bovine TB (bTB) and badgers

Notes on TB, bTB, cattle, badgers, vaccination - the great debate...

What is bovine TB?

Bovine tuberculosis (bTB), as the name suggests, is a strain of TB which mainly affects cattle and is caused by Mycobacterium bovis (M. bovis). Unfortunately this strain of the disease also causes TB in badgers, as well as other wildlife and domestic animals, and causes ~0.5% of the human TB cases in the UK.

A brief history of bovine TB and government policy?

How is M. bovis spread?

M. bovis lives in the lungs of the infected species, and is normally passed on by aerosol droplets in breath so usually requires very close contact. M. bovis can however live outside of a host for some time in wet conditions (saliva, wet grass, drinking trough ...) but would usually have to be breathed into the lungs to cause a new infection. Transmission via urine, faeces etc.is also possible. Transmission only occurs from animals with active TB. It is thought that one third of the human population carries TB, however most of this is latent. The exact mechanisms and transmission paths of M. bovis are not known, however it is known that bTB is passed from...
cattle to cattle - lots of regular very close contact and easily passed by cattle movement between herds. Cattle bTB incidents roughly doubled during the foot and mouth outbreak when very little bTB testing and slaughtering was done
badgers to badgers - mostly within badger communities and not widely transmitted
cattle to badgers - when in bTB cattle increased during the foot and mouth outbreak, bTB in badgers increased. (both approximately doubled)
badgers to cattle - with shared territory this will happen, however tightened biosecurity measures on farms reduces this, and in N Wales bTB in cattle has been reduced just by increased biosecurity during 2010 and early 2011

The Randomised Badger Culling Trial (RBCT) and the Independent Scientific Group (ISG)

The RBCT was conducted in 10 sets of three 100km2 areas of high bTB occurence (triplets). In each triplet, one area was subject to an annual badger cull (proactive), one area had badger culling in and around bTB outbreaks (reactive) and one area was a control where monitoring only occured. The trial ran for 5 years, and follow up monitoring has taken place in the years since the trial.
The trial had two general conclusions
1. "... badger culling is unlikely to contribute positively, or cost effectively, to the control of cattle TB in Britain"
2. "... there is substantial scope for improvement of the disease through the application of heightened control measures directly targetting cattle..."
Roughly 10,000 badgers were culled in the trial.
M. Bovis prevalence in badgers rose in response to culling - thought to be due to perturbation.
The reactive culling trial was ended early as it was clearly causing an increase in herd bTB breakdowns.
Note that the reference areas for the 'Irish four areas trial' were in fact reactively culled, so the comparison data from this Irish trial yielded over optimistic results as to the benefits of culling.
Post trial analysis (Donnelly et al Feb & Jul 2010) showed that over the total period from trial start to July 2010, inside the trial areas confirmed breakdowns were 28.3% lower than in survey only areas, and in areas up to 2km outside the culling area confirmed breakdowns were 9.0% higher than the same area outside survey only areas.
The trial was government funded, run by the ISG, and is the best scientific data available on the effects of badger culling. Culling is not the answer to bTB in cattle.

BCG Vaccine

Bacillus Calmette-Guérin (BCG) is an attenuated strain on M. Bovis which is widely used around the world as a vaccine against human TB. It protects mainly against severe childhood forms of TB. Recent study and field trials published by Mark A Chambers et al. 2010 demonstrated that intramuscular BCG vaccination of badgers reduced the severity and progression of bTB infections. It concluded "...BCG vaccine... could provide a new and important component of a comprehensive programme of bovine TB control for cattle in UK and Ireland. Several more trials have started up, by DEFRA, the National Trust, the Gloucestershire Wildlife Trust and others - this must be better than culling.

bTB Statistics

Cattle slaughtered as a result of bTB testing

2011 figures are provisional and relatively more cattle were tested in this period

Graph below clearly shows that the rises suffered started with the almost complete lack of cattle TB testing and slaughter during the 2001 foot and mouth outbreak.