The effect of cowpox virus infection on fecundity in bank voles and wood mice.

Although epidemic infectious diseases are a recognized cause of changes in host population dynamics, there is little direct evidence for the effect of endemic infections on populations. Cowpox virus is an orthopoxvirus which is endemic in bank voles (Clethrionomys glareolus), wood mice (Apodemus sylvaticus) and field voles (Microtus agrestis) in Great Britain. It does not cause obvious signs of disease nor does it affect survival, but in this study we demonstrate experimentally that it can reduce the fecundity of bank voles and wood mice by increasing the time to first litter by 20-30 days. The pathogenic mechanisms causing this effect are at present not known, but this finding suggests that natural subclinical infection could have a considerable effect on the dynamics of wild populations.

Cowpox in British voles and mice.

Serosurveys indicate that bank voles, field voles and woodmice are probably reservoir hosts of cowpox virus in western Europe, although virus has not yet been isolated from these species. In this study, bank voles, field voles, woodmice and laboratory mice were shown to be susceptible to combined intradermal and subcutaneous inoculation with 3-20 plaque-forming units (pfu) of cowpox virus. Bank and field voles, but not laboratory mice, were also susceptible to combined oral and nasal inoculation with 50 pfu. Few clinical signs were seen and virus was generally recovered only from inoculation sites. Bank voles were not susceptible to injection of ectromelia virus (5000 pfu) into the skin (as described above). These results provide information on which further pathogenesis and transmission studies can be based, and support the view that the orthopoxvirus antibody detected in British wild voles and woodmice indicates infection with cowpox virus. However, further investigation of the pathogenesis of cowpox in these species is needed to understand better the epidemiology of the disease.