Bovine TB: spread from badgers to cows?

Bovine TB; this disease of cattle is caused by Mycobacterium bovis, a close relative of human TB. And because of the threat to human health, the livestock industry operates under a regimen of regular testing and strict controls. But bovine TB is also found in badgers. So do the cows give it to the badgers; or do the badgers give it to the cows? Or are both true? To get to the bottom of this, and speaking with Chris Smith, Edinburgh’s Rowland Kao has been conducting a very comprehensive genome screening programme to track how the disease spreads…

Rowland - What we know is that there's a lot of controversy regarding the control of the disease, whether you should control it in cattle, in badgers, or in both. And the fundamental question of that is essentially how often does one of those species infect the other, and vice versa. And what we were aiming to do is to try to get at that precise question,

Chris - Because, of course, what we don't know is whether it's one way traffic or two way traffic. Do the Badgers give it to the cows, or the cows give it to the badgers, or does it go round in one giant whirlpool?

Rowland - Absolutely I mean we have lots of indirect evidence to show that each species is important in the transmission process. But what we've not been able to do until now is to get any idea of just how much is going on in both directions.

Chris - So how did you solve it?

Rowland - A few years ago working with some colleagues in Northern Ireland, we did a relatively small study looking at the bacteria which causes bovine tuberculosis. So this bacterium, called Mycobacterium bovis, is very closely related to human TB, but is more specifically oriented around cattle. Now, what we did, was we took what's called the whole genome sequence of these bacteria - so essentially you take the entire genetic code - and, by tracing changes in the genetic code - essentially how is the bacteria evolving from individual to individual - you can get quite a good idea of who is infecting whom.

Chris - How does this solve the problem though? Because, obviously, if I can match a sequence in a badger with some cows, how do I know that the badger gave it to the cows, or the cows gave it to the badger? I just know that they're infected with a genetically extremely similar strain.

Rowland - Absolutely, and people are probably familiar with DNA fingerprinting, which tells you whether or not two individuals are closely-related to each other, or where a genetic sample might come from a person. Now that technology is now relatively established, but only gives you a very vague idea. Whole genome sequencing does the same thing only it's much more precise. And what we can use is mathematical and statistical models to analyse relationships to track the bacteria as it evolves from individual to individual, herd to herd, badger group to badger group and determine the rate at which it's most likely that each one transmits to the other.

Chris - And where did you get the raw data from and how were those data collected?

Rowland - So we were really fortunate that we had contacts with a group of scientists who work in a place called Winchester Park, in Gloucestershire, and they've been studying badgers there since the 1970s; they regularly trap badgers, sample them for bacteria, and then release them again. We also have similar information from the cattle every single time they test them. So we have a nice comprehensive dataset looking at how the bacteria evolves in both populations,

Chris - And what does it show?

Rowland - We have on the order of about 100 samples from both populations. And what they show is the vast majority are very very similar to each other. And it appears to be that there's more diversity existing in the Badger population.

Chris - And would that suggest then that the badges are the main host and that they're injecting certain strains into the cows. Or, is there evidence for a feedback loop so the cows have it for a while, they give some back to the badgers, the Badgers then recirculate, amplify, maybe ditch some strains, amplify others. What does the dynamic now look like?

Rowland - There's a field of study called phylodynamics, where what people aim to do is try to integrate different sources of information - not just the genetic data, but the information you have about the populations - to try to understand exactly those kinds of questions. And for this particular population what those analyses show us is that, on the whole, transmission of the disease within each species appears to be greater than the transmission between species. But we do get some transmission in both, and the badgers are giving it more to the cattle.

Chris - Because at the moment people are saying this apparent relationship is grounds for various culling manoeuvers and things like that. If you then consider culling in your model, would that make a difference?

Rowland - I'm sure it would. I mean one thing about the Winchester Park area in particular in this timeframe is there was no official culling going on. So it's a relatively undisturbed badger population. Now, if you were to cull them, what you might expect to see, the spatial relationships - so essentially if I took two samples one from a badger on one from a cow - I'm asking the question how closely related are they. Does their distance from each other geographically tell us something about that we'd expect those relationships to change because they change they would also change the outcome of the what we think the transmission rates are. So I think if I were to ask what the important message to get from this is, in this area it shows that badgers are important. What it allows us to do, if we can so studies in other areas, is to refine the way we consider the control of both populations. Because, obviously, the disease in cattle for farmers is extremely important. It's hugely debilitating to the farm To have this going on. At the same time. badger welfare and badger conservation is important. So if we can tailor the way we do the control, identify areas where badgers are important and where they are not, that helps us to do the most scientifically sensible approach to controlling the disease.