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.