may be helpful.  The rash may be prevented by toweling off vigorously and showering promptly after freshwater bathing. 

In this particular case, because the lesions were very pruritic, she was treated with hydrocortisone valerate 0.02% cream, bid for 4 days with complete resolution.

Reference:
Pocket Guide to Cutaneous Medicine and Surgery byDover, Arndt, Le Boit, Robinson, Wintroub.  Swimmer’s Itch, p. 462.

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