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


Annex

RESPONSE TO "TUBERCULOSIS IN CATTLE AND BADGERS: A REPORT BY THE CHIEF SCIENTIFIC ADVISER"

FJ Bourne, CA Donnelly, DR Cox, G Gettinby, JP McInerney, WI Morrison and R Woodroffe

  Former members, Independent Scientific Group on Cattle TB

SUMMARY

  ISG1  The Independent Scientific Group on Cattle TB (ISG) was an independent body charged with developing science-based policy options for the control of tuberculosis (TB) in cattle. As the ISG was dissolved in June 2007 following publication of its final report (Bourne et al, 2007), this response to the Chief Scientific Adviser's recent report (King et al, 2007) presents the views of the seven former members of the ISG.

  ISG2  A key conclusion of our work was that "badger culling can make no meaningful contribution to cattle TB control in Britain" (Bourne et al, 2007). This conclusion is based on the analysis and interpretation of data derived from nearly 10 years' scientific research by ourselves and our colleagues. Our major scientific findings, and their interpretation, have been published in top-quality peer-reviewed journals, and our conclusions are widely accepted within the scientific community. We are surprised, therefore, that King et al (2007) reached a different conclusion, namely that "the removal of badgers could make a significant contribution to the control of cattle TB in those areas of England where there is a high and persistent incidence of TB in cattle".

  ISG3  We believe that a key reason for these differing conclusions is that King et al (2007) were constrained within their terms of reference, which prevented them from fully evaluating policy options. While we aimed "to present Ministers with a range of scientifically-based policy options which will be technically, environmentally, socially and economically acceptable" (Bourne et al, 1998), King et al (2007) were "...asked to make comment on scientific issues..."; their "...brief did not extend to economic or other practical issues" (Environment, Food and Rural Affairs Committee, 2007). Unfortunately, the complex relationship between badger abundance and cattle TB risks, as revealed by our work, means that "economic [and] practical issues"—which determine how, where, when, and on what scale badger culling might be conducted—are absolutely critical in determining whether culling would reduce or increase the incidence of cattle TB. By excluding consideration of such issues from Sir David King's remit, Ministers severely hampered his ability to inform policy development.

  ISG4  In addition to this broad concern about King et al remit, we have identified a large number of scientific problems with their report, which have led them to draw conclusions from our work which are not consistent with the data available. In particular, King et al (2007) dismiss as "unsound" our finding that badger culling increases TB risks for cattle on neighbouring unculled land, yet their conclusion is undermined by (i) incorrect interpretation of statistical confidence intervals; (ii) exclusion of data accrued between the first and second proactive culls, even though this cannot be justified either by statistical bias or by the time taken for changes in the badger population to cause detectable effects in TB risks for cattle; and (iii) incomplete consideration of ecological data consistent with detrimental effects observed among cattle. In addition, misinterpretation of our mathematical modelling work is likely to have led King et al (2007) to under-estimate the likely benefits of improved cattle-based controls.

  ISG5  Given these concerns, we are not persuaded by the arguments in King et al (2007) report and stand by our published recommendations concerning the control of cattle TB in Britain.

INTRODUCTION

  ISG6  The Independent Scientific Group on Cattle TB (ISG) worked from its outset to build and interpret a science base to inform the control of tuberculosis (TB) in cattle. As members of the ISG, we analysed and interpreted data from the Randomised Badger Culling Trial (RBCT) and related studies. The ISG was dissolved in June 2007 following publication of its final report (Bourne et al, 2007). This response to the Chief Scientific Adviser's recent report (King et al, 2007) thus represents the views of the seven former members of the ISG.

  ISG7  Our primary findings, including their interpretation, were peer reviewed, both before publication in scientific journals, and by the Defra-appointed statistical auditor; comments were also sought from colleagues at Defra and its associated agencies, some of whom were co-authors on our papers. We have throughout our work encouraged informed debate and discussion, and continue to welcome further constructive comment and dialogue on any aspect of our work.

  ISG8  Like King et al (2007), we recognised from the start of our work that "the overriding aim is to control TB in cattle". Indeed, the opening sentences of our final report noted that "Bovine TB is a serious infectious disease of cattle. It has public health implications, has major economic consequences for Government and the farming industry, and causes distress to farmers and their families" (Bourne et al, 2007). Hence we firmly agree that "...strong action needs to be taken now to reverse the upward trend of this important disease" (King et al, 2007). Our recommendations, summarised in our final report (Bourne et al, 2007) on the basis of data published in a broad array of peer-reviewed papers, therefore represent our views of the best way to achieve control of cattle TB using methods currently available.

  ISG9  A key conclusion of our work was that "...while badgers are clearly a source of cattle TB, careful evaluation of our own and others' data indicates that badger culling can make no meaningful contribution to cattle TB control in Britain. Indeed, some policies under consideration are likely to make matters worse rather than better" (Bourne et al, 2007). We note that our broad conclusions regarding the role of badger culling are consistent with views expressed previously by experts on the ecology of TB in badgers such as Dr Chris Cheeseman,[7] Professor David Macdonald[8] and Professor Tim Roper,[9] as well as with recent statements by leading scientists charged with past independent reviews of the issue such as Lord Krebs.[10] However, our conclusion contrasts with King et al (2007) recommendation that "the removal of badgers could make a significant contribution to the control of cattle TB in those areas of England where there is a high and persistent incidence of TB in cattle" (King et al, 2007).

THE IMPORTANCE OF TERMS OF REFERENCE

  ISG10  At the start of our work, our stated aim was "to present Ministers with a range of scientifically-based policy options which will be technically, environmentally, socially and economically acceptable" and the RBCT was therefore designed as a trial of potential policy options (Bourne et al, 1998). Our approach contrasts with the remit of King et al. (2007), who were "...asked to make comment on scientific issues..."; their... brief did not extend to economic or other practical issues" (Environment, Food and Rural Affairs Committee, 2007). We consider this distinction critical, as we believe it partially explains the difference between our conclusions and those of King et al (2007).

  ISG11  We recognise three major reasons why the "economic [and] other practical issues" excluded from King et al (2007) terms of reference should be given detailed consideration in any scientific discussion of future TB control policy. (i) Although our findings suggest that, in principle, modest reductions in the overall incidence of cattle TB would result from simultaneous, coordinated and repeated culls of badgers over extremely large areas of the countryside, using skilled staff and ideally within geographical barriers to badger movement, trying and failing to achieve this is likely to make matters worse, increasing the incidence of disease in cattle and spreading infection to new areas. As discussed in our final report (Bourne et al, 2007), it is highly unlikely that coordinated culls could be conducted simultaneously and repeatedly across hundreds of square kilometres—especially since Defra's Wildlife Unit was dissolved—whereas our work shows that culling which is asynchronous, patchy, small scale or discontinuous is likely to increase rather than reduce the incidence and spatial spread of disease (Donnelly et al, 2007; Donnelly et al, 2003; Woodroffe et al, 2006b; Jenkins et al, 2007). (ii) There are too few natural geographical barriers to badger movement in TB-affected areas of Britain to contribute to national control strategies, while building and maintaining artificial barriers would be extremely costly and highly impractical on the scale at which TB control is needed (Bourne et al, 2007; Poole et al, 2002). (iii) Even if benefits were achieved by culling, our results indicate that these would be modest in comparison with the substantial financial costs of conducting the widespread culls that would be required. These three issues illustrate the critical importance of the "economic [and] other practical issues" excluded from Sir David King's remit. Our own consideration of these issues was based on systematic evaluation of ecological and epidemiological data derived from the RBCT and other studies, and our conclusions were reinforced by economic data. We feel that it would have been very difficult for Sir David and his team to reach meaningful policy recommendations without similarly detailed consideration of such issues, and it is therefore unfortunate that their terms of reference were so narrow.

INTERPRETATION OF SCIENTIFIC DATA, STATISTICAL ANALYSES, AND MODELLING RESULTS

  ISG12  In addition to this broad concern about the Chief Scientific Adviser's terms of reference, we wish to express six major concerns about the scientific basis of King et al (2007) report. These concern (i) incorrect use and interpretation of statistical confidence intervals in subgroup analysis; (ii) inappropriate exclusion of data accrued between the first and second proactive culls; (iii) failure to consider or cite the ecological data and analyses relevant to our conclusions; (iv) misunderstanding of our mathematical modelling work and its implications; (v) incorrect interpretation of our conclusions regarding temporal trends in the effects of culling; and (vi) over-reliance on assumptions concerning the effects of culling on transmission among badgers. These major issues are addressed below; a point-by-point response to King et al (2007) report is provided in an Appendix to this document. This report is also accompanied by copies of three scientific papers describing findings from the RBCT, which have been published in the peer-reviewed literature since publication of our final report (Jenkins et al, 2007; Pope et al, 2007b; Woodroffe et al, 2007). Key findings reported in these three papers were cited in our final report (Bourne et al, 2007), referring to the papers as "in press" (in the case of Woodroffe et al), "in review" (in the case of Jenkins et al) or citing a report to Defra (in the case of Pope et al).

(i)  Interpretation of statistical confidence intervals in subgroup analysis

  ISG13  King et al (2007) place great weight on interpretation of results presented in Figure 5.2B of our final report (Bourne et al, 2007). This figure shows the results of a subgroup analysis, stratifying overall RBCT results into beneficial and detrimental effects recorded at different distances from the boundaries of proactive trial areas. As is often the case in scientific studies, each effect was presented as a point estimate, associated with 95% confidence limits. The latter give a measure of the uncertainty associated with the point estimate; there will be substantial uncertainty when small sample size prevents a precise estimate from being obtained from a subset of the data.

  ISG14  In paragraph 43 of King et al (2007), the detrimental effect of badger culling on cattle TB incidence outside culling area boundaries is dismissed since "Three out of four [confidence intervals] go through zero (ie one cannot be confident that the overall effect is detrimental)". This interpretation is incorrect, as it omits one very crucial proviso in the interpretation of our subgroup analysis. The limits attached to any subgroup of data concern what can be learned from that subgroup of data on its own. While it is useful to know this, it is rarely the primary focus of analysis, and was not so in this case. Indeed if the data are broken into a large number of small subgroups, each on its own will have substantial uncertainty and be indecisive on any issue of concern, even if the overall picture is entirely clear on the point under study, as was the case here. The overall picture must be studied, not fragments of it. This is what we have done in all our analyses, subject to tests of the uniformity of the effect under study.

  ISG15  In the interests of consistency, we note that the same analysis showed that all five of the subgroup estimates of beneficial effects of culling inside proactive areas had confidence intervals which included zero. Thus, were the same (erroneous) interpretation to be placed on those findings, evidence of the beneficial effects of culling should likewise have been dismissed.

(ii)  Exclusion of the first year of data post-culling

  ISG16  In our published accounts of the effects of proactive culling on cattle TB incidence (Donnelly et al, 2007; Donnelly et al, 2006), we provided estimates of culling effects following completion of the first proactive cull, and also following the second cull; the latter analyses excluded roughly a year of data accrued in each triplet between the first and second culls. King et al (2007) chose, where possible, to present results from the second cull only, noting in paragraph 29 that our use of the full data set to assess detrimental impacts outside culling areas might "overestimate the effect". In fact, there is no such bias toward overestimation, nor do King et al (2007) put forward any case for such a bias. Since cattle in RBCT areas were tested annually but not simultaneously, some breakdowns detected in the first year would have originated from infections which occurred prior to culling, and some infections which occurred in the first year would not be detected as breakdowns until the second year. Contrary to King et al (2007) statement, this would make culling and no-culling areas appear more similar in the first year: the estimated effects on the incidence of detected breakdowns—whether beneficial or detrimental—would be smaller than the underlying effects on the incidence of new infections in this time period. This is important because the exclusion of this initial time period, which cannot be justified on the basis of any bias toward overestimation, appears to have contributed to King et al (2007) tendency to downplay the importance of detrimental effects.

  ISG17  Paragraphs A16 and A41-A44 in the Appendix give more in-depth interpretation of results from different time periods.

(iii)  Failure to consider ecological data

  ISG18  King et al (2007) dismiss detrimental effects of badger culling on cattle TB as "hard to interpret" and "unsound", noting that they were "not fully persuaded by" our explanation for these effects. In this context, it is unfortunate that King et al (2007) appear to have considered only a part of the wealth of ecological data that we have published on the impacts of culling on badger abundance, distribution, ranging behaviour and infection status, citing none of our primary papers on these issues. This work shows that badger culling prompted immigration into culled areas (Woodroffe et al, 2007) as well as disruption of badger territories and expanded ranging (Pope et al, 2007b; Woodroffe et al, 2006a). These ecological changes were associated with reduced clustering of infection in both badgers and cattle (Jenkins et al, 2007), and also with elevated prevalence of M bovis infection in badgers in both proactive and reactive culling areas (Bourne et al, 2007; Woodroffe et al, 2006b). These ecological findings are consistent with, and hence support, our observation of elevated cattle TB incidence on land neighbouring proactive culling areas, and in reactive areas.

  ISG19  King et al (2007) failure to give full consideration to the ecological data is important as it may have given them an incomplete or biased picture of badger ecology and M bovis epidemiology, leading to inappropriate conclusions and recommendations. For example, the recommendation to "reduc[e] the migration of badgers into the removal area by... soft boundaries (such as arable land with no cattle)" is unsupported by any ecological data and highly unlikely to be effective.

(iv)  Conclusions of our mathematical modelling work

  ISG20  King et al (2007) discuss the conclusions of our mathematical modelling work (Cox et al, 2005), on which we based our conclusion that "the rising incidence of disease can be reversed, and geographical spread contained, by the rigid application of cattle-based control measures alone" (Bourne et al, 2007). We used this model essentially to estimate how close the epidemic is to criticality and hence to infer the likely consequences of policy action, including controls aimed at cattle. Of course all such modelling is based on highly idealised assumptions but it, combined with careful study of all the relevant data that have been collected, is the only rational basis for assessing the likely consequences of policy actions not yet undertaken.

  ISG21  Unfortunately, King et al (2007) have misinterpreted this work. Cox et al (2005) present an explicitly two-species model and the R0 estimates obtained do not, as stated in paragraph 11 of King et al (2007), refer to cattle-to-cattle transmission alone. Rather, these estimates are for cattle within the badger-cattle disease system, and thus represent contributions from both cattle-to-cattle and badger-to-cattle transmission. Indeed, equations 13 and 14 in Cox et al (2005) present analytical solutions based on two different assumptions regarding disease levels in the badgers, the first that the badger disease levels stay constant and the second that they follow the increasing pattern observed in cattle. These equations give indistinguishable fits to the data, and so the true situation cannot be inferred. The important issue for the conclusions presented in Cox et al (2005) is that the estimates of R0 are essentially identical. We agree with King et al (2007) that the levels of badger-to-cattle and cattle-to-cattle transmission are likely to vary considerably throughout the UK. However, if the incidence of cattle TB in low incidence areas is driven largely by the epidemic in the South-West of England then the growth rate, and thus the R0 estimates, for both areas will be similar.

  ISG22  We are concerned that the failure by King et al (2007) to fully appreciate the structure of this important modelling work, and the estimates obtained from it, will have led them to under-estimate the potential for reducing the incidence of cattle TB in Britain using cattle-based measures alone.

(v)  Conclusions regarding temporal trends in the effects of culling

  ISG23  In paragraph 37 of their report, King et al (2007) raise the possibility that detrimental effects of badger culling on badger ranging and disease transmission might be "transient", indicating that we did not consider this possibility. This is incorrect; several of our papers present and discuss evidence for temporal trends in the effects of culling. This issue is discussed in detail in paragraphs A28-A30 below.

  ISG24  King et al (2007) restrict their discussion to the transience (or otherwise) of disruption to badger spatial organisation; as detailed in paragraph A30 below, there is evidence to suggest that such ecological effects were sustained in proactive culling areas. However, as discussed in Donnelly et al (2007), the possibility remains that detrimental effects on cattle TB could be transient—or at least of smaller magnitude following later culls when sustained reductions in badger densities have been achieved—despite continued disruption of badger ecology. We believe that the likely outcome of future TB control options involving badger management can be reliably predicted only through separate, but parallel, consideration of effects on cattle and badgers.

(vi)  Capacity to reduce badger-to-badger transmission by culling

  ISG25  King et al (2007) note that "the likelihood of uninfected badgers being exposed to infectious badgers will... be reduced [by culling]" (paragraph 38). While such an effect would be expected in the randomly-mixing populations assumed in simple epidemiological models, more complex effects can occur in socially structured populations (Keeling and Eames, 2005). In fact, published data suggest that the substantial reductions in badger density which were achieved by proactive culling (Woodroffe et al, 2007) increased badger-to-badger transmission of infection rather than reducing it (Bourne et al, 2007; Woodroffe et al, 2006b). The likely reasons for this pattern are detailed in paragraph A31 below.

  ISG26  Although suppression of badger densities to extremely low levels (substantially lower than those achieved in the RBCT) would be expected to reduce badger-to-badger transmission of infection, no data are available on the densities at which this might be achieved in TB-affected areas of Britain. However, evidence for widespread cattle-to-badger transmission (Jenkins et al, 2007; Woodroffe et al, 2006b) suggests that even extremely low-density badger populations would rarely remain TB-free. Hence, data do not support King et al (2007) characterisation of culling as an "intervention... [to] reduce the prevalence of disease in... wildlife" (paragraph 5).

CONCLUSIONS

  ISG27  In summary, King et al (2007) appear to have given incomplete consideration both to the scientific data and to the wider issues critical for determining the likely outcomes of particular approaches to the management of bovine TB. In addition, we have major concerns about their interpretation of our scientific data. Thus, we are not persuaded by the arguments in their report and stand by our conclusion that "badger culling can make no meaningful contribution to cattle TB control in Britain" (Bourne et al, 2007).



7   Dr Chris Cheeseman, Oral evidence to Environment, Food & Rural Affairs Committee 2006 "I would venture to suggest now that I do not believe that any culling policy is sustainable in the long term." http://www.publications.parliament.uk/pa/cm200506/cmselect/cmenvfru/905/6020704.htm Back

8   Prof David Macdonald, Letter to The Guardian 2006 "The evidence is that a badger cull on a scale or level of efficiency that seems feasible will not solve cattle farmers' problem" http://www.guardian.co.uk/environment/2006/mar/10/guardianletters.conservationandendangeredspecies Back

9   Professor Tim Roper, Press release on behalf of the Mammal Society 2006 "While we understand the farming community's concerns, we believe the available evidence does not justify a policy of badger culling" http://www.abdn.ac.uk/mammal/badgercull-press.shtml Back

10   Lord Krebs, Discussions in House of Lords 2007 "We now know from reading the report of the Independent Scientific Group that culling is not a viable policy option. There is no wriggle room." http://www.publications.parliament.uk/pa/ld200607/ldhansrd/text/70726-0001.htm Back


 
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