Cowpox virus infection is an uncommon skin condition, usually affecting 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.
Cow pox virus
There are a variety of pox viruses that are able to infect a number of different animal species. Although cowpox virus infects cattle, cattle are actually rarely implicated as a source of infection for cats, and cows are not the natural reservoir for this virus. Cowpox infection has been reported in domestic cats and cheetahs, cattle, and occasionally humans, dogs, elephants and horses. The natural host for this virus is thought to be wild rodents.
Cat with feline cowpox virus infection: lesions on the head with focal encrusted plaques
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), among others, which are known to carry the infection. Such rodents don’t usually show any signs of infections. The disorder is reported in many European countries including Austria, Belgium, France, Germany, The Netherlands, UK, Scandinavia and western states in the former Soviet Union.
Infection in cats
The prevalence of cowpox infection in cats is considered to be relatively low but varies geographically. There is no age, breed or sex predisposition for infection but there may be an increased risk in seasons where rodents are most prevalent (e.g. the autumn). Infection is primarily seen in outdoor cats that are active hunters.
Clinical signs in cats
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.
Cowpox lesion on limb of cat. The hair has been removed to show the typical shape of a plaque lesion with crust removed
Once inside the cat’s body a viraemia will develop (the virus will be present in the blood within circulating cells) and it may spread to the lungs, nasal passages, mouth, gastrointestinal tract and various lymphoid tissues. There may be oral and gastrointestinal ulceration, nasal discharge, pneumonia and diarrhoea. However, in most cases the signs of viraemia are mild but may include pyrexia (high body temperature), depression and a poor appetite.
10 days to several weeks after the initial infection, numerous further skin lesions may develop and these are often the most obvious sign of infection. These consist of small oval to circular ulcerated papules (small bumps) and plaques (flat swollen 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 crusts which will fall off as the infection resolves. These lesions may be observed on any part of the body.
Infection usually resolves itself over six to eight weeks. However, if glucocorticoids (steroids) are administered (which suppress the cat’s immune response) or if they are immunosuppressed (e.g., infection with FeLV or FV), then severe generalised systemic infection may occur, often with fatal pneumonia.
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. Infection can be confirmed by:
- A positive antibody titre (detection of sufficient antibodies against the virus in a blood sample) is supportive of recent infection (usually within the previous six months), and cats will be antibody positive within 7-14 days after initial exposure
- Virus isolation or PCR – detecting the genetic material of the virus – from crust material taken from skin lesions. These are regarded as the gold standard tests although a specialist laboratory is needed for these investigations
- Skin biopsies from affected areas will also demonstrate characteristic changes within the infected cells, and if needed, virus can also be detected within the skin biopsies
Treatment of infected cats
In most cases the lesions will heal without intervention within a few weeks of initial infection, and in mild cases nothing more may need to be done. In more severely affected cats, and in those with secondary bacterial infections antibiotics, may be needed and lesions may need to be bathed.
Cats that are not eating may require hospitalisation with intravenous fluid therapy and nutritional support. It is vitally important to avoid administration of glucocorticoids because the clinical signs may become much worse. Severe cases that have developed respiratory involvement and viral pneumonia have a poor prognosis.
Spread of infection and risk to humans
Cat-to-cat transmission may occur rarely but this 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. However, up to 50% of human infections may be acquired from cats. Although the risk is low, cats with suspected cowpox infection should be isolated from other cats where possible, and handled by people with a low risk of infection (e.g., avoid children and elderly people or immunocompromised people handling the cat). Also, wearing disposable gloves when handling the cat and keeping it in an environment that is easy to clean is sensible as the virus can survive quite well at room temperature. Hypochlorite-based disinfectants are likely to be effective in destroying the virus.