Select Committee on Environment, Food and Rural Affairs Minutes of Evidence


Examination of Witnesses (Questions 440 - 459)

WEDNESDAY 24 OCTOBER 2007

PROFESSOR JOHN BOURNE CBE, PROFESSOR PROFESSOR CHRISTL DONNELLY AND PROFESSOR ROSIE WOODROFFE

  Q440  Chairman: The message I am getting from that is that you adjudge, as somebody who is a real expert on the statistical modelling in the context of this work, that you had a very sound methodology and that you firmly rebut the line of criticism which Sir David King has identified.

  Professor Donnelly: In terms of the unsoundness or weakness in the external effect, yes.

  Q441  Chairman: Would you agree, just looking at it as a layman, that it is possible for somebody looking at your data to draw the kind of alternative conclusions which Sir David King has done, because the thing that I struggle with in this is that we are not dealing with absolutes. There is not, for example, an absolute measure of temperature that there is in certain scientific experiments. What you are dealing with is your interpretation of some field data, so is it unfair for anybody to come to a different conclusion than you did?

  Professor Donnelly: I think it is unfair for them to come to a different conclusion on the basis of counting how many of these confidence intervals include zero. There are always going to be uncertainties in terms of epidemiology. When you find an increased heart attack risk associated with some risk you cannot identify usually which individuals had the extra heart attacks and it is similar here in that you cannot identify which of the herd breakdowns are the extra ones that would have become breakdowns had we not done culling or got the extra breakdowns, so there is that difficulty. However, the consistency between the strength of the statistical results is then strengthened considerably by the analysis of ecological data, which is what I think Rosie can speak to more clearly.

  Chairman: I just want to stop you there because there are going to be limits to what I and my colleagues can absorb for this and I want to give them a chance.

  Q442  Mr Williams: The article in one of the veterinary academic journals by some Irish authors, I think, was critical of your report on the basis that the perturbation effects could not possibly have taken place in such a short time as you have indicated on the basis that the time the badgers take to move, the time that they need to infect other badgers and for those badgers to infect other cattle and for that infection to be evidenced in tests would have been a very long time and much longer than you claimed in your work. That was not something that was actually raised by the Government Chief Scientific Adviser.

  Professor Woodroffe: We are in the process of responding to that paper and I am very happy that you have raised it. There are two separate issues here. One is saying, was there a detrimental effect, and I think that the statistical evidence that there was a detrimental effect is very strong. What they were criticising in that case was our interpretation of the mechanism leading to that detrimental effect. We had proposed in brief that when you cull the badgers that disrupts their territorial organisation, they mix around with one another, range more widely, meet more cattle herds, and if they are infected they have got the opportunity to infect more cattle. The contention was that we could not possibly see such a rapid effect because that chain of events could not take place fast enough.

  Q443  Mr Williams: Was that in The Veterinary Record?

  Professor Woodroffe: That was in The Veterinary Record, precisely. This is an issue that has been raised in the past, and in fact when we published these detrimental effects around proactive culling areas in Nature in 2005 we addressed exactly this issue in the supplementary information. I sympathise with Sir David King and his committee at having to read our fat report, and also not just all of our scientific papers but also many pages of supplementary information that were provided, and perhaps the need to read such a large volume of information might explain some of the deficiencies that we have seen in their report. The series of events is, you cull the badgers. How quickly do they start ranging more widely? Essentially, in ecological terms, immediately; it happens within a few days. We know that. In fact, Professor Tim Roper was one of the advisers to Sir David King, he was the person that did that work, so we know very confidently that badgers ecologically respond rapidly. Then they are presumably contacting cattle and that is the sort of area where you are not quite sure how often they are contacting cattle but, after being exposed to M. bovis, evidence from experimental infections carried out, I believe, at the Institute for Animal Health suggests that responsiveness to the tuberculin test takes about three weeks to develop. Therefore, in theory, within a couple of months you should be able to detect an elevated number of infections, so we have contested very strongly, and are continuing to do that, the idea that these results are implausible. They are perfectly plausible based upon the evidence that is available to us.

  Professor Bourne: We deal with this in the report. The suggestion is that taking experimental data from cattle with respect to the timing of development of a tuberculin reaction is not the thing to do; we should use field material. Well, of course, you cannot use field material because you do not know when these animals have been infected. I suppose the nearest we can come to this is an experiment that was done at CVL over the last 12 months when they took a group of tuberculin-positive cattle and housed these with sentinel cattle, and within seven months 50% of the sentinel cattle were infected.

  Q444  Chairman: But I can equally quote you an example that we heard when we went to Devon last week of 29 infected calves within a closed herd being put in with other cattle and there was no cattle-to-cattle transmission.

  Professor Bourne: I can also quote you a situation in Australia where infection in cattle took seven years to be detected by the tuberculin test. That probably tells me more about the tuberculin test than anything else. We can use only experimental data, and that is experimental data we have, which are consistent with this finding.

  Q445  David Taylor: Professor Donnelly, I saw on page 99 of the report and the bottom half of the portrayed data that it says the distance from the trial area boundary is four to five kilometres in a 100 square kilometre area. In a 300 square kilometre area you cannot get much more than eight to nine kilometres, I would calculate, from that boundary. Is it an incorrect inference to think that that graph might extrapolate down slightly as you get further and further away from a boundary and that the beneficial effect would get significantly closer to 100%? It stops at 50% in that bottom graph, does it not, at the five-kilometre point?

  Professor Donnelly: Yes, about that. If you are doing it linearly you have got a pretty good idea that you get ten in and then get 100%, which would not be plausible if we think that badgers are responsible for—

  Q446  David Taylor: No, exactly.

  Professor Donnelly:— less than 100%. While you could do that, as a statistician I would be extremely cautious about doing that. For one thing, the linear trend, even among the data that we have observed, is just on the borderline of significance, so it is not overwhelming even for the data that we have and then extrapolating. The difficulty is, of course, that that would be extrapolating a linear trend on that basis but what you really want to extrapolate is the ecology.

  Q447  David Taylor: So how has David King come up with 300 square kilometres as the magic area?

  Professor Donnelly: I think he is looking at the graphs that we did. We did two different scenarios.

  Professor Bourne: This is the graph on page 105.

  Professor Donnelly: You have the full report there. It shows two scenarios, the bottom of which assumes a constant benefit across the whole culled area, no matter how big it is, and then counters that with a detrimental effect outside. The one above, A, assumes that you have this trend with distance from the boundary, so the lower one is more conservative but the reality is that it is difficult to say beyond four to five kilometres, which is what we are able to observe.

  Q448  Sir Peter Soulsby: I want to take you back to your very helpful explanation of the data and what it meant and take you particularly to what has already been mentioned as to what Sir David King said about perturbation. He said—let me see if I can get this right—that there is a reasonable possibility that the disruption is transient, and then he takes things on from there and develops that quite significantly. Is he right? Is there a reasonable possibility from what you said to us and what your studies show that as a disruption the perturbation is merely transient?

  Professor Donnelly: It is quite possible. I will read you the sentence of the results. Having looked at this detrimental effect outside, it looks, like a linear would be, quite reasonable; it wobbles around a bit but it has a general linear trend down. The linear trend suggested a 7.3% decrease in the detrimental proactive effect with each cull, but the P value for that is 0.17. Statisticians for significance produce 0.05, so it is a suggestion of the transients, but it is far from convincing in and of itself. That is when you would also look toward the other ecological data.

  Professor Woodroffe: Yes. If I can just come in on that, it was quite frustrating, reading this report, where they say that the possibility that this effect was transient was something which the ISG ignored, when we had made repeated mention of it, and that paper that Professor Donnelly has just circulated actually had quite a detailed ecological explanation for why you might expect to see these effects diminish over time. I can go into the ecological explanation if you like but it is a little intricate. Suffice it to say that there are suggestions of reasons for thinking that you might see such a reduction over time, but, as Professor Donnelly says, it is not a statistically significant effect. One issue that I did want to raise is that later on in their report, buried somewhere deep inside, there is discussion of reopening the door on localised reactive culling. We have already discussed this issue of whether the first year is irrelevant or not. Sir David King's group admitted claiming that we could not really interpret anything from the first year of data, and that happens to be the year when you see these particularly severe detrimental effects, and ecologically that is exactly what you would expect. That is the period when you have caused the massive disruption in badger society. As Christl has argued, if anything, that might even be an underestimate of how severe the detrimental effects are because it has to some extent been diluted by what happened the year before, before you started culling. If you are going to do culling over many years, for example, over the first few years of the study—and Christl will correct me on the numbers—that initial detrimental effect is so severe that you get a statistically significant effect even over two, three, four years. It is only when you start going for much longer periods that it begins to make the overall effect non-significant. If you were, say, going to do 15 years of culling, you could begin to say, okay, this is a cost you have to pay early on, but when it is diluted over many years perhaps you can discount it or think it is going to get diluted. If you go back and think about something more like reactive culling, something more like licensed culling to individual farmers, which is likely to be small-scale and, critically, was always conducted as one-off culls, all you are ever having is the first year when you have these very severe effects, so it is very important to bear in mind that, even if you think there is an argument for discounting the first year when you consider proactive culling, which I would contest, at least on the sorts of timescales we are talking about, it is absolutely critical for understanding what the effects of localised culling would be.

  Q449  Chairman: If you put it the other way round that reinforces one of your principal findings, that if it is going to work at all it has got to be big.

  Professor Woodroffe: Yes.

  Q450  Chairman: Big and continuous.

  Professor Woodroffe: Big and continuous. These detrimental effects in the first year are so severe that they overwhelm any of the benefits and the net impact is negative. That graph that you have seen is based upon assuming that you cull over five years. If you were to do the same thing for one or two years you would see an overwhelming detrimental effect more or less, almost however big an area you cull.

  Professor Bourne: But there are other criteria for culling and one is that it should be done simultaneously.

  Professor Woodroffe: Yes.

  Q451  Mr Cox: It should be co-ordinated.

  Professor Bourne: Yes, absolutely. Also, Sir David says here that the removal process must be effectively done by competent operators.

  Chairman: I am going to draw matters to a conclusion. Two more colleagues have caught my eye.

  Q452  Lynne Jones: My question was really to follow up on the same thing. Sir David King only says that there is a reasonable possibility that the disruption is transient and that the detrimental effect appeared to reduce. In a way, that is true, is it not?

  Professor Woodroffe: Yes.

  Q453  Lynne Jones: And he acknowledges that the reduction in the detrimental effect is not statistically significant, so I cannot see anything wrong with what he is saying there. What I think is wrong is then to take that particular statement and say that as a result of that culling is a reasonable strategy.

  Professor Woodroffe: Absolutely. If I could just raise one issue that I think is very important and that I would like you all to think about, it is comparing what we have been saying with what Sir David King has been saying. In the report he says, and I quote early on in the report, I think at point four, "TB control will require interventions that reduce the prevalence of disease in both cattle and wildlife". That is what they are hoping to achieve, and certainly when Sir David King was sitting in this seat here and talking about this, he repeatedly talked about the need to eliminate TB and that unless you deal with the issue in badgers you are never going to eliminate TB. Unless you eradicate badgers you cannot eliminate TB in badgers by culling them. There is no current method. This paper, which was published in Proceedings of the National Academy of Sciences of the United States of America and which I will provide to you, is called Culling and cattle controls influence tuberculosis risk for badgers, and shows very clearly that culling increases the prevalence of disease in badgers, and it is very consistent and there is really very little debate about it. I think it is quite a fundamental misunderstanding that when you read this report you could come away with the idea that you can eliminate TB in the cattle/badger system by culling badgers. You cannot.

  Professor Bourne: "Elimination" is not a word we have ever used.

  Professor Woodroffe: Unless you eliminate badgers completely.

  Professor Bourne: No. Defra eliminated the word from their vocabulary two years ago.

  Q454  Chairman: But I do not think in either of the reports, whether you cull and on whatever basis, anybody has actually ever said you eliminate TB. In fairness to both sets of reports, that is not a conclusion you drew.

  Professor Woodroffe: Sir David King used it just this afternoon, and if you check the transcript you will see.

  Q455  Chairman: One has got to be fair on what he said because I read his report and that is not what he said.

  Professor Woodroffe: He said it this afternoon.

  Chairman: He may have said those words but that is not what is written in his report. The argument is about effectiveness within the scenarios of culling. I am pleased to see that the handout in this day and age of electronic education has not gone away. Your lectures must be full of people with bits of paper.

  Q456  Mr Drew: The ISG work was heavily criticised throughout for not taking out sufficient numbers of badgers to really be able to feel that there was a robustness in the way in which you were trying—and I apologise if I used the word "experiment"—to carry out the particular study that you were carrying out, and we got that last week in our trip to Devon where there were lots of accusations that you did not take out enough badgers regularly enough to get a real purchase on what you were trying to achieve. Is trapping an effective enough way to get the sort of removal that the King report is alluding to, or have you got to consider other measures?

  Professor Bourne: We were committed on welfare grounds to using cage traps, as you know, with the expectancy that we could achieve an 80% trapping efficiency, and that is consistent with the data we have, that we did trap around that percentage and we achieved something like 75% or a little less removal of badgers at the end of the day, taking into account badgers coming back into the piece. Much of the confusion with respect to culling activity in the trial resulted from figures that were presented in the consultation document which were erroneous. I think there has been an attempt by authors such as Chris Cheeseman and Graham Smith to put the record straight on that, using a model based upon first cull only, and we have hard data to support our evidence. It is described in this document as a guess. I think that is a silly statement because the data we have is the data that you would collect to give you an assessment of numbers of badgers on the ground and we have done it by using three or four criteria to give us that figure. With respect to other methods of trapping, in Ireland, as you know, they use snares. We were told we could not use snares and we did not use snares. With respect to gassing, just consider the Thornbury area where we do have experience. This was an area of 110 square kilometres with a much lower badger population than we are experiencing now, maybe a quarter of that we are experiencing now but certainly very much lower. It took them over five years to clear that area of badgers using cyanide gas, involving over 500 gassings, so I think to achieve elimination or near elimination is extremely difficult. To achieve the level of removal that we did in the trial is extremely demanding and requires skill from the operators doing it, given the amount of land that one does not have access to, and I have no reason to believe that that will change. Thirteen per cent of the inaccessible land was because one could not identify the owners, so it was illegal for us to get onto that land. To answer your question, there may well be better trapping methods but we do not have any data on that. We can only recall what has happened elsewhere.

  Professor Woodroffe: If I could just embellish what Professor Bourne has said, in chapter 10 of the final report, which I am sure you have got copies of, we thought in great detail about precisely this issue and an array of other issues. We thought of every way that we could to improve the effectiveness of badger culling in the broadest sense, or other badger management methods, and everything that has ever been suggested to us we went through and thought very carefully, based upon our own and others' evidence, would this work, would this make it better. Clearly, improving trapping efficiency was one of the first things to think about, and, as Professor Bourne says, we achieved about a 70% reduction in badger activity, and multiple different measures of badger activity triangulate and show the same sort of effectiveness. The other thing that that evidence showed was evidence of substantial badger immigration into the trial areas, and in fact there is a paper which has just come out today in Molecular Ecology showing genetic evidence of very widespread movement of badgers into these areas over substantial distances, and I am sure we will be very keen to share that recently published paper with you. As we culled these areas successively, so an increasing proportion of badgers would be getting caught on the boundaries, showing that there was immigration into these areas. What that means is that if, say, rather than removing 80% of the badgers by cage trapping, you used snaring and removed 90%—I am making these numbers up—that does not mean that you are going to have an equivalent reduction in the standing crop of badgers because every time you remove badgers from an area others are coming in from the outside, so if you remove a higher proportion of what was there it may be that you would just have even more coming in.

  Q457  Lynne Jones: What about boundaries?

  Professor Woodroffe: Precisely, which is why we then looked at boundaries. The sorts of boundaries which Sir David King and his team discussed in their report, motorways, rivers, coastline, came from us and we showed indirect evidence suggesting that what we called hard boundaries, which are pretty substantial boundaries to badger movement, do seem to have an effect because these increases in the prevalence of infection in badgers, which are described in that paper I just circulated, are most substantial where those boundaries are missing, so you do not see such a big increase where there are boundaries, suggesting that badgers immigrating in are contributing to this perturbation and therefore to the increased prevalence of infection. At the same time we did another analysis in which we looked at soft boundaries, other smaller water courses and other little boundaries, and found no effect from those. That was why we said ourselves in the final report that you have in front of you that perhaps culling badgers would be more effective if you could do it within these geographically circumscribed areas, which is what was done in Ireland. The four Irish areas were chosen to be these areas which were comparatively isolated by geographic boundaries to badger movement. They are not representative of the broader areas where TB is an issue, either in Ireland or in Britain, but if you look at a map of the south west area and you look at the distribution of motorways and large rivers, there are not that many.

  Q458  Mr Cox: I can think of one straightaway. There is a very large area between the north coast, the Tamar and the A30.

  Professor Woodroffe: The northern reaches of the Tamar—actually, that is not a very big river, and at the southern end of the Tamar is a big estuary. I am Cornish.

  Q459  Mr Cox: You know it as I do. I represent that area and I can tell you that I would not like to swim it. Have you done it?

  Professor Woodroffe: I have not swum the Tamar, no. There are very few areas which are geographically defined in that way, and if you are looking for a policy which is going to affect a large proportion of Britain's cattle farmers that is not a solution.


 
previous page contents next page

House of Commons home page Parliament home page House of Lords home page search page enquiries index

© Parliamentary copyright 2008
Prepared 27 February 2008