Louping-ill and Semliki forest virus infections in the short-tailed vole Microtus agrestis (L.).

A large proportion of short-tailed voles survived i.c. and footpad inoculations of louping-ill virus and many developed neutralizing antibodies. A short-lived viraemia developed after footpad inoculation, but the levels of virus were low and it seems unlikely that the vole could act as a principal reservoir host to the virus in infected areas. In initial experiments in which a low passage strain of Semliki Forest virus was inoculated i.c. into voles virus multiplication occurred in the brain and moderately severe cerebral lesions developed but neuronal degeneration and necrosis were not seen. Mortality was low and the infection appeared to be self limiting. When a high passage strain of virus was inoculated i.c. into voles high titres of virus were found in the brain. Lesions of neuronal degeneration and necrosis developed and all the inoculated voles died. In later experiments with the low passage strain of virus a higher mortality occurred and the death rate varied from experiment to experiment. The mortality varied independently of virus dosage and the age of the voles. In voles which died high titres of virus were found in the brain and neuronal necrosis was present. In these voles the infection resembled that which followed the inoculation of the high passage strain. By plotting mortality on a monthly basis periods of increased and decreased mortality became evident. Although these periods corresponded roughly with the seasons, the evidence for a seasonal variation was not conclusive.