Cowpox virus infection is an uncommon skin condition that usually affects cats which enjoy hunting small rodents. The skin lesions resulting from infection usually disappear on their own with time. However, when a cat's immune system is suppressed by medicines or illness, then the infection can develop in a severe and generalised way.
There are a variety of pox viruses that can infect a number of different animal species. Although cowpox virus infects cattle, cattle are rarely implicated as a source of infection for cats. Cowpox infection has been reported in domestic cats and cheetahs, rodents, occasionally humans, and infrequently, in recent years, cattle, dogs, elephants and a foal. Cats infected with cowpox virus are usually avid hunters that come into contact with prey, especially the bank vole (Clethrionomys glareolus), field vole (Microtus agrestis) and the wood mouse (Apodemus sylvaticus) which are known to carry the infection. The infection can occasionally be found in the house mouse (Mus domesticus). Such rodents don't usually show any signs of infections. The prevalence of cowpox infection in cats is considered to be relatively low but varies geographically. There is no age, breed or sex predilection for infection and clinical disease.
The disorder is reported throughout western countries in Europe, including Austria, Belgium, France, Germany, The Netherlands, and Norway and western states in the former Soviet Union. In the UK infection is most commonly seen from July to November when rodents are most common. Infection may also be reported throughout the remainder of the year.
Cat with feline cowpox virus infection: Lesions on
the head with focal encrusted plaques
Focal secondary cowpox virus lesion. The hair has been clipped to show the typical shape of a plaque lesion with crust removed
Feline cowpox infection with secondary lesions on the
hind limbs and tail, with focal crusted plaques
Pictures courtesy of Aiden Foster
Cowpox virus enters the skin through a bite wound from a rodent usually on the cat's head, neck or limb. The viral infection becomes apparent after a few days as a small nodule with ulceration (broken and infected skin). This may be followed by secondary bacterial infection, producing cellulitis (inflamed red and sore skin) and an abscess may form. Once inside the cat's body the virus will multiply in the lungs, nasal passages and various lymphoid tissues for about five days producing what is know as a transient systemic virus infection or viraemia. The mouth, nasal passages, lungs and gut may be affected with oral and gastrointestinal ulceration, nasal discharge, pneumonia and diarrhoea. Usually the signs of viraemia are mild and include serous (clear) to mucopurulent (containing mucus and pus) discharge, pyrexia (high body temperature), depression and a poor appetite.
After initial infection, 10 days to several weeks later, there may be numerous skin lesions consisting of oval to circular ulcerated papules (small bumps) and plaques (flat areas) up to 1 cm in size. Pruritus (itching) is not a major feature of this disease and this may help to distinguish it from other conditions. The plaques are usually covered with crust material which will separate as the infection resolves. There may be residual scarring and hair loss (alopecia). These lesions may be observed on any part of the body.
Infection usually resolves itself over six to eight weeks. Occasionally the lesions can be mistaken for other skin conditions such as eosinophilic granulomas and glucocorticoids are administered. This may lead to severe generalised systemic infection. Concurrent infection with feline immunodeficiency virus (FIV) or feline leukaemia virus (FeLV) may affect the rate of recovery from cowpox virus infection.
Blood or skin samples can be used to confirm a diagnosis of cowpox virus infection. These are used to check for the presence of specific antibodies or the cowpox virus itself. Antibodies are proteins found in blood that are produced by the body in response to infection.
Cowpox virus infection can be confirmed by:
• A positive antibody titre (detection of sufficient numbers of antibodies) is supportive of recent infection in at least the previous six months. Antibodies are not cowpox specific but indicate a pox virus infection. There are, however, no other orthopoxviruses that infect cats in the UK. Titres may be positive seven to 14 days after initial exposure.
• Crust material taken from skin lesions and sent for analysis to the laboratory in viral transport media or a sterile container can be used for viral culture (growing virus in the laboratory). It can take three to 10 days for positive results.
• The diagnosis may also be confirmed, in many cases, with skin samples (biopsies) especially of the generalised skin lesions, because there are characteristic changes occurring within the infected cells. Other causes of skin diseases including, for example, bite wounds and skin tumours can also be eliminated through microscopic examination of changes in the cells.
• Crust material submitted in viral transport media or a sterile container can be examined by electron microscopy for detection of typical pox virus particles, in many cases. This is a quick technique compared with culture.
• The crust from skin lesions can be used for detection of orthopoxvirus DNA by a molecular technique called polymerase chain reaction (PCR). This highly sensitive technique is not routinely available.
• The lesions usually heal without intervention within weeks of initial infection.
• Where there is secondary bacterial infection of older and generalised lesions, broad-spectrum antibacterial therapy can be useful.
• Cats that are not eating may require hospitalisation and intravenous fluid therapy.
• There are no specific antiviral agents recommended for this condition and response to such therapy (eg, interferons) may be difficult to evaluate when most cases will spontaneously recover.
• Severe cases with respiratory involvement may have a poor prognosis.
• It is very important to avoid administration of glucocorticoid therapy because the clinical signs may become much worse.
Spread of infection
Cat-to-cat transmission may occur rarely but does not seem to be associated with overt clinical signs. In general, cowpox virus infection of humans is uncommon and its infectivity is considered to be low. There are, however, occasional reports of human patients who became infected after playing with an infected cat and being scratched. These cases are usually very young or elderly people receiving immunosuppressive therapy or afflicted with immunosuppressive conditions, or individuals with severe skin disease. Sadly in one case the infection became generalised and the patient died. As a precaution cats with suspected cowpox infection, where possible, should be isolated from other cats and handled by people not at risk of infection, wearing gloves and avoiding contact between infected material and skin wounds or the eyes.
The virus can survive for months to years at room temperature and is resistant to many disinfectants, although hypochlorite-based products should be effective.
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