Declining field vole (Microtus agrestis) and bank vole (Clethrionomys glareolus) populations were sampled (117 field voles and 34 bank voles) in south-central Finland during the winter of 1988-89. The last surviving field voles 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 material, 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 prevalences of Frenkelia sp. in the brain and Sarcocystis sp. in leg muscles were low (both 6%). Bordetella bronchiseptica was commonly (31%) isolated from field vole lungs and Listeria monocytogenes from the intestines (34%). Salmonella spp. could not be found. The dynamics and abundance of inflammations in the lungs and intestines, as well as B. bronchiseptica isolations from the lungs, indicate that obvious epidemics took place in declining vole populations. Of the Luhanka subsample of 16 field voles brought to the laboratory in April, one died of listeriosis, two of Bordetella, and five died for unknown reasons. Even if small mustelids are the driving force in microtine cycles, it is possible that diseases also contribute to the decline.
Les populations en declin de campagnol des champs (Microtus agrestis) et de campagnol roussatre (Clethrionomys glareolus) ont ete echantillonnees dans la partie sud de la Finlande centrale pendant l'hiver 1988-1989 (117 campagnols des champs et 34 campagnols roussatres). Les derniers campagnols des champs survivants ont ete attrapes en avril et les campagnols roussatres en fevrier. Un echantillon de 16 campagnols des champs attrapes en avril a ete apporte au laboratoire pour subir une immunosuppression. L'histopathologie des principaux organes internes ainsi que la presence de bacteries aerobies et de certains parasites ont ete etudiees. Au niveau des poumons, la decouverte la plus commune (25% de l'ensemble des specimens) etait la multiplication de tissu lymphoide, probablement causee par des infections. Les frequences de l'ensemble du materiel Chrysosporium sp. et Pneumocystis carinii etaient de 13 et 10% chez les campagnols des champs et de 9 et 0% chez les campagnols roussatres. Les kystes de Taenia mustelae (9 et 27%) etaient les changements pathologiques les plus communs au niveau du foie. L'enterite etait egalement relativement commune (14 et 34%). Chez les campagnols des champs la frequence de Frenkelia sp. dans le cerveau et de Sarcocystis sp. dans les muscles des pattes etait basse (les deux de 6%). Bordetella bronchiseptica etait communement identifiee dans les poumons (31%) chez les campagnols des champs ainsi que Listeria monocytogenes dans les intestins (34%). Des cas de Salmonella spp n'ont pas ete detectes. La dynamique et l'abondance des inflammations dans les poumons et les intestins ainsi que l'isolement de B. bronchiseptica dans le cerveau indiquent l'existence d'epidemies nettement perceptibles chez les populations en declin des campagnols et des campagnols roussatres. Parmi les 16 campagnols des champs de l'echantillon de Luhanka apportes au laboratoire en avril, un est mort de listeriose, deux de Bordetella et cinq autres de causes inconnues. Meme si les cycles microtines sont essentiellement provoques par de petits mustelides, il est possible que les maladies contribuent egalement au declin constate.