Badger badger badger badger CULL badger badger badger TRIAL

June 1st, 2013 by Ben Goldacre in bad science | 13 Comments »

Reading about the badger cull today, I noticed this column – on the evidence for badger culls – never got posted. Here it is!

Ben Goldacre, The Guardian, Saturday 23 July 2011

Squabbles between farmers and animal rights’ protesters bore me senseless. This week, environment secretary Caroline Spelman announced that the scientific evidence supports her new policy of farmers killingbadgers to prevent bovine TB. It’s an overstatement, but more importantly, this story walks through several important issues in science.

Firstly, what works in principle may not work in practice. Bovine TB is a massive problem (and one reason why we pasteurise milk). Around 25,000 cattle were slaughtered last year because of it, and the cost to the taxpayer, since we compensate farmers, was £90m. Badgers carry TB, and about half of all cattle infections come from a badger source. It makes perfect sense that killing some badgers should reduce the number of cattle infections.

To test this hunch, 10 years ago the government took a very unusual step, and set up a proper trial: the Randomised Badger Culling Trial. This was a huge project, running from 1998 to 2007, in 30 separate 100km2 areas around England.

These study zones were grouped together into triplets. One zone in a triplet got repeated culling, roughly once a year (“proactive culling”). Another saw local badger culling after any TB outbreak in cattle (“reactive culling”). And a final zone was kept as a “control” region, with no culling. TB rates were surveyed in all of the areas.

You’d have thought this culling should do some good, or at least no harm. In fact, the “reactive culling” was stopped after a few years when the rates of cattle TB infections in these areas turned out to be higher than areas with no culling, by about 20%.

One suggested explanation was “perturbation”. Badgers live in small groups, with territorial boundaries; if you kill some, the groups are disrupted, and the animals wander further afield, spreading infections more widely. But the results from the “proactive culling” were more interesting. In the 100km2culling zone, cattle TB infections fell by about a quarter. But in the 2km-wide “ring” around the proactive culling zone, the number of TB infections in cattle rose by about a quarter, perhaps, again, because of “perturbation”.

A 2km ring becomes less important when the culling area is larger, and mathematical modelling suggests that after 150km2, the extra TB infections in the ring are outweighed by the benefits in the cull zone. At this size, you prevent 23 of the 187 expected herd outbreaks, and so save £600,000 ( outbreaks each cost £27,000). The cost, however, using the cage-trapping method used in the trial, is £2.14m. This is why people concluded it wasn’t worth the effort.

Here is the second science bit. The government is now introducing a kind of farmer-led culling. This costs around £500,000 for the same size area, and so now a cull becomes cost-effective, by a hair. But we also end up several steps away from the scientific evidence. First, we’re assuming that results from small cull zones scale up neatly into larger ones, and that killing can be done uniformly without local perturbation.

But more importantly, the trial loses what evidence nerds call “external validity”: the ideal perfect intervention, used in the trial, is very different to the boring, cheap, real-world intervention that the trial is being used to justify.

This is a common problem, and the right thing to do next is a new trial, this time in the real world, with no magic. The intervention could be the thing we’re doing, and the outcome could be routinely collected bovine TB data, since that’s the outcome we’re interested in. This gives you answers that matter, on the results you care about, with the intervention you’re going to use.

People worry that research is expensive, and deprives participants of effective interventions. That’s not the case when your intervention and data collection are happening anyway, and when you don’t know if your intervention actually works. Here, though, as in many cases, the missing ingredient is will.

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13 Responses

  1. Labrat said,

    June 1, 2013 at 8:24 pm

    “the ideal perfect intervention, used in the trial”

    Little discussed fact was that the perfect intervention in the trial, due to various factors (foot and mouth, protester disruption etc.) was disrupted so much that only one of the proactive test areas reached the protocol cull ratio. As always in science, garbage in, garbage out.

  2. gijoel said,

    June 2, 2013 at 12:41 am

    Awesome Brian May/Brian Blessed collaboration.

  3. beefbreeder said,

    June 2, 2013 at 5:06 pm

    The randomised badger culling trial was subject to political interference from the outset.
    As a result the cull only lasted for 8-11 days per year and significant areas within the 100 sq km were not included due to lack of landowner consent.
    Even though the rbct estimated badger numbers at about 15-18 per sq km only an average 1.8 badgers per year were removed.
    The rbct has been subject to much criticism from both farmers and vets.
    Previous smaller scale culls such as Thornbury and Hartland managed to reduce TB in cattle by 90-100% although a similar percentage of the local badger population had to be culled to achieve this.
    Make no mistake – properly run badger culls work extremely well in reducing TB, The question is more to do with is the political will there to see them through.

  4. skeptic reader said,

    June 4, 2013 at 8:40 am

    In reply to beefbreeder, note that the short period of culling in the rbct was to avoid culling immigrant badgers, and that this short period removed an average of 64-77% of the population, not the 10% suggested. The level of landowner consent and removal rates in the proposed cull mirror those in the rbct. The main difference is that the proposed cull will last 6 weeks (maximum) and will be based on shooting rather than trapping. There is no evidence that this will be better or worse than the rbct, but lots of opinion. Beefbreeder is right in that the near total removal of badgers will lead to a substantial reduction in tb in cattle, but the results from the earlier trials referred to are no longer valid, since cattle rearing and cattle movement rates are now so different from the 1970s and 1980s.

  5. beefbreeder said,

    June 4, 2013 at 12:39 pm

    With all due respect to skeptic reader, rbct’s own figures show a maximum of 2059 badgers culled over 1000sq km putting the highest cull rate at 2.1 badgers per sq km a year.
    The 64-77% you mention were for the entire duration of the cull not per year and are disputed as being overestimates by more knowledgeable people then me.
    Please see

  6. Mojo said,

    June 9, 2013 at 11:40 am

    @skeptic reader:

    “In reply to beefbreeder, note that the short period of culling in the rbct was to avoid culling immigrant badgers…”

    Was that just to annoy the Daily Mail?

  7. ironduke said,

    June 10, 2013 at 1:28 pm

    There is a bigger issue related to TB control rather than simply culling badgers, which as noted, is not particularly effective or cost effective. This is: why bother trying to control bovine TB? It is not a risk to human health, with universal pasteurisation of milk and diary products. The number of TB cases attributable to a bovine source and contracted in the UK is vanishingly small (probably less than 20 cases over the last couple of decades). Unlike foot and mouth disease it doesn’t cause an economic loss to farmers (it is the control measures that causes the loss); it isn’t a problem with animal welfare (except for badgers!) as cattle are culled before they become symptomatic, either for beef or because of age related milk production decline. The only remaining rationale is we would be prevented from exporting live animals (at a total loss of around £2million per year) yet we would be saving nearly £100 million by abandoning the intensive control measures we have. What futile waste of money. Bovine TB should be treated like other animal diseases, farmers take out insurance for it, culling of symptomatic herds and release us taxpayers from having to pay for this nonsense.

  8. Silenus said,

    June 10, 2013 at 3:15 pm

    New Zealand has a bit of experience concerning possums carrying bovine tb.
    Maybe their experience could be used ?
    I appreciate that the situation isn’t the same, but some of the problems are similar.
    article example:
    “The formulation of a national strategy for biological control of possums and bovine Tb
    P. H. Atkinson a & D. E. Wright b, 1993” (haven’t read it yet myself)

    And are there any efforts made towards a vaccination strategy ?

  9. Joe Choi said,

    June 13, 2013 at 5:55 am

    Is that “will” from donors or from politicians? The latter are actually surprising hard to convince.

  10. prof-quatermass said,

    July 5, 2013 at 11:11 pm

    Hmm didn’t ‘New Scientist’ do an article years ago on curing Badgers TB?

    Didn’t they say that trials adding potassium supplements to their food greatly reduced them getting TB?

    Also, how does a Badger give a cow TB?
    Via their Dung I suppose?

    Maybe Farmers could do some early morning pick and scoop protocols like dog owners do. 😉

  11. lspall said,

    August 4, 2013 at 8:00 pm

    Sorry I saw this blod so late!
    some clarifications: the increased incidence of TB in the ring zone referred to TB in the badger population; an increase in TB in cattle occurred in the reactive cull areas were the trial was stopped both within and outside the cull area. In time the incidence of TB in cattle after the period included in the trial gradually declined and stayed low for a number of years, showing an overall positive and sustained effect of the cull and blunting the perturbation effect.
    The major objection raised by the panel involved in the trial was on the sustainability of a cull over a large area by the method employed (cage trapping and shooting) NOT on the effect on reducing TB incidence in cattle!
    Hence the new trial over a large area and allowing free shooting (cheaper)
    In reply to prof quatermass badgers excrete M bovis in huge numbers through all excreta (faces, urine, nasal discharge); this can contaminate cattle feed (they do enter farms food stores) or pasture; little evidence exists of how this happens in the real world!
    Truth is the scientific world is divided on this:
    prof Krebs of the original badger trial thinks there still is no robust evidence to go ahead and the current trial is a waste of money; most cattle vets and farmers see it as the only way to give respite to a battered industry; other vets, in particular those involved with wild animals (zoological vet society for instance) are against it.
    For the ordinary person, trying to formulate an unbiased view cutting through the enormity and complexity of the data is a mammoth task.

  12. lspall said,

    August 4, 2013 at 8:12 pm

    By the way a few refs:
    These are open access articles and 1 abstract
    Good reading!

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