Disease patterns in field and bank vole populations during a cyclic decline in central Finland

Declining field vole (Microtus agrestis) and bank vole (Clethrionomys glare olus) populations were sampled (117 field voles and 34 bank voles) in south -central Finland during the winter of 1988-89. The last surviving field vol es were caught in April and bank voles in February. A subsample (16) of the April field voles were taken live to the laboratory for immunosuppression. The histopathology of the main internal organs and the presence of aerobic bacteria and certain parasites were studied. In the lungs, an increase in lymphoid tissue, probably caused by infections, was the most common finding (52% of all individuals). The prevalences in the voles, in the whole mater ial, of Chrysosporium sp, and Pneumocystis carinii in lungs were 13 and 10% in field voles, and 9 and 0% in bank voles, respectively. Cysts of Taenia mustelae (9 and 27%) were the most common pathological changes in the liver . Enteritis was also rather common (14 and 34%). In field voles the prevale nces of Frenkelia sp. in the brain and Sarcocystis sp. in leg muscles were low (both 6%). Bordetella bronchiseptica was commonly (31%) isolated from f ield vole lungs and Listeria monocytogenes from the intestines (34%). Salmo nella spp. could not be found. The dynamics and abundance of inflammations in the lungs and intestines, as well as B. bronchiseptica isolations from t he lungs, indicate that obvious epidemics took place in declining vole popu lations. Of the Luhanka subsample of 16 held voles brought to the laborator y in April, one died of listeriosis, two of Bordetella, and five died for u nknown reasons. Even if small mustelids are the driving force in microtine cycles, it is possible that diseases also contribute to the decline. (C) 20 00 Elsevier Science Ltd. All rights reserved.