Schistosomal dermatitis (swimmer's itch, cercarial dermatitis) is usually caused by cercarial trematodes of the Trichobilharzia, Gigantobilharzia or Austrobilharzia species (e.g. T. regenti or A. variglandis). The species includes different types of trematodes. Birds serve as the definitive host and gastropods as the intermediate host.
Humans cannot serve as hosts but the cercarias may penetrate the skin and cause both an immediate and a delayed hypersensitivity reaction. Usually only persons who have during earlier swimming been exposed and later sensitized to cercariae of the same species will have strong symptoms.
Because the life cycle of the trematodes requires both birds (ducks) as definitive hosts and infected gastropods, schistosomal dermatitis is only encountered in scattered locations and from time to time.
Symptoms
A maculopapular, itching rash appears particularly on the feet after wading.
Symptoms may start as a prickly or tingly feeling already within 4-20 minutes after exposure as a consequence of an allergic reaction.
Symptoms typically start within a day after exposure and may last one week or even longer, gradually abating.
Diagnosis
Based on a typical history.
A cercaria less than 1 mm in length may sometimes be detected from a papule by microscopy but skin sample is not needed because the diagnosis is clinical.
Serological tests are not of use.
Treatment
Itching can be alleviated with skin ointments and oral antihistamines. Mild glucocorticoid creams may also be temporarily used (e.g. 1% hydrocortisone).The disease resolves spontaneously usually in a few days (range one day - 3 weeks).
Prevention
Drying of the skin after swimming by careful rubbing with a towel
Avoidance of swimming or wading in shallow waters, e.g. lakes or ponds favoured by ducks and geese
References
Horák P, Mikeš L, Lichtenbergová L et al. Avian schistosomes and outbreaks of cercarial dermatitis. Clin Microbiol Rev 2015;28(1):165-90. [PubMed]