To maintain the vaccine in an available state during the voyage, the physician recruited 22 young boys who had never had cowpox or smallpox before, aged three to nine years, from the orphanages of Spain. During the trip across the Atlantic, de Balmis vaccinated the orphans in a living chain. Two children were vaccinated immediately before departure, and when cowpox pustules had appeared on their arms, material from these lesions was used to vaccinate two more children.In 1796, English medical practitioner Edward Jenner tested the theory that cowpox could protect someone from being infected by smallpox. There had long been speculation regarding the origins of Jenners variolae vaccinae, until DNA sequencing data showed close similarities between horsepox and cowpox viruses. Jenner noted that farriers sometimes milked cows and that material from the equine disease could produce a vesicular disease in cows from which variolae vaccinae was derived. Contemporary accounts provide support for Jenners speculation that the vaccine probably originated as an equine disease called "grease". Although cowpox originates on the udder of cows, Jenner took his sample from a milkmaid, Sarah Nelmes.Jenner extracted the pus of one of the lesions formed by cowpox on Nelmes to James Phipps, an eight-year-old boy who had never had smallpox. He eventually developed a scab and fever that was manageable. Approximately six weeks later, Jenner then introduced an active sample of the smallpox virus into Phipps to test the theory.
Human cases are very rare (though in 2010 a laboratory worker contracted cowpox) and most often contracted from domestic cats. Human infections usually remain localized and self-limiting, but can become fatal in immunosuppressed patients. The virus is not commonly found in cattle; the reservoir hosts for the virus are woodland rodents, particularly voles. Domestic cats contract the virus from these rodents. Symptoms in cats include lesions on the face, neck, forelimbs, and paws, and, less commonly, upper respiratory tract infections. Symptoms of infection with cowpox virus in humans are localized, pustular lesions generally found on the hands and limited to the site of introduction. The incubation period is nine to ten days. The virus is most prevalent in late summer and autumn. Immunity to cowpox is gained when the smallpox vaccine is administered. Although the vaccine now uses vaccinia virus, the poxviruses are similar enough that the body becomes immune to both cow- and smallpox. Citations
General sources
Peck DR (2002). Or Perish in the Attempt: Wilderness Medicine in the Lewis & Clark Expedition. Farcountry Press. ISBN 978-1-56037-226-4.