COWPOX

DEBRA BOURNE

Wildlife Information Network, Twycross Zoo — East Midland Zoological Society, Atherstone, UK

The virus of cowpox is somewhat misnamed. Although it can cause lesions in cattle, the main hosts are thought to be small rodents, and it has been suggested that the true reservoir hosts may be bank voles (Microtus agrestis) and field voles ( Clethrionomys glareolus).

In the UK, there is serological evidence of infection in bank voles, field voles, wood mice (Apodemus sylvaticus) and house mice (Mus musculus), with seroconversion peaking in autumn¹-⁴⁸’⁴⁹). In Norway, serological evidence suggests that bank voles, wood mice and Norway lem­mings (L emmus lemmus) may be reservoir hosts (serologi­cal prevalences 17%, 30% and 56%, respectively), with occasional seropositive individuals in field voles, brown rats (Rattus norvegicus) and common shrews (Sorex aranus'f^a'). Additionally, orthopoxvirus DNA was detected in one or more organs, particularly the lungs, of bank voles, grey-sided voles ( Clethrionomys rufocanus), northern red-backed voles ( Clethrionomys rutilus), wood mice and common shrews in Norway(⁵¹).

Experimental studies support the view that the orthopoxvirus found in voles and other small mammals in Europe is cowpox virus and not ectromelia virus, because bank voles are resistant to ectromelia virus (no clinical signs nor seroconversion following inoculation). Inocula­tion of cowpox virus into voles produced only localized lesions (footpad swelling, sometimes ulceration) and (54)

seroconversion ’.

Although previously seen as a disease of dairy cows, cowpox is now seen as a clinical disease mainly in domestic cats and occurs most commonly in autumn. Typically, a single primary lesion (ulcerated nodule with crust) is seen on the face or forelimbs of the cat, with secondary lesions appearing 1 to 2 weeks later, as macules or small erythema­tous nodules, which ulcerate then scab over and heal; scars are left. In about 20% of cases there are concurrent oral lesions. Sometimes there are systemic signs — low-level pyrexia, depression and occasionally diarrhoea, with pneu­monia a less common but more serious complication. Atypical presentations have included oral lesions without skin lesions, large areas of skin necrosis, severe oedema and loss of digits⁽⁵⁵⁾. Cowpox has not been reported as a cause of clinical disease in wild European felid species, but it has been seen in various large felids in zoos and should be considered as a possible diagnosis if similar lesions are seen on lynx (Lynx lynx or L. pardinus) or wildcats (Felis sylves- tris) in the areas where the ranges of these species coincide with the ranges of the reservoir host rodents. Antibodies to orthopoxviruses (probably cowpox virus) have been detected in lynx (Lynx lynx) in Sweden and Finland⁽⁵⁶⁾.

Diagnosis of cowpox in cats is based on characteristic histopathological findings with eosinophilic intracytoplas- mic inclusion bodies in keratocytes (e.g. on biopsy), with confirmation by immunohistochemistry, electron micros­copy, PCR or culture.

Cowpox is a zoonosis, usually acquired from cats or from rodents, with one or two cases per year in humans in the UK, for example. Usually only a single lesion devel­ops, on the hands or face, although in about 25% of cases there are multiple lesions. Occasional severe generalized infections occur and fatalities have been reported.

Natural infection does not cause obvious clinical disease in rodents; however, it is possible that infection may have a complex effect on rodents at the population level, through effects on individual survival and fecundity. Experimen­tally infected bank voles and wood mice showed a signifi­cant delay in the onset of reproduction¹-⁵⁷) and reduced survival has been noted in naturally infected field voles⁽⁵⁸⁾, although an increased survival was noted in infected bank voles, possibly due to host resources being directed towards immune response and away from reproduction¹-⁵⁹). It has also been suggested that increased infection rates in rodents following population peaks may be associated with both increased contact rates and increased vulnerability to infec­tion associated with higher host abundance¹-⁵⁸’⁶⁰).