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expert reaction to study on rates of cattle TB and the badger cull

Research, published in Scientific Reports, reports a reduction in bovine TB rates in areas surrounding badger culls.

 

Prof Rosie Woodroffe, Senior Research Fellow, Zoological Society of London (ZSL), said:

“The Randomised Badger Culling Trial, conducted between 1998 and 2006, showed clearly that killing badgers has the capacity to both reduce and increase the incidence of tuberculosis in cattle. It would not be surprising, therefore, for this subsequent (and necessarily less robust) study to find either increases or reductions, or both.

“This study represents a valiant attempt to quantify the impacts on cattle TB incidence of management strategies implemented in a small number of areas, for relatively short periods, using un-culled comparison areas which were gradually being eroded by the roll-out of culling across the countryside. Not mentioned in the paper is the policy of implementing improved cattle TB testing in areas which have received two or more years of badger culling1; it would have been helpful to comment on whether (or not) improved cattle controls might have confounded the impacts here attributed entirely to killing badgers.

“The statistical analyses are complex. Rather than fitting a single variable to describe the badger culling treatment (which would be conventional), the authors use a separate cull variable for each intervention area. The effects of these variables suggest falling cattle TB incidence in two areas, but potentially increasing cattle TB in a third. I can’t help wondering what the results would have shown if a single cull variable had been used. Likewise, the caution that should be exercised in interpreting analyses based on small sample sizes is illustrated by the fact that, while this paper reports on data collected up to autumn 2017, more recent data1 up to autumn 2018 suggest a recent upswing in cattle TB in the area which here appears to show the strongest reduction. Repeating the same analysis on more recent data would therefore be expected to show weaker impacts of culling.

“The statistical models include a string of covariates, some of which, to my knowledge, have no prior link to cattle TB, such as flood risk and the presence of motorways, and several of which are included in some models but not others. A previous paper2 showed that estimates of culling impact were sensitive to the inclusion or exclusion of some of these covariates, but no similar sensitivity analysis seems to be included in this paper.

“In contrast with the findings of the Randomised Badger Culling Trial, this study showed little or no evidence of increased cattle TB on land adjoining culling areas. While data are extremely limited, it is interesting to speculate on why such a difference might occur. One possibility could be that reductions in badger density might have been smaller than those in the previous trial, leading to less of a “vacuum effect” drawing badgers into the cull zones from outside. Studies of badger populations would be needed to evaluate this speculative idea.

“Despite all these caveats, a reduction in cattle TB inside at least some of the culled areas would be expected, based on the findings of the Randomised Badger Culling Trial. The question remains as to whether culling is a cost-effective tool in the fight to eradicate bovine TB. This study suggests that culling is not working consistently across all TB-affected areas of England. Previous studies showed that culling did not lead to the sustained reductions in TB that would be required from an eradication tool, partly because it consistently increased TB prevalence in the badger population3, even as it reduced badger numbers. The authors are right, therefore, to highlight that “Culling badgers will not provide the entire solution to the cattle TB problem in Great Britain and the impact of the policy needs to be evaluated alongside other TB controls.” This statement echoes the conclusions of an independent review of TB policy4, which a year ago called for a side-by-side comparison of badger vaccination against culling, but which has thus far received no government response. Badger vaccination is cheaper, more humane, and less environmentally damaging than culling, and might also be a more effective tool in the long-term battle to eradicate TB5.”

1APHA. Bovine TB in cattle: badger control areas monitoring report for the period 2013-2018.  (2019);

2Brunton, LA et al. Assessing the effects of the first 2 years of industry-led badger culling in England on the incidence of bovine tuberculosis in cattle in 2013-2015. Ecology and Evolution 7, 7213-7230 (2017);

3Woodroffe, R et al. Culling and cattle controls influence tuberculosis risk for badgers. Proceedings of the National Academy of Sciences of the United States of America 103, 14713-14717 (2006);

4Godfray, HCJ et al. Bovine TB Strategy Review – Report to Rt Hon Michael Gove MP, Secretary of State, Defra.  (https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/756942/tb-review-final-report-corrected.pdf, 2018);

5Zoological Society of London. Eradicating TB from cattle and badgers – a review of evidence.  (https://www.zsl.org/sites/default/files/media/2018-09/ZSL_Eradicating_TB_Report_final_24Sep18.pdf, 2018).

 

Professor Lord John Krebs FRS FMedSci, Emeritus Professor of Zoology, University of Oxford, said:

“The latest analysis of the farmer led culls in Gloucestershire and Somerset suggests that large scale and long term killing of badgers can lead to a reduction in TB in cattle.  This is consistent with the results of the Randomised Badger Culling Trials (RBCT).  However, as the authors point out, the farmer led culls are not set up as a proper experiment and therefore one cannot be sure of the interpretation of the results. Furthermore, the authors report that in the RBCT under 6% of cattle infections came from badgers. All the recent scientific evidence points to a central role of cattle to cattle transmission, and focusing on killing badgers is probably a distraction from the main story”

 

Prof James Wood, Head of Department of Veterinary Medicine and Alborada Professor of Equine and Farm Animal Science, University of Cambridge, said:

“This is a robust scientific evaluation of the overall impact of farmer led badger culls in the first three areas that the policy was implemented in, comparing rates of disease in cull zones and their periphery with those in comparable other parts of the country. Importantly, the analysis takes into account known confounders although it cannot distinguish the impact of culling itself from the other measures that farmers may have implemented in cull zones. Overall, the incidence of cattle TB on farms was significantly and substantially lower in Gloucestershire and Somerset, but not significantly reduced in Dorset, in areas where badgers were culled by farmers. Interestingly, there was no increase in rates of cattle disease in zones around the cull zones, but rather a significant decrease in Gloucestershire and Dorset. This paper thus describes a positive impact of the overall policy, with evidence that the increased movement reported elsewhere in surviving badgers is not significantly increasing the disease in cattle. The variation in impact between cull zones is not surprising given the results of the earlier randomised controlled badger trial.”

 

Assessing effects from four years of industry-led badger culling in England on the incidence of bovine tuberculosis in cattle, 2013–2017’ by Downs et al. was published in Scientific Reports at 10:00 UK time on Friday 11th October. 

DOI: 10.1038/s41598-019-49957-6

 

Declared interests

Dr Rosie Woodroffe: “I have previously received research funding from Defra, and receive in-kind support in the form of badger vaccine for my ongoing research.”

Prof James Wood: “I am directly involved in Defra policy, as an independent scientist member on their TB Eradication Advisory Group, also as a  member of Defra Science Advisory Council. My work is on infectious diseases of animals and I have been involved in research on bovine TB in the UK and Ethiopia for more than 10 years, including receiving UK government (UKRI, Defra and DfID) funding, as well as support from the Gates Foundation.” 

None other received. 

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