Pulmonate snails (Physa acuta) were sampled monthly in ponds at the Horsehead Sanctuary Farm of The Wildfowl Trust of North America (Eastern Shore of the Maryland) and (P. gyrina) in The Baltimore Zoo. These areas are used as a stopover places by wildfowl which migrate along the Chesapeake Bay and the eastern shore of the USA. Snails were examined for pre- and patent infections with trematodes. Avian schistosome larvae (Trichobilharzia spp.) were found to be under the strong dominance of redial trematode species and only to dominate those other species which develop sporocysts. The pre- and patent infections exhibited a definite temporal (seasonal) and spatial heterogeneity influenced by the local visiting patterns of migratory birds and the snail population dynamics.

During host-finding behavior avian schistosome cercariae screen the water. They can be trapped by a substrate with a synthetic compound of human/bird skin surface lipid -Linoleic acid (Shiff et al. 1993. J. Parasit.), which imitates the host skin surface. Linoleic acid affects the swimming pattern of cercaria and stimulates larvae for attachment, creeping and penetration the substrate. They can be visualized by the water-soluble stains and counted under the microscope in the substrate.

Avian schistosomes are strongly pathogenic to wildfowl. The death of one Olor columbianus and two Mergus merganser was noted in the areas under investigation. Birds exhibited typical schistosomiasis syndromes. There were high numbers of granulomas in the livers and intestinal mucosa as well as giant cells and macrophages in granulomas. The schistosome eggs were normal, with clearly seen lateral spine indicating Trichobilharzia sp. as well as in various disintegrated forms. Adult trematodes were detected in mesenteric vessels of mergansers. Eosinophilic reactions in granulomas were not as strong as in newly acquired infections. The gross pathology indicated a chronic infection with progressive character.

In the wild, at the intermediate host level avian schistosomes are under dominance of the redial trematode species, which are common and slightly pathogenic to wildfowl and mammals. The dominance of avian schistosomes is temporal; however, and may result in severe infections and death of birds. Presently developed method for cercaria detection along with the analysis of competitive interactions between early trematode larvae are useful tools for artificial manipulation within the guild of parasites in a particular habitat.