In this mild, eruptive disease of dairy cows, lesions are seen on the udder and teats. Although once common, cowpox is now extremely rare and reported only in western Europe (also see Cowpox Virus Infections in Cats and Other Species).
The virus of cowpox is closely related antigenically to vaccinia and smallpox viruses. Indeed, the first two can be differentiated only by sophisticated laboratory techniques. Before vaccination of the general population against smallpox was discontinued, some outbreaks of cowpox in cows in North America and Europe were due to infection with vaccinia from recently vaccinated persons. Vaccinia-related viruses continue to cause occasional outbreaks of teat infections in dairy cattle in South America and buffalo in the Indian subcontinent. These viruses often spread to people in contact with cattle. The epidemiology of these viruses is unknown, but it has been suggested that they are vaccine viruses that spread to animals during the smallpox vaccination campaigns.
The disease spreads by contact during milking. After an incubation period of 3–7 days, during which cows may be mildly febrile, papules appear on the teats and udder. Vesicles may not be evident or may rupture readily, leaving raw, ulcerated areas that form scabs. Lesions heal within 1 mo. Most cows in a milking herd may become affected. Milkers may develop fever and have lesions on the hands, arms, or face. Occasionally, cowpox in people can cause generalized disease, and fatalities have been recorded.
Cowpox or vaccinia infection may be confused with bovine herpes mammillitis (see Diseases of Bovine Teats and Skin); because the lesions of these two conditions are superficially similar, laboratory confirmation is required. The viruses of cowpox and vaccinia can be easily visualized by electron microscopy. Although they cannot be distinguished from each other, their morphology by electron microscopy is distinct from that of pseudocowpox virus and bovine herpes mammillitis virus. Both vaccinia and cowpox viruses grow readily in cell cultures.
Measures to prevent spread of cowpox within a herd must be based on segregation and hygiene. Cowpox and vaccinia viruses are important causes of zoonoses.