Cowpox virus is an example of a virus with a misleading name. It’s place in history is from Jenner’s observation that milkmaids who had been infected with cowpox were resistant to smallpox, leading to the use of cowpox (which causes very mild disease) to protect against smallpox (which is very, very bad). While cattle can be infected, they are not the true host of this virus, and infections in cattle are actually quite rare.

Various rodents are the true reservoir of cowpox. Other species can be infected from contact with infected rodents, including people and pets. Among pets, cats are most commonly infected, with most reports coming from central Europe. Cats may be infected more often because they may more often have close encounters with rats, but they are probably also inherently more susceptible to the disease than dogs.

Cowpox infections in dogs are very rare, but a case was recently described in Veterinary Dermatology (von Bomhard et al 2011). It involved a five-month-old Rottweiller from Germany that developed a very mild case of cowpox, with a single nodule on its muzzle. The dog recovered uneventfully, but it was an interesting case of a rare disease in a dog, and one that has some human health considerations.

People can be infected with cowpox from pets. In particular, infections from pet rats have been a problem in central Europe over the past few years. Outbreaks of human infections have been identified associated with widespread dissemination of infected rats from infected breeding or distribution facilities. Infections have also been reported from cats, and cats are a significant concern because of their ability to be a bridge between wild rodents and people, and because of the close contact they tend to have with people.

It’s not surprising that disease was so mild in this Rottweiller puppy, and the risk to people in contact with the dog was probably limited because of the mild nature of the infection. No human cases were reported associated with this dog. Human infections from dogs have not been reported, largely because the disease is so rare in dogs and perhaps because when they are infected, dogs tend to have very mild disease. Cowpox is of minimal concern for most pet owners, but it something to be aware of when obtaining a new rodent, especially in regions where cowpox is an issue, and when dealing with cats in areas where cowpox is endemic in wild rodents. Some basic preventive measures include:

• Pet rodents and cats (especially newly obtained rodents) that develop skin lesions should be handled with care and be examined promptly by a veterinarian.
• Wild rodents should never be caught and kept as pets (for various other reasons, as well).
• Contact between domestic pets (particularly pet rodents and cats) and wild rodents should be prevented.