Swimmer's itch is a zoonotic disease caused by certain digenean trematodes, in Europe most noticeably by bird schistosomes of the genus Trichobilharzia. These parasites require waterfowl and aquatic snails as final and intermediate hosts, respectively, to complete their life cycle. Swimmer's itch occurs when the free-swimming larvae emitted from snails, the cercariae, accidentally infect humans. Here the parasites cannot complete their life cycle but can cause allergic inflammatory responses of the skin. In the context of the joint BMBF project 'Sichere Ruhr' (Safe Ruhr), which evaluates the Ruhr River as a potential bathing water, the occurrence of the causative agents of swimmer's itch in Lake Baldeney was studied. A total of 1741 snails was examined for the presence of trematode infections, including bird schistosomes. Snails infected with Trichobilharzia spp. were found at three sampling locations but showed low overall prevalences (0.6-3.0%). Based on parasite and host biology, risk factors were evaluated and discussed in the context of the potential use of Lake Baldeney as a bathing water. Although bird schistosomes only constitute a fraction of the trematode diversity occurring in natural snail populations and show low prevalence, they still pose an infection risk due to the high emission rates of cercariae from individual snail hosts. A wide variety of often interacting biotic and abiotic factors, as well as personal behaviour have an effect on the likelihood and severity of a human infection. Based on these risk factors, a number of possible preventive actions aiming at the disruption of the life cycle, or personal protective measures can be suggested. While absolute protection is impossible (unless swimming in natural waters is altogether avoided) some preventive measures can reduce the risk of human infections.

Department of Aquatic Ecology and Centre for Water and Environmental Research University of Duisburg Essen Universitätsstraße 5 D 45141 Essen Germany

Department of Zoology University of Johannesburg PO Box 524 Auckland Park 2006 Johannesburg South Africa

Institute of Parasitology Biology Centre of the Czech Academy of Sciences Branišovská 31 370 05 České Budějovice Czech Republic

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