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.

Parapoxviruses

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.

Signs and Their Pathogenesis

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