Eimeria-parasites are associated with a lowered mother's and offspring's body condition in island and mainland populations of the bank vole.

This study, based on correlative data, tests the hypothesis that infections with Eimeria spp. parasites exert a significant loss of fitness of bank voles (Clethrionomys glareolus) reflected in lower reproductive success and survival, declining host population densities and are associated positively with population size. The study was conducted in 20 mainland and 27 island populations in central Finland during May-September in 1999. Faecal samples showed that 28% of 767 individuals were infected with Eimeria spp. The presence of Eimeria parasites was higher in dense mainland populations than in sparsely populated islands. Eimerian infections increased during the course of the breeding season, probably as a result of the high infection rate of young individuals. Accordingly, the body masses of bank voles were negatively related to the presence of Eimeria spp. Reproductive output, as measured by the breeding probability of females and litter size, was not associated with the presence of eimerian infection. Interestingly, the body condition of the infected mothers appeared to be low. Moreover, mother's body condition was the single most important variable studied that showed a positive correlation to pup's body condition at birth. On small islands (< or =3.2 ha) that were comprehensively trapped, the mean number of Eimeria spp. in the bank vole population was negatively related to density changes of the bank vole population during the study. Our data are consistent with the idea that infection with coccidian parasites may be one of the factors responsible for declining host populations in small, isolated populations.