In the cat, infections by viruses of the family Poxviridae, genus Orthopoxvirus are rare and usually limited to subjects that live in the country or that have temporary access to environments visited by the wild animals that are the natural reservoir of the infection. Nevertheless, cases have been reported relatively frequently in continental Europe (in Italy particularly in the north-west) and the United Kingdom and have usually been due to infections by Cowpox virus. These infections are extremely rare in North America and Asia. One case of infection by Racoonpox virus has been reported in Canada.
Poxvirus infections can occur in any period of the year although the autumn is the season at highest risk because the population of natural hosts is largest in this period. The route of transmission of the virus is generally percutaneous and less frequently oronasal. The virus initially replicates in the site of inoculation, causing a primary lesion from which the virus spreads, via the lymphatic system, throughout the whole body causing secondary lesions.
Poxvirus infection is a zoonosis. The infected cats are potentially contagious for other cats/dogs and humans. In order to minimise the risk of contagion, cats with symptoms suspicious of poxvirus infection should be kept isolated and always handled with gloves; when possible the areas used by the cat should be disinfected with a solution of sodium hypochlorite 5%.
CLINICAL SIGNS
Cats are usually infected through wounds caused by wild rodents. The virus replicates in the wound, producing a primary lesion which is usually single, located on the head, neck or anterior limbs, and presents as an erythematous macule that rapidly evolves into a papule-plaque-nodule with ulceration and crusting. In many cases this primary lesion is found incidentally and is often neglected. After 1 to 3 weeks, the secondary lesions appear: these are erythematous macules, papules and nodules (sometimes more than 1 cm in diameter), with a variable distribution, which readily ulcerate and develop haemorrhagic crusts. In most cases the lesions dry gradually and resolve spontaneously within 3 to 8 weeks, although areas of hairless scarring remain.
Affected animals can develop vesicles and/or ulcers of the oral cavity as well as generalised pruritus. Atypical presentations of poxvirus infection include ulcers limited to the lips and oral cavity, ulcerative stomatitis not associated with skin lesions, and oedema and cutaneous necrosis which may be generalised or localised to the limbs with possible loss of the toes.
Infected cats may have mild signs of upper respiratory tract involvement, conjunctivitis, anorexia, depression and moderate fever, but the infection often evolves almost without symptoms. Severe systemic symptoms, particularly of the respiratory system (viral pneumonia), sometimes with a fatal outcome, can develop in subjects with concomitant diseases that compromise the immune system, such as Feline immunodeficiency virus (FIV) infection, in subjects that develop secondary bacterial infections or in animals inappropriately treated with glucocorticoids.
DIFFERENTIAL DIAGNOSIS
The main differential diagnoses include other viral diseases, such as infections by Herpesvirus, Calicivirus and FIV, deep fungal and bacterial infections, infections by atypical mycobacteria, nocardiosis, diseases associated with eosinophilic granuloma complex and neoplasms.
DIAGNOSIS
When a poxvirus infection is suspected based on the history and the clinical examination of the patient, the diagnosis can be confirmed through histological examination of the skin lesions and/or ancillary tests, such as polymerase chain reaction analysis and immunohistochemical studies. Positive serology for poxvirus can support a diagnostic suspicion but is not sufficient to confirm the viral aetiology of the lesions.
In the case of suspected poxvirus dermatitis it is always wise to contact the local reference laboratory to organize the procedures for collecting and sending the samples. Generally speaking it is advisable to include margins of the most recent ulcers and dry crusts among the samples.
The histological diagnosis can only be made by identification of eosinophilic inclusion bodies in the cytoplasm of the keratinocytes (type A inclusion bodies). These structures are very large and easily recognized. Their viral nature can be confirmed by immunohistochemical studies, polymerase chain reaction analysis or electron microscopy. This last technique can be used on skin samples fixed in paraffin and enables both the elements that characterize replication of Cowpox virus to be seen; that is, mature virions within the inclusion bodies and immature particles free in the cytoplasm.
Cytological examination of samples obtained from the ulcerated-crusted lesions by direct apposition or scraping can lead to the identification of inclusion bodies by an expert cytologist but do not exclude the need to carry out histological studies to confirm the diagnosis. Isolation of the virus from skin biopsies is possible but requires a specialised laboratory and prolongs the time required to make the diagnosis.
TREATMENT
There is no specific treatment. The administration of broad-spectrum antibiotics is indicated to avoid the development of secondary bacterial infections. Treatment with L-lysine, interferon-alpha and imiquimod can be used in the same way as in Herpesvirus infections. Given that cats infected by poxviruses are very highly contagious, great care must be taken during local treatment of the lesions.
A patient with extensive ulcers of the oral cavity must be given nutritional support and intravenous fluid therapy. The use of glucocorticoids is contraindicated. The prognosis is usually good in the cases with mild-moderate systemic symptoms, but can be very poor in animals that develop pneumonia or oedema and generalised necrosis.
Suggested readings
- Carletti F, Bordi L, Castilletti C et al. Cat-to-human orthopoxvirus infection, Northeastern Italy. Emerging Infectious Disease 2009; 15:499-500.
- Godfrey DR, Blundell CJ, Essbauer S et al. Unusual presentation of cowpox infection in cats. Journal of Small Animal Practice 2004; 45:202-5.
- Yager JA, Hucthison L, Barret JW. Raccoonpox in a Canadian cat. Veterinary Dermatology 2006; 17:443-8.
- Nowotny N, Fischer OW, Schilcher F et al. Poxvirus infection in the domestic cat: clinical, histopathological, virological and epidemiological studies. European Journal of Companion Animal Practice 1997; 7:19-26.
- Schiavi S, Vercelli A, Cornegliani L et al. Un caso fatale di infezione da poxvirus in un gatto. Veterinaria 2010, 3:57-60.