Swimmer's itch or schistosome cercarial dermatitis is a skin reaction that certain people have to the entry of a larval stage of certain flatworms into the epidermal layer of the skin. After the parasite enters, it dies and may cause dermatitis in individuals who have been previously sensitized. It is widely scattered throughout Michigan, but uncommon in the thumb region.
Major outbreaks occur on the larger recreational lakes in the northern part of the Lower Peninsula.
Nearly every lake in Michigan has the potential to support the snails and birds that host the schistosomes that cause swimmer's itch. No, cases have been reported from nearly every state as well as from all of the provinces of Canada. In North America, most of the problems of swimmer's itch occur in the northern tier of states, north to Alaska. In addition, swimmer's itch has been reported from more than 30 countries. Usually within 30 minutes, a small red spot appears at the site where the cercaria penetrated.