Bovine tuberculosis – cattle, badgers and culling

The British farming community suffers considerably from the epidemic of bovine Tuberculosis (TB) in cattle, with estimated future costs to the GB economy on the order of £100m per year. While bovine tuberculosis can be an important zoonosis, in GB this is largely controlled by milk pasteurisation, and so much of this impact is due to the effect on international trade. For the farmer, requirements put in place following the identification of a cow that tests positive on a farm include movement restrictions and the slaughter of all test positive cattle. The latter in particular can make this an extremely traumatic event for the farmer, and one which, for some at least, there is little or no hope that it will get better in the forseeable future. For these reasons, bovine TB is regarded by Defra as the most pressing animal health problem in Britain today.

circulating btb

Badgers and cattle circulate bovine TB in both directions. The problem is, we don’t know how much.

How should we control it? This much we know: badgers can infect cattle with bovine tuberculosis, and this contributes to the epidemic of bovine TB in England and Wales. This much we also know: cattle can infect badgers, and likely do so on a regular enough basis so that the cessation of cattle testing during the 2001 foot-and-mouth disease epidemic resulted in a spike of badger infection. Unfortunately, beyond that the scientific evidence is sufficiently opaque that there is no clear support for any particular policy towards its control: each of EnglandWales, and Northern Ireland (NI) have adopted different strategies, reflecting a combination of epidemiological circumstance and socio-economic factors specific to each jurisdiction (Scotland is officially TB free). Meanwhile all three jurisdictions can look on with some envy at the Republic of Ireland (ROI), where badger culling combined with intensive cattle testing and controls has resulted in a dramatic decline in cattle incidence. Whether or not the current measures lead to eradication is yet to be determined.

Unfortunately, a comparison with the ROI is not a simple one. Badger densities are lower on average compared to England and Wales, and the cattle industry enormously important to the Irish economy. In contrast, in the UK both dairy and beef cattle numbers are in decline, while the 1992 Protection of Badgers Act reflects the status of the badger in GB as a well-loved national icon. This affection is not shared in the ROI to the same extent, and in ROI official culling has been conducted continuously since the 1980’s, leading to a very different badger population compared to that in GB. Thus while the ROI can provide valuable insights that can be useful to the British situation, it cannot be used as a model for control.

And that leads us to the badger cull. At the end of August, trial culls of badgers were initiated in England, using hunting rifles as the method of choice to undertake the cull. In a statement last year from Prof. Ian Boyd (Defra’s Chief Scientist) and Mr. Nigel Gibbens (Chief Veterinary Officer), “culling … will enable us to test our assumptions about the effectiveness, safety and humaneness of culling by means of controlled shooting.” The latter two of these three points represent important considerations, so long as culling by shooting is a seriously considered option. Keeping in mind that the ethical questions associated with the widespread depopulation of a wildlife species is also a critical consideration (but a matter that lies outside this blog), many are asking whether or not the science supports the cull. Keeping in mind that effectiveness in this case relates solely to whether culling of badger populations is ‘sufficient’, there remains the question of whether or not the proposed sufficiency for a cull would reduce the incidence in cattle. Most of the evidence for bovine TB in England is based on the randomised badger culling trial (RBCT), a controlled field ‘experiment’ of massive proportions. An enormous amount was learned from the analyses conducted during and after the RBCT, and this forms the basis of most of our knowledge of bovine TB epidemiology in GB today. Extensive though the areas of the trial were, they were not comprehensive, especially considering the increase in extent of areas of high bovine TB incidence compared to when the culls were started in the late 1990’s. Thus we are largely reliant on extrapolation to understand how these data relate to disease progression elsewhere. These extrapolations have thus far been largely based on statistical arguments – at their core, they do not directly consider the complex ecology underlying the spread of the disease. Even if ‘enough badgers are culled, the result could be a reduction in cattle TB, or an increase. Because it will not be possible to compare cull areas directly to a non-cull area in a controlled fashion, it will be difficult to identify in a scientific way, the cause behind any change. Thus perhaps the best that can be said (in either direction) is that, far from either supporting or opposing the cull, the science is simply is not yet sufficiently mature to properly inform it.

Science and policy make strange bedfellows. Ironically while science is responsible for some of the most radical changes we see in society today, it is inherently conservative, classically requiring ‘95% confidence’ in order to make any decision. On the other hand, policy often simply needs a decision and that decision must take into account many factors outside of science, including socio-economics, ethics and politics. Thus it would be foolish to ever think that a matter as controversial as the control of bovine TB would ever be decided solely on the science. Despite this, science can and does play an important role in developing the evidence base for making decisions; while not science based, policy decisions must nevertheless be scientifically sound. The science behind the cull is therefore important.

Over the next several weeks and months, I’ll be publishing a series of blog articles about bovine Tuberculosis in Great Britain. In these, we’ll look at various aspects of the science that can help us understand both how bovine TB is maintained in British cattle and badgers, and how it might help us understand how to control it.

Advertisements

2 comments

    1. A bit of a late response to Tom’s reply on this – yes, just because a difference is ‘significant’ doesn’t mean its meaningful. A very good question to ask policy makers is the balance between surety of outcome and impact of a positive outcome. In a recent conversation with Dom Mellor, he brought up the recent vaccination campaign for toddlers against flu. A clear winner in that it is almost certainly to have some effect, but would the money be better spent on something with potentially greater impact?

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s