Abstract
Virus
‘Cowpox’ virus is a member of the family Poxviridae, subfamily Chordopoxvirinae, genus Orthopoxvirus. Poxviruses are among the largest animal viruses. They are the only animal DNA viruses that induce ‘viroplasma’ zones in the cytoplasm of infected cells, which appear as inclusion bodies by light microscopy. Orthopoxviruses show broad antigenic relatedness.
Epidemiology
Poxviruses are ubiquitous among mammals; ‘cowpox’ is a misnomer, the virus occurs as an inapparent infection predominantly in small rodents, which are considered the natural reservoir.
The host spectrum is wide. Infections have been seen in exotic felids (having been fed laboratory rats), anteaters, elephants, rhinoceroses and okapis in zoos in Europe.
The infection occurs sporadically, but transmission between cats has been reported.
Pathogenesis
Infection usually starts with head lesions inflicted by the struggling rodent and then spreads to other body parts, notably the paws and ears (Figure 1), during grooming. After local replication, the virus causes a generalised infection with viraemic spread and multiple skin lesions. Virus has been isolated from the thoracic and peritoneal cavities. Neutralising and haemagglutination-inhibiting antibodies appear 2 weeks after infection.
Cowpox virus induced skin lesions on the ear. Note that gloves should be worn when examining a cat with a suspected poxvirus infection. Courtesy of Marian C Horzinek
Clinical signs
In most cases, rodent (rat) contacts are reported anamnestically. Cats display skin lesions, which are followed by inflammation, the foci later being covered by crusts (Figure 2). Often itching and poorly resolving ulcers (diameter 3–5 mm) with hard margins are noted. 5 Lesions are predominantly found on the face and paws; in severe cases progressive proliferative ulcerations ensue. The animal appears healthy if lesions are not superinfected by bacteria. Sometimes the mucosae of the pharynx and oesophagus are affected. Pneumonia, at times with exudative pleuritis and atelectasis, has been described.6–8
Skin lesion covered by crusts. Courtesy of Marian C Horzinek
Solitary superficial lesions usually heal spontaneously within 4–5 weeks in well-fed animals. 5 Generalised cowpox infections are fatal in kittens, and in cats being treated with corticosteroids.
Diagnosis
Cells from biopsy material taken from the marginal zones of inflammation contain Cowdry type-A inclusions (homogeneously dense, intracytoplasmic eosinophilic bodies); immunofluorescence tests are specific, quick and reliable. For virus isolation (using embryonated eggs or cell culture) scab material can be shipped dry (cooling is not necessary) or small quantities of exudate can be dried onto cover slips for shipping. Using negative-stain electron microscopy, evidence of brick-shaped virions is sufficient for diagnosis. Polymerase chain reaction allows the detection of viral nucleic acids and subsequent genetic and phylogenetic analyses. Paired serum samples can be used for retrospective diagnosis (seroconversion). Evidence of antibody in an ‘acute’ sample of serum from animals with characteristic lesions is strongly indicative of recent infection. 5
Disinfection
The virus is rather resistant to physical and chemical inactivation. For disinfection, sodium hydroxide solution (0.8%), sodium hypochlorite (1%), quaternary ammonium compounds, chloramine T (0.2%), iodine and phenolic compounds (3%), as well as detergents (sodium deoxycholate, Nonidet P40) and in general all disinfectants which have been tested for their efficacy (such as DVG-listed commercial products) are recommended. Alcohol (and also ethyl ether) is not suitable. In dry scabs and crust material, viral infectivity is maintained for months. 5 Heating to >80°C leads to rapid inactivation.
Disease management
Therapy should focus on cleaning and treating the ulcerated areas, with the primary objective of preventing secondary infection. The use of corticosteroids must be avoided [
Owners of affected cats and pet rats need to be alert to the risk of infection for humans.
There are no vaccines available.
Footnotes
Key Points
Funding
The authors received no specific grant from any funding agency in the public, commercial or not-for-profit sectors for the preparation of this article. The ABCD is supported by Merial, but is a scientifically independent body.
Conflict of interest
The authors do not have any potential conflicts of interest to declare.