Study Tracks TB Transmission in Badgers and Cattle

Bovine tuberculosis, or bTB, is caused by a bacterium called Mycobacterium bovis.  M. bovis is closely related to the species that causes human TB (Mycobacterium tuberculosis) and is responsible for outbreaks in wildlife, as well as domestic cattle herds.  Not only badgers but rats, rabbits and other wildlife can be infected by bTB.

For years it has been suggested that badgers in Great Britain are a vector for the spread of bTB in cattle herds, but there has been no direct evidence that establishes a link.  In November 2012 an article was published in the journal PLOS Pathogens and reported in the Science Daily that provides possible genetic evidence for a link between the bTB in badgers and that in cattle.

In this study, the researchers sequenced genomes of bacteria that were isolated from cows and badgers that came from farms in Northern Ireland where there was a history of repeated bTB outbreaks. The results of this study showed that the bacteria isolated from badgers and cattle were very similar, which indicates that they are closely related. The most telling evidence was that the TB bacteria in badgers and cattle from the same farm were more similar than the TB DNA in bacteria from cattle on other farms. If the means of transmission was cow to cow, the cattle TB DNA should have been more similar to each other than to the badger TB.

This is bad news for the badgers of the United Kingdom because there has been a push to cull badgers for years, and it is the lack of hard data to support the badger vector theory that has prevented this from happening on a wide scale. One badger culling study, called the Krebs experiment, has been highly controversial and has produced very little hard evidence either way. Badgers are an important part of the natural ecosystems of Great Britain, and this has to be considered, as well as the possibility that they are spreading this disease. 

There is no doubt that bovine TB is a deadly pathogen and must be controlled for both animal health and economic reasons. However, neither of the studies mentioned above can actually show which way the transmission occurs. In fact it may be that the cattle are passing the TB bacterium to the badgers rather than the badgers passing it to the cattle. This seems more likely since badgers have very little direct contact with cattle but do rummage around in cow pats to find beetles and other food items. Since the TB bacterium can survive in cow pats, it is possible that the badgers are becoming infected this way. 

Before there is a rush to kill the badgers, it is important to learn more about the actual method of transmission of this disease. Just because the DNA of the TB bacteria in badgers is very similar to that in nearby cattle populations is no proof that the badgers are responsible for the presence of TB in the cattle. It may be that there are other, more important, factors causing the disease in cattle, such as overcrowding and poor nutrition. The main known way of TB infection is through the air by coughing and sneezing of those infected and then those uninfected breathing in the bacteria. It does not seem likely that this method could occur between cattle and badgers given their very different lifestyles, even though they may live in the same paddocks. More studies are needed before conclusions can be drawn.