Patterns of trematode parasitism in lymnaeid snails from northern and central Finland

A species rich and prevalent larval digenean fauna was found in lymnaeid sn ail populations from two lakes in northern Finland (Kuivasjarvi and Pyykosj arvi populated by L. stagnalis and L. peregra, respectively) and ten years later in a third lake in central Finland (Kuuhankavesi populated by L. stag nalis). Echinostomes were common to all lakes, one species overlapping betw een Pyykosjarvi and Kuuhankavesi (Echinostoma revolutum). All three lymnaei d populations also harboured xiphidiocercariae of Plagiorchis elegans, furc ocercariae of Diplostomum pseudospathaceum and Trichobilharzia ocellata and the monostome Notocotylus attenuatus. A consistent seasonal pattern of inf ection was recorded for four of the parasites in Pyykosjarvi, although annu al variation in mean prevalence was generally significant. Annual parasite species composition within the three lakes was relatively stable despite th e long dormant winter. This may be due to the over-wintering of larval dige neans in infected snails that then act as a source of reinfection of bird d efinitive hosts each spring.