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Editor's Correspondence
January 27, 2003

Cowpox—Not Cows but Cats

Arch Intern Med. 2003;163(2):249. doi:10.1001/archinte.163.2.249

The review article by Kravetz and Federman1 on cat-associated zoonoses emphasized several infectious diseases associated with cat ownership. With discussion about smallpox being at the forefront in the media at present, it is worth adding the role of cats as carriers of cowpox, another poxvirus. In spite of its name, human cases of cowpox have tended to have a feline association.25 Most patients with cowpox tend to have contact with cats, with disease starting as a few macular lesions at the site of inoculation, which progress to pustular "pox" lesions. These lesions occur generally in the extremities or the face; however, in some patients the lesions may be disseminated all over the body.4,6,7 Patients also complain of systemic symptoms, which gradually improve with eschar formation. Rarely, fatalities have occurred in immunocompromised individuals.5,6 In the current era of heightened awareness of smallpox, it is worth remembering that disseminated cowpox in rare cases may mimic smallpox, and electron microscopy cannot differentiate between cowpox and smallpox viruses. The use of polymerase chain reaction may aid in rapid differentiation of the 2 viruses, though classically, the differing growth characteristics of the 2 viruses on chorioallantoic membrane have been used to make the difference, with cowpox showing a bright red hemorrhagic pock on the chorioallantoic membrane compared with a small white pock caused by smallpox virus.8 Treatment in limited cowpox disease has generally been supportive, though in severe cases antivaccinia gamma globulin may be used. The role of antiviral drugs with potential activity against cowpox virus such as cidofovir and ribavirin needs to be explored. In conclusion, it is worth adding this disease to the vast number of diseases associated with cats.

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