Mycobacterium bovis... don't blame the cows?
I recently wrote about Mycobacterium bovis, the cause of bovine TB and a pathogen that can be transmitted to people and rarely to pets. A reader made the following comment:
"Having come across your very interesting blog, I was questioning/wondering whether your statement regarding Mycobacterium bovis, "whose main natural reservoir is cattle", is in fact actually so any longer; if ever. . In the UK all cattle herds were once declared clear of this disease by testing and culling and the gassing of badgers, until the government protected the badger over here to appease animal rights activists and gain a few extra votes. Now it is rife again. These people somehow believe badgers have 'rights' to life above farmers' cattle.
I often wonder if the 'bovine' association is simply because the bacterium was first isolated in cattle as they were obviously captive and there to be investigated. Could it just as easily have been called Mycobacterium meles? As I understand it, Mycobacterium tuberculosis came first and originated in humans and then developed as Mycobacterium bovis in animals"
Good question. Just because a disease is named after a particular species does not mean that it's the main source or original source of the pathogen responsible. Cowpox virus is a good example - cows aren't actually the reservoir of this virus, rodents are. However, because cows are more closely observed or monitored than rats, it was originally associated with cows and thus named cowpox.
We are more likely to detect diseases in humans first, followed by domestic animals, followed by wildlife. Severe acute respiratory syndrome (SARS) is a good example of this. This disease was first found in people. It was then linked to civets and raccoon dogs. However, civets and raccoon dogs aren't the true reservoirs, nor where the disease originated. It appears that the reservoir is in fact bats. Bats are a lot harder to investigate than captive animals, so even though they are now the presumed reservoir, it took a while to figure that out.
Back to Mycobacterium bovis... It's impossible to say for certain where it originated. Since it is thought to have evolved from Mycobacterium tuberculosis, the reservoir of which is people, it makes sense that the evolution of M. bovis would involve a domestic animal species instead of a wildlife species. Cattle have much closer contact with people than do wildlife such as badgers, opossums and deer. So, since M. bovis has historically been most strongly associated with cattle, and cattle live in close contact with humans, I woudn't be surprised if they are the true orgin in this case. However, since M. bovis can infect a very wide range of species, we can never really know.
Cowpox from rats
.jpg)
Cowpox is a disease caused by cowpox virus. This virus is most famous for being used by Edward Jenner to vaccinate people against smallpox (which is caused by a related virus). However, despite the name, rodents are the natural host of cowpox virus, not cows. Most human cases of cowpox are associated with contact with rodents. Cats are another important source of human cowpox infection in some areas.
Cowpox infections in people are uncommon. Most cases occur in Europe, western parts of the former Soviet Union and parts of Northern and Central Asia. Outbreaks of cowpox can be associated with infection of pet rats at central breeding or large housing facilities, which results in large numbers of infected rats that are then shipped to many different places. Recently, outbreaks of rodent-associated cowpox have been reported in Germany and France. It is suspected that these infections are associated with a large rat breeder in the Czech Republic, however this has not been confirmed.
Cowpox causes typical "pox" lesions, like those that are seen with chickenpox. Only a single lesion is present in most cases, but multiple lesions can develop. Flu-like symptoms such as fever and fatigue, as well as enlarged lymph nodes, are also common. Severe, even fatal, disease can occur in individuals with a weakened immune system, but in most people cowpox infection causes only mild illness and resolves without treatment.
Cowpox is not a concern in many areas of the world. In regions where it is present, basic, common-sense precautions can reduce the risk of infection.
- Contact with infected animals should be avoided. Cats are an important source, and cowpox should be considered in any cat with appropriate skin lesions in an endemic area. The same should apply to rats with skin lesions.
- Gloves should be worn when handling cats and rats with skin disease, and hands should be washed immediately after glove removal.
- If skin lesions are identified on a rat, particularly over the feet, ears or tail, the animal should be examined promptly by a veterinarian.
- Don't buy any rat with skin lesions, and if possible, avoid buying rats that were bred and/or housed in facilities with a very large number of other rats.
