Cowpox is a viral disease caused by the cowpox virus (CPXV), a member of the Orthopoxvirus family, which includes the smallpox virus. This zoonotic disease can transfer from animals to humans. Historically, cowpox gained prominence due to its connection to the first successful vaccine against smallpox. In 1796, Edward Jenner observed that dairymaids who contracted cowpox appeared immune to smallpox, leading him to develop the smallpox vaccine using cowpox material. While originally named for cows, human infections today are rare and acquired from infected domestic cats, not directly from cattle.
Visual Characteristics of Human Cowpox
Cowpox infection in humans begins with a localized skin lesion at the site of virus entry, often on the hands, arms, or face. After an incubation period of about 9 to 10 days, the initial sign is an inflamed, reddish, flat spot known as a macule. This macule rapidly progresses, between days 7 and 12, becoming a raised, firm bump called a papule.
The papule then develops into a fluid-filled blister, or vesicle, which can range from a few millimeters to 1-2 centimeters in diameter. By days 13 to 20, the vesicle transforms into a pus-filled pustule. This pustule may then ulcerate, forming an open sore.
Between three to six weeks after infection, the ulcerated lesion develops into a deep, hard, black crusty scab, known as an eschar. This eschar is surrounded by redness and swelling, and can be quite painful. The lesion heals within 6 to 12 weeks, with the eschar flaking off, and leaves a scar. Affected individuals may also experience general symptoms like fever, tiredness, and swollen, painful lymph nodes near the infection site.
How Cowpox Appears in Animals
The natural reservoirs for the cowpox virus are woodland rodents, such as voles and mice. These rodents can then transmit the virus to domestic animals, with cats being the most common source of human infection. In cats, the virus enters the skin through a bite wound, most often observed on the head, neck, or a forelimb.
The primary lesion in cats appears as a small, ulcerated nodule or a scabbed wound, which can progress into a larger abscess. About 7 to 10 days later, secondary lesions may emerge across the body as small, circular, ulcerated papules, measuring 0.5 to 1 centimeter in diameter. These lesions then become covered by scabs. Beyond skin manifestations, infected cats might show signs such as fever, depression, reduced appetite, and enlarged lymph nodes.
While less common today, cowpox can also affect cattle, where lesions are found on the udder and teats. These appear as small, raised papules that can quickly develop into vesicles before rupturing. The ruptured lesions then form raw, ulcerated areas that crust over with scabs, healing within a month.