Memorandum submitted by the Independent
Scientific Group on Cattle TB (ISG) (BTB 15)
SUMMARY
1. Over the next 12-15 months the ISG will
be working towards finalising its scientific advice to Ministers
to inform the development of an effective and sustainable policy
to control TB in cattle.
2. We have already reported that localised
reactive culling of badgers in response to TB being confirmed
in a cattle herd was associated with an estimated 25% increase
(95% confidence interval, CI: 2.6 to 52% increase) in the incidence
of confirmed TB herd breakdowns, and concluded that reactive culling
as performed in the Randomised Badger Culling Trial cannot contribute
constructively to the control of bovine TB in Britain. We have
also reported that a strategy of proactive culling of badgers
indicates an incidence of confirmed TB herd breakdowns 19% lower
(95% CI: 6.2 to 30% lower) in the proactive culling areas than
in trial areas where no culling took place, but that analyses
also revealed that the incidence of confirmed TB herd breakdowns
was 29% higher (95% CI: 5.0 to 58% higher) on land neighbouring
proactive culling areas, relative to land neighbouring trial areas
where no culling took place.
3. Despite this evidence having been presented
to Ministers in advance we are concerned to see that the badger
culling consultation then proposed two strategies that scientific
findings show will increase rather than decrease cattle herd breakdowns.
Scientific evidence suggests that the third strategy option in
the consultation—a general cull—could in principle
reduce herd breakdown rates, but only if systematic and prolonged
culling could be achieved, and over a larger area than that proposed
("at least 100 km2"). But this, in itself, presents
serious implementation difficulties, and its cost effectiveness
must be seriously considered.
INTRODUCTION
4. The ISG is a group of independent scientists
who advise Ministers on how best to tackle the problem of cattle
TB. Set up in 1998 following the acceptance by Ministers of the
recommendations contained in the Krebs Report (1997), the role
of the ISG is to provide the scientific evidence base for an effective
policy to control TB in cattle.
5. The ISG's Terms of Reference are to advise
Ministers on the implementation of the Krebs Report on bovine
TB in cattle and badgers by:
(a) overseeing the design and analysis of
the randomised trial to test the effectiveness of badger culling
as a means of controlling bovine TB;
(b) regularly monitoring the progress of,
and outputs from, the trial and assessing any important differences
in results between the treatments;
(c) monitoring data on the Mycobacterium
bovis situation in areas and species outside the trial;
(d) reporting to Ministers on progress; and,
(e) advising, as requested, on related issues.
THE RANDOMISED
BADGER CULLING
TRIAL
6. The Randomised Badger Culling Trial (RBCT),
one component of the research effort put in place by Defra on
the advice of the ISG, and which we have directed since 1998,
is now nearing completion: the badger culling programme ended
in 2005 and the final trial surveys are in progress.
7. The RBCT was designed to test the effect
on the incidence of cattle TB of two different approaches to badger
culling, each of which represented a possible policy option, as
well as to provide a range of other epidemiological information.
8. The RBCT investigates 10 matched triplets
each consisting of three trial areas of approximately 100 square
kilometres. The 30 trial areas are located in areas of the highest
TB incidence in cattle in England. Within each triplet, trial
areas were randomly allocated to one of three experimental treatments:
proactive culling; localised reactive culling in response to TB
being confirmed in a cattle herd; or no badger culling (ie survey-only,
this being the scientific control against which the findings of
the other two treatments are measured). A calculation made when
the RBCT was planned showed that the data from 50 triplet-years
(eg 10 triplets over a five-year period) should provide sufficient
precision to detect a reduction in TB incidence as small as 20%.
9. We have presented four annual reports
and a number of progress reports to Ministers during the course
of our study and plan for a final report, reviewing the trial
and other related research, to be ready in 2007. This will conclude
our work.
THE SCIENCE
10. From the outset the ISG has adopted
a holistic approach, recognising that sustainable control policies
could only be achieved through a better understanding of the epidemiology
of TB in cattle and wildlife reservoirs. Implicit in our approach
was the requirement to consider badger welfare issues related
to the trapping and killing of badgers and that the widespread
elimination of badgers from large tracts of the countryside would
not be politically or socially acceptable. Hence we have sought
to explore a much wider consideration of the problem and possible
solution(s).
11. Statistical analyses of the effect of
culling treatments have been undertaken at intervals since 2000
and the findings reported to the independent statistical auditor.
Analyses of data on herd breakdowns up to 31 August 2003 revealed
that reactive badger culling was associated with an estimated
increase of 27% (95% CI: 2.4% decrease to 65% increase) in the
incidence of confirmed herd breakdowns. This was the first time
that any clear indications with potential implications for policy
had emerged from the RBCT and, subsequently, Ministers decided
that the reactive element of the trial should be suspended with
effect from November 2003.
12. In October 2005 we published a paper
"The impact of localised reactive badger culling versus no
culling on TB incidence in British cattle: a randomised trial".[1]
The paper extended and supported the findings of our paper entitled
"Impact of localized badger culling on tuberculosis incidence
in British cattle"[2]
published in Nature in December 2003. Our 2005 paper reported
that localised reactive badger culling was associated with an
estimated 25% increase (95% CI: 2.6 to 52% increase) in the number
of cattle herds disclosing TB. This lent further support to the
view that localised reactive culling, as carried out in the RBCT,
could not contribute constructively to the control of bovine TB,
and would make matters worse.
13. In order to meet the timetable of the
2005 Autumn Ministerial announcement the findings of the proactive
element of the RBCT, based on data available up to 4 September
2005, were released earlier than we had planned. However, since
data for only another five of the planned 50 triplet-years remained
to be collected, the results were near final and certainly not
considered to be either provisional or uncertain. Details of our
correspondence and advice to Ministers and published papers in
the period 5 September to 14 December 2005 can be found in the
"Latest news" section of the ISG web site, under the
"15 December 2005" heading. [3]
14. The findings reported to Ministers were
subjected to stringent peer review and published in Nature
in December 2005 to coincide with the Autumn Announcement. That
paper, "Positive and negative effects of widespread badger
culling on tuberculosis in cattle"[4]
reported on the proactive element of the RBCT. The paper concluded
that the incidence of confirmed TB herd breakdowns has been 19%
lower (95% CI: 6.2 to 30% lower) in proactive trial areas than
in survey-only trial areas. Analyses also revealed that the incidence
of confirmed TB herd breakdowns was 29% higher (95% CI: 5.0 to
58% higher) on land neighbouring proactive areas, relative to
land neighbouring survey-only trial areas.
15. A supporting publication "Effects
of culling on badger Meles meles spatial organization:
implications for the control of bovine tuberculosis",[5]
published simultaneously in the Journal of Applied Ecology,
demonstrated that culling badgers profoundly alters their spatial
organisation as well as their population density, with badger
home ranges found to be consistently larger in culling areas.
These changes have the potential to influence contact rates between
cattle and badgers, both where culls occur and on adjoining land.
These results may help to explain why localised badger culling
appears to have failed to control cattle TB, and should be taken
into account in determining what role, if any, badger culling
should play in future control strategies.
16. The wealth of other data, such as on
pathology and infection prevalence in badgers, and spatio-temporal
data, have not yet been fully analysed but these are expected
to shed further light on our understanding of disease dynamics.
Further analysis of cattle breakdown incidence and other data
from the RBCT will continue throughout 2006 and early 2007, during
which time we will submit reports to Ministers and prepare reports
for publication in peer-reviewed scientific journals. We do not
expect any major change in the outcome with respect to the decreased
incidence inside culled areas (about 20% lower) and the increased
incidence in neighbouring areas (the so-called "edge effect")
(about 30% greater) although there may be sharpening of confidence
intervals.
17. The availability of new data (up to
early December 2005) recently allowed us to update our analyses.
These indicate a significant 20% reduction (95% CI: 8.5 to 31%
lower) in cattle TB incidence within proactive trial areas, but
a 30% increase (95% CI: 5.2 to 67% higher) in cattle TB incidence
in areas neighbouring the proactive trial areas.
THE CONSULTATION
18. We have read with interest the consultation
paper "Controlling the Spread of Bovine Tuberculosis in Cattle
in High Incidence Areas in England: Badger Culling". It contains
valuable suggestions, but it is inaccurate in important respects.
In view of this the ISG was obliged to write to the Parliamentary
Under-Secretary (Commons) on 11 January 2006, in advance of a
more detailed submission that the ISG will make later.
19. The comments in paragraphs 21 and 22
below closely reflect those contained in our 11 January letter
to the Minister, and those in a letter we sent to a number of
stakeholders and interested parties on 20 and 23 January, and
in an open letter that we included in the delegates pack for our
25 January 2006 Open Meeting.
20. In particular there is solid scientific
evidence that two of the culling strategies proposed will increase
rather than decrease cattle breakdown rates, yet the consultation
paper fails to make this finding clear. The remaining strategy,
intensive culling over a large area and over an extended time
period, could in principle reduce cattle breakdown rates, although
clearly there are substantial logistical and other difficulties
in implementing such a strategy.
21. At this stage the ISG is not in a position
to offer specific comments on the questions asked in the consultation
paper. Nevertheless, we would wish to make the following initial
observations in respect of the scientific findings highlighted
in the consultation paper, and have already made the same points
to Ministers:
(i) The findings of the analysis of proactive
culling, and associated research, based on data available up to
4 September 2005, were released earlier than initially planned
to meet the timetable for the December 2005 announcement. However
only another five of the originally planned 50 triplet-years remained
to be included and while some sharpening of the conclusions may
be possible it is very unlikely that any major change will occur.
The conclusions we reported should not be regarded as provisional
or uncertain. The scientific findings were subjected to stringent
peer review and published in highly respected scientific journals,
Nature and the Journal of Applied Ecology. These
findings clearly demonstrate that two of the proposed culling
options on which stakeholder opinion is sought, licensed and targeting
culling, would seriously worsen the situation;
(ii) All of the culling strategies proposed
would suffer from the edge effects described in the scientific
papers and these would be maximal in localised culling, as envisaged
in the option proposed for licensed and targeted culling. There
are no scientific data to support suggestions in the consultation
paper that targeted culling with coordinated farming groups would
reduce the risk of perturbation or that culling efficiency would
decrease the edge effect in non-culled adjoining areas. The suggestion
in the consultation paper that the edge effect may be due to differences
in cattle management in the different treatment areas is without
foundation; and,
(iii) The general cull strategy does have
supporting scientific evidence if culling were to be conducted
systematically over very large areas and, on a prolonged time
scale. However the suggestion that 100 sq km is a big enough area
is at variance with the scientific findings, which indicate a
negligible impact overall when operated on this scale. It is true
that the relative impact of the deleterious edge effect in un-culled
adjoining areas would decrease as the size of the culled area
increased. Systematic and prolonged culling extending to areas
of 300 sq km or more could be expected to have an overall positive
impact on cattle herd breakdown rates, if adequately resourced
and coordinated to ensure high coverage, though the benefits may
not exceed the costs. An informed cost benefit analysis is necessary.
CONCLUSION
22. The ISG has worked closely with Defra
over the past seven years to provide what is now a solid base
of scientific evidence for Ministers. Our findings provide an
explanation for the failure of past TB control policies and contribute
to a science base that is now robust enough, for the first time,
to inform future cattle TB control policy. This evidence clearly
indicates that two of the proposed culling strategies cannot be
justified on scientific grounds.
23. Extrapolation from our results suggests
that the general cull strategy could in principle have a positive
impact, but presents serious implementation difficulties that
would need to be overcome if this approach is to make a direct
contribution to the control of cattle TB.
24. Further information on the ISG, our
reports and the scientific evidence we have made available can
be found on the ISG web pages at: http://defraweb/animalh/tb/isg/index.htm
February 2006
1 Andrea M. Le Fevre et al. Available at http://www.defra.gov.uk/animlh/tb/isg/pdf/lefevre1005.pdf Back
2
C A Donnelly et al, Nature 2003 426 834-837 doi:10.1038/nature02192 Back
3
http://www.defra.gov.uk/animalh/tb/isg/index.htm Back
4
Christl A Donnelly et al, Nature, 2005, doi:10.1038/nature04454 Back
5
R Woodroffe et al, Journal of Applied Ecology, 2005 doi:10.1111/j.1365-2664.2005.01144.x Back
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