Swimmer's itch (SI) or human cercarial dermatitis is caused by free-swimming larvae of bird parasites of the family Schistosomatidae (Trematoda) which have penetrated thorough the skin. Sometimes, mainly during first infections, the larvae do not cause any symptoms but if trapped by the immune system of the host each larva causes a maculopapular eruption. So far, five bird schistosome species have been reported in Iceland. Cercariae are shed by the freshwater snail Radix peregra but adults live in anseriform birds; one nasal Trichobilharzia species occurs in the nasal cavity of mallards, two visceral species have been found in veins of whooper swans and a visceral species has been found in greylag goose and in mallards, respectively. Experiments have shown that developing schistosomulae are able to survive for days or even weeks in mammals. Long term pathologic effects on the host are unknown. During the second half of August 2003 thousands of bathers got SI in a slowly streaming brook with geothermally heated groundwater in Landmannalaugar, the most frequently visited area in the interior of Iceland. The number of cercariae in the water and SI cases decreased until October but still in December and in late winter 2004 SI cases were reported. In August 2004 SI again started in the area but the density of cercariae in the water seemed to be less than in the previous year. The prevalence of snails shedding Trichobilharzia cercariae on the bathing site never exceeded 1%. The rapid increase of cercariae in the water by the middle of August in 2003 and 2004 were caused by a breeding mallard female and its ducklings which were raised on the bathing site during summer. All the ducklings had nasal- and visceral Trichobilharzia infections which they must have acquired soon after hatching. Three weeks later the adult worms could have started egg-laying. Consequently, emerging miracidia infected the snails which finally started shedding the cercariae by middle of August in 2003 and 2004. In future years this rapid increase of cercariae could be avoided if ducklings are not allowed to have access to the bathing site and the adjacent brook. Retrospective enquiries suggest that some visitors have occasionally got SI after bathing in the brook in preceding decades. Low number of papules, however, indicates a low density of cercariae in the water.
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