Cowpox virus infection
Edited June 2018
The Cowpox in cats guideline was first published in the J Feline Med Surg 2013; 15: 557-559; the present update has been authorised by Karin Möstl et al.
Cowpox virus has a wide host spectrum including man (zoonosis!) and occurs predominantly in small rodents.
Cats with rodent contact are at risk to become infected.
Skin lesions are predominantly found on the head and paws. They usually heal spontaneously, in severe cases progressive proliferative ulcerations ensue.
In kittens and immunosuppressed cats generalized cowpox infections take a fatal course.
Corticosteroids facilitate virus generalisation and are contraindicated.
Biopsy and/or scab material is suitable for diagnosis.
Owners of affected cats (as well as of affected pet rats) must be informed about the zoonotic risk.
”Cowpox“ virus is a member of the family Poxviridae, subfamily Chordopoxvirinae, genus Orthopoxvirus; poxviruses are amongst 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. The Orthopoxvirus genus comprises the species variola (smallpox) virus, with humans as only susceptible hosts. The monkeypox, cowpox, vaccinia, and camelpox viruses amongst others are other members of the genus and show broad antigenic relatedness. The zoonotic risk is considerable.
Poxviruses are ubiquitous amongst 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. In addition to bovines, infections have been seen in exotic felids (after having been fed laboratory rats), anteaters, elephants, rhinoceroses and okapis in zoos in Europe.
In domestic cats, the infection occurs sporadically, but transmission between them has been reported.
Human cases caused by virus transmitted from cats (fatal in immune-suppressed individuals; Czerny et al., 1991) and from pet rats (e.g. in France and Germany; Ninove et al., 2009; Vogel et al., 2012) have been reported. These sources of cowpox virus infection for humans have to be kept in mind, especially as variola vaccination has been discontinued and populations are now vulnerable (Willemse and Egberink, 1985).
Infection usually starts with head lesions inflicted by the struggling rodent and then spreads to other body parts, notably paws and ears (Fig 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 about two weeks after infection.
In most cases, contacts with wild rodents (rats) or hunting are reported anamnestically. Cats display skin lesions, which are followed by inflammation, with subsequent crusting of the lesions (Fig 2). Often itching and poorly resolving ulcers (diameter 3 – 15 mm) with hard margins are noted (Gaskell et al., 1987). Lesions are predominantly found on the face and paws; in severe cases, progressive proliferative ulcerations ensue. The animals appear 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 (Schöniger et al., 2007; Schulze et al., 2007; Herder et al., 2011).
McInerney et al. (2015) reported five cases of pneumonia, of which three had pleural effusions and two had mixed infections (feline herpesvirus, Bordetella bronchiseptica and Mycoplasma spp). Four cats also presented with skin lesions.
Smith and Sloan (2017) described a fatal necrotising bronchopneumonia in a 2.5 years old cat, that exhibited fever, dyspnoe and tachypnoe, but did not show any skin lesions. This case shows the importance to consider a cowpox virus infection in cats with bronchopneumonia, especially if lung lobe consolidation is observed, even in the absence of skin lesions.
Breheny et al. (2017) reported two unusual cases of cowpox virus infection. The first presented with inspiratory dyspnoea and stridor; laryngeal oedema and a 1 cm para-laryngeal mass were identified (without any other abnormalities). The second case displayed non-specific clinical signs, multiple nodular lesions and ulcerations and central neurological signs at the terminal stage.
Solitary superficial lesions usually heal spontaneously within 4 to 5 weeks in well-fed animals (Gaskell et al., 1987). Generalised cowpox infections are fatal in kittens and cats undergoing treatment with corticosteroids.
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.
McInerney et al. (2015) isolated cowpox virus from bronchoalveolar lavage fluid, oropharyngeal swabs or pleural fluid from cats with pneumonia. They also found typical inclusion bodies in fine needle aspirates from the lung.
Using negative-stain electron microscopy, evidence of brick-shaped virions is sufficient for diagnosis (Fig. 3). 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 (Gaskell et al., 1987).
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 were tested for their efficacy (such 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 (Gaskell et al., 1987). Heating to >80°C leads to rapid inactivation.
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 (Gaskell et al., 1983; 1987). The prognosis is good, except when the lungs are affected.
McInerney et al. (2015) reported improvement of the condition after treatment with interferon omega in two of four cats with pneumonia. This observation needs to be confirmed.
Cowpox is a zoonosis – owners of affected cats and pet rats must be alerted of the zoonotic risk.
There are no vaccines available.
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