Science Questions

What is the link between badgers and bovine TB?

Sat, 22nd Sep 2012

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Christina asked:

I've been a big fan of your show for a few years now and really enjoy how you explain everything scientific so that anyone can understand.


When it came to a friend of mine, who is a farmer, talking about the UK Badger cull and the Badger TB crisis I couldn't get my head around all the arguments. Would it be possible for you to explain the science behind why we need the cull? And what ways we could help both the badgers and the farmers?



Kat -   Okay, this is a very complicated and very emotive issue, and it mixes science, cute furry animals, commercial interest and politics, and with this kind of complicated issue there's obviously no really simple answer to it. 

Now, the UK government are proposing a badger cull in England that could see as many as 100,000 badgers killed and thatís a third of the national population.  So, thatís quite a big thing and not great if you're a badger or someone who loves badgers.  Now, on the farmers side, they're arguing that the wild badger populations harbour the bacteria responsible for bovine TB, and this is a germ called Mycobacterium tuberculosis.  Itís very closely related to human TB and it can also cause TB in humans although itís a very low risk.  You really have to get very close to cows with it.  But it is a big problem for cattle farmers and itís responsible for a huge number of animals having to be destroyed every year.  There is some evidence that it can get into milk, but the thing is, if you pasteurise milk, then thatís absolutely fine and there's no risk to consumers.  But they say that badgers do spread the TB germs around through the badgerís urine and faeces and cows only need to pick up a relatively small dose of bacteria to be infected.  So in theory, getting rid of badgers would stop them bringing TB onto farms and it would protect cows, it would protect the farmerís interests, and the money that it costs to control TB, and all that kind of stuff, and that would be great.

Now on the other hand, there's a lot of people who say that a cull would not be effective.  There was a big randomised trial of badger culling and it showed that culling is only really going to make a small difference to TB infections in cows and it might actually make the problem worse.  Because if you start disturbing badgers and trying to kill them, theyíll scurry away into other locations.  And also, if you do manage to kill a lot of them, then you have all these empty badger sets that could potentially be infected and then new badgers will come in, start interacting with the population there, and itís just going to spread, and potentially, it could get worse.  There's also no way of telling whether a random badger has TB or not, so they're proposing that you just shoot badgers on sight.  So the problem is, you're not selecting between healthy badgers and diseased badgers.  So thatís a problem and there's also a public risk of having people out at night with shotguns.  Again, itís not a great idea.  The only way that it could work to do a cull would be, you'd have to kill a huge number of badgers in a very big area or at least 150 square kilometres in a very short time.  So thatís at least in two weeks, in less than two weeks.  And this is going to be very difficult, very expensive.  Itís not going to completely reduce TB outbreaks anyway in cows and it would cost farmers probably more than they would lose form bovine TB.  Itís also worth noting that other animals can actually spread bovine TB and it includes deer and foxes, so itís a bit of an unwinnable battle.

Now, where do we go from here?  How can we help farmers?  How can we help badgers?  There's an argument to say that actual better farming practices could do a lot to curb infection.  So, controls on moving cattle around, doing more to keep badgers out of farms and out of the food sheds where they store cow food, and actually, there's a lot of work going on into vaccinating badgers.  There's currently an injectable vaccine.  Itís obviously quite hard to trap and inject quite a lot of badgers, but there are proposals that really should be worked on an oral vaccine for badgers so you could leave out in their food, and that would probably be a really good way forward without having to kill lots of them.


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The science behind a badger cull is not terribly clear.

I think it is a subject that could prove to be a fascinating study into the use of "annecdata" and lobbying in the formation of public policy.  I think a second study of the general public's reactions to the treatment of iconic megafauna would also be terribly interesting.

The original link between badgers and bovine TB (bTB) was made 90 years ago because the south west of England has the highest incidence of TB positive badgers and the highest incidence of bTB in cattle.  Most of the studies have focused on this hypothesis. 

The limited culling trials in the 1990's found that TB levels fell by around 15% in the cull areas - confirming some link -  but also suggesting that the issue is far more complicated than simply the presence or otherwise of badgers.  The same studies found a small increase in TB levels outside of the cull areas which has been attributed as being caused by disruption of the badger population. 

It is interesting to note that in 2011 Lord Krebbs who reported these findings to Parliament criticised the large scale cull planned, suggesting it would be ineffective and money would be better spent on developing a vaccine.  This has been countered by saying that the trial cull areas were too small and that a larger area would see greater reduction in incidence and proportionately less ďperturbationĒ problems from disrupted badger populations.  For obvious reasons this has not be scientifically studied (to date) and has been criticised by some ecologists, although these ecologists are more likely to be critical of modern intensive farming practice. 

Badgers are not the only carriers of TB, with vermin and deer also carrying it. 

Some research from Ireland suggests that is TB in healthy Badgers is not particularly infectious.  Similar studies have noted that the condition of the cattle - vitamin levels, immunity levels being a factor which are primarily dictated by the weather and the intensity of the farming - was an important factor.  Cows in good condition are less likely to become infected.  However, when bTB infects a herd, it spreads much more rapidly.  It is not inconceivable that poor biosecurity at livestock markets may also be a factor in the spread of the disease.  This links to another possible issue is modern agricultural practice, which can see animals moved all around the country.  For example to be called ďScottish BeefĒ the animals only have to be finished  for a relatively short time in Scotland Ė they do not have to be born/ raised there.
Mazurka, Fri, 24th Aug 2012

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