Feline Poxvirus Infections

Chapter 24

Feline Poxvirus Infections

Etiology and Epidemiology

Cowpox Virus

Cowpox virus is a member of the Orthopoxvirus genus within the family Poxviridae, which are large, enveloped, double-stranded DNA viruses.2 Other orthopoxviruses include the viruses of smallpox (now eradicated), vaccinia (the smallpox vaccine), monkeypox (which is endemic to central Africa but was recently introduced to, and eradicated from, North America), and a range of other mammalian orthopoxviruses. All orthopoxviruses are closely related genetically and antigenically; therefore, vaccinia virus can be used to immunize against all orthopoxvirus infections. Cowpox virus has the largest genome of all the orthopoxviruses. Different strains of cowpox virus exist, as determined by both biologic characteristics and nucleotide sequence. Whether these merely reflect geographic variation of the virus (and therefore its main hosts and pathogenicity) or a range of independent virus species is still to be determined. For the purpose of this account, however, they are regarded as a single viral species.

Despite its name, cowpox is rarely described in cattle; cowpox virus circulates primarily in wild rodents. It has a wide host range that includes not only rodents, cats, dogs, cattle, and human beings, but a variety of captive wildlife mammals. The main route of transmission among rodents is not known, although, as for many orthopoxviruses, the respiratory route is probably important. In most accidental hosts, which include domestic animals and human beings, the most frequent source of infection is through a break in the skin. This gives rise to the “primary” lesion. The main reservoirs of cowpox virus are voles and, to a slightly lesser extent, Apodemus mice. As these rodent species undergo annual population cycles, breeding mainly in the summer and fall months, viral transmission and prevalence has strong seasonality. Transmission to accidental hosts such as cats, dogs, people, and zoo animals therefore also occurs mainly (but not exclusively) in the late summer, fall, and early winter. Cats probably become infected directly from rodent hunting, whereas infection in other species is often acquired from a “liaison” or “amplifying” host. These are accidental hosts in which the virus replicates to higher titers than in the reservoir rodent hosts; thus they bridge the epidemiologic gap between the reservoir and the host of interest. The main liaison hosts for human cowpox are the domestic cat (and, in the past, possibly cattle), followed by peridomestic rodent species such as rats. Rats can also be a source of infection for captive wildlife species. The virus is very hardy and survives in scabs for weeks or months, including in the environment, where it may be a source of human infection.

Raccoonpox Virus

Raccoonpox virus is also an orthopoxvirus. Remarkably little is known about its epidemiology, or indeed about the other known North American orthopoxviruses, skunkpox, and volepox viruses, which together form a distinct clade. Raccoonpox virus appears to be fairly host specific but was isolated from a cat with cowpox-like disease in Canada.3

Raccoonpox virus was originally isolated from the lungs of apparently healthy raccoons in the northeastern United States, and serologic surveys in the same area suggested that more than 20% of wild raccoons had antibody to the virus. Experimental infection of raccoons also caused no obvious disease.4 The narrow host range, low zoonotic and pathogenic potential, and endemicity of raccoonpox virus to North America has led to its development as a vector for recombinant vaccines for use in American wildlife species.


The parapoxviruses comprise a different genus from the orthopoxviruses in the family Poxviridae. Parapoxviruses are genetically very similar to each other (but significantly different antigenically and genetically from the orthopoxviruses), and differentiation of individual species is difficult.5 Parapoxvirus infections have been described in cats in several countries, and when characterized, the causative virus has been shown to be orf virus.6 Orf occurs worldwide, mainly in sheep and goats, and is readily transmissible to human beings.

Parapoxviruses, like many other poxviruses, survive in the environment for long periods—possibly months or even years under the right conditions. So although cats can become infected through direct contact with sheep or goats, it is more likely that infection occurs through contact with a contaminated fomite—perhaps a piece of barbed wire or fencing that contacts a preexisting wound. The rarity of parapoxvirus infection in cats makes its study very difficult.

Clinical Features

Signs and Their Pathogenesis

Cowpox Virus

In many hosts, such as human beings, dogs, and cattle, the primary lesion of cowpox virus infection, plus perhaps some virus replication in draining lymph nodes, may be as far as disease progresses. In cats and many captive wild mammalian species, however, the virus replicates in monocytes and macrophages and spreads to multiple organs, particularly the lungs and the spleen. Here, the virus replicates further, giving rise to a secondary viremia, which is often associated with pyrexia and mild or occasionally severe systemic signs. Pneumonia can itself be fatal, but usually the secondary viremia leads to a “secondary rash” of lesions distributed across the skin and mucosae.

The signs of cowpox vary according to the host species. The classical presentation in cats is of a primary skin lesion, often with secondary bacterial infection, usually on a forelimb or the face, thought to arise from the inoculation of virus into a wound—perhaps a bite wound—while hunting infected wild rodents. In some cases, severe necrotizing facial dermatitis develops. The primary lesion may be followed by lethargy and possibly pyrexia for up to 10 days, after which multifocal skin lesions appear. These first develop as papules (Figure 24-1), which become ulcers up to 1-cm in diameter that crust over and heal over 4 to 6 weeks. This leaves small patches of alopecia until the hair regrows. In cats such as Siamese, the hair can grow back a darker color, and scars are often visible for years in cats if their hair is clipped. Infection of dogs is less common, and in dogs, lesions are usually more localized and the systemic disease much less severe.68

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Jul 10, 2016 | Posted by in INTERNAL MEDICINE | Comments Off on Feline Poxvirus Infections
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