March 18, 2022

expert reaction to a study looking at the impacts of badger culling and cattle controls on bovine tuberculosis in England

A study published in Vet Record looks at the impacts of badger culling and cattle controls on bovine tuberculosis in cattle in England.

Prof James Wood, Head of Department of Veterinary Medicine, University of Cambridge, said:

“The authors have made a substantial effort to analyse the data that are publicly available on the impact of ‘industry-led’ badger culling operations since 2013. They have analysed data on bovine TB breakdown incidents from unculled and culled areas (based on data published by Defra) and by comparing the same data from counties that have had different culling strategies. While it is essential that there is a detailed analysis of the effects of the culling on the rates of disease in cattle, any analysis should take into account known facts of the impacts of culling and also be carried out in a rigorous manner.

“A considerable challenge regarding an independent analysis of industry led culling is that the data on where culling has taken place is not publicly available.

“Data from the now 20 year old randomised badger culling trial showed that the impact of culling on cattle disease was not significant in the first 2 years but was significant in years 3 and 4 as well as for some time after the cull (e.g. see Godfray et al 2013 http://doi.org/10.1098/rspb.2013.1634). Data from previous analyses of industry culling (Downs et al 2019 https://doi.org/10.1038/s41598-019-49957-6) have also shown that culling in 2 of the first 3 areas was associated with a significant reduction in the rates of cattle disease, both in the rate in the culled areas and in the ‘3km buffer area’ around the culled zone. In their comparison, Langton et al combined data from all years of culling undertaken from 2013 to 2018, including some areas where culling had only been undertaken for one year. They also included data from the buffer areas in the unculled areas, even though this industry culling had been shown to have reduced the rates of disease in some of these. Thus, if there were any positive impact of culling, its apparent impact would have been diluted by using years where no impact was expected and comparing it to areas which included buffer areas, likely substantial in some years, where the rates would have also been expected to reduce due to the culling. Unfortunately, no statistical analysis in the world can fully account for starting biases in data.

“The authors rightly pick out the cyclical nature of regional bovine TB statistics, a feature that has never been fully explained, but evident over years in some government statistics for the disease; care needs to be taken in interpreting crude, uncontrolled trends in data.

“In their separate comparison of county level statistics, in which they compared ‘heavily’ with ‘lightly’ culled counties, the authors were comparing counties that had variable but sometimes substantial unculled areas in them, so this is not a direct evaluation of culling impact and so is not particularly insightful.

“The paper, taken as a whole, seems to have failed to do what it set out to do, namely to determine what the impact of the industry led badger culling is. Any conclusions therefore seem unjustified.

“A detailed analysis should be undertaken as a matter of some urgency, with direct controlled comparison over time of the rates of disease in culled and unculled areas of the country.”

Prof Rowland Kao, the Sir Timothy O’Shea Professor of Veterinary Epidemiology and Data Science, University of Edinburgh, said:

“The study compares year on year differences between culled and unculled areas. However, the area under culling increased substantially over the period of study. This means the bulk of the analysis compares primarily recently culled areas to unculled. As the impact of culling in Tb is slow this means it’s not surprising no effect was seen. It does not mean that there is no evidence that culling has an impact. It does highlight that bovine tb is a disease that does not easily fit into a one size fits all approach to control as any single policy requires patience.  As such it will require a multifaceted approach to control that balances evidence based wildlife measures with cattle controls.”

 ‘Analysis of the impacts of badger culling and cattle controls on bovine tuberculosis in cattle in the high-risk area of England, 2009–2020’ by Thomas E.S. Langton et al. was published in Vet Record at 14:00 UK time on Friday 18th March.

DOI: 10.1002/vetr.1384

Declared interests

Prof James Wood: “I have researched bovine TB for nearly 15 years and have, at different times, received funding for this work from Defra, UKRI, DfID and The Bill & Melinda Gates Foundation. I have sat on Defra’s TB Eradication Advisory Group from 2013-2021 and am now a member of Defra’s TB Partnership group as an independent scientist.” 

Prof Rowland Kao: “I am a member of Defra’s bovine tb partnership and science advisory council but am writing in a personal capacity.”