Letter: In debating any proposed cull, it must be recognised that the proportion of badger to cattle TB transmission events may be small - but important

https://www.newsletter.co.uk/news/environment/uspca-chief-warns-of-biggest-animal-welfare-tragedy-if-proposed-badger-cull-goes-ahead-4263568

Speaking as chair of the TB Eradication Partnership, I couldn’t agree more. It is therefore with sadness I must write in reply to a number of assertions made in her letter which are incorrect and misleading.

She states that recent research published by the Agri-Food and Biosciences Institute (AFBI) published in June, found that ‘bovine TB is 800 times more likely to be transmitted from cow-to-badger,’ and that ‘the transmission rate from badger-to-cow was negligible.’ No-where in that research paper is this stated. The 800 figure is a profound misreading of the research. It appears that Ms Smith has mistaken the comparative number of genetic changes in the TB organism isolated from different lineages of the organism as it moves between and within species, known as transitions, and assumed this reflects the degree of transmission within and between cattle and badgers. It simply does not.

In fact, the authors are quite clear about their estimates of the proportion of between species transmission, stating that this was probably in the range of 3-10% of all transmission events in the study area. Much larger than what Ms Smith states. These estimates by AFBI make a great deal of biological sense. One would expect that most transmission would happen between those animals most closely associated with each other, for example within a herd of cattle. However, this doesn’t answer the key question which is whether the estimated proportion of between species transmission events is important? In certain circumstances it is. Not every transmission event carries the same importance. In any new outbreak of an infectious disease, the most important step is often the first, if it then leads to a cascade of further infection transmissions later. For example, if one badger, or indeed one bovine, were to introduce infection into a herd by infecting one cow and that cow then led to a cascading network of infections within the herd, then the total number of transmission events would be dominated by cattle to cattle within the herd. However, crucially, if the first introduction of infection had not happened all of the consequential infections would not have happened either. In other words, while the total proportion of badger to cattle transmission events may be small this does not imply that badgers are not playing an important role.

This is consistent with previous studies in England where it was estimated that even where the direct badger-to-cattle contribution was estimated at 5.7%, this could amplify to ~52 % of all transmission events – if the 5.7% was prevented from happening it could lead to an overall halving of all transmissions. These are considerations that the authors of the AFBI paper clearly acknowledge when they say, ‘Our data were consistent with badgers playing a smaller role in transmission of M. bovis infection in this study site, compared to cattle. We hypothesize, however, that this minor role may still be important for [infection] persistence.’

Ms Smith states that similar badger culls in England and the Republic of Ireland have not had their ‘intended effect.’ Once again, the evidence contradicts this assertion. In the Republic of Ireland, the most recent herd incidence of TB is 4.6%, compared to NI where it is currently 10.8%. TB programmes in both jurisdictions comply with EU legislative requirements with the single largest difference being a badger intervention in RoI.

That badgers play a role in bovine TB in not disputed in the scientific literature from RoI. For example, a scientific review paper states, ‘There is little doubt that badgers are a maintenance host with spillback to cattle—essentially, an upstream driver of infection. Substantial supporting evidence is now available.’ In England the Chief Scientific Advisor to DEFRA as well as the Chief Veterinary Officer for England in reviewing English badger culls identified ‘a clear reduction in TB cattle breakdowns, relative to unculled areas, in culled areas from cull year 2 onwards.’

Further to this Ms Smith states that, ‘In Wales, the levels of TB are reducing at a similar rate to that in England and they have not culled a single badger.’ Once again this is not true. It is crucial that medium to long term trends in infection rates are assessed to look for any changes that might be taking place – rather than cherry picking data points that support a particular narrative. For example, the herd prevalence of TB in Wales in 2017 was 5.5% and in 2022 was 5.3% (5.4% in qtr 1 2023). In England in 2017 it was 6.2% and in 2022 was 4.5% (4.3% in qtr 1 of 2023).

The culling of badgers is of course emotive. Ms Smith decries a cull as being by definition cruel. She asserts that, ‘many badgers will not die right away and could suffer a slow, painful death.’ If this were to happen it would be unacceptable. However, the reality of the culls in England is that only shooters that are trained and monitored are permitted to operate. They are required to have suitable equipment such as night vision and demonstrate that they can work safely and accurately such that badgers are humanely culled. Anyone who does not meet these requirements does not get licensed to operate.

Underpinning much of what Ms Smith discusses is the error of extrapolating the findings of research in one area to other areas. The authors of the AFBI paper make this important point when they state, ‘Comparison to other areas suggests that M. bovis [the cause of bovine TB] transmission dynamics are likely to be context dependent, with the role of wildlife being difficult to generalize.’ In other words, the things that drive bovine TB infection are very likely to vary from area to area and region to region. It is simply inappropriate to assume the drivers of infection transmission will be the same everywhere one looks. This is illustrated in studies in the Woodchester Park area in England where badger to cattle transmission rates were found to be much higher than cattle to cattle rates. It would be wrong to extrapolate from this study to say this is true everywhere, just as it is wrong to extrapolate from the study area AFBI conducted their research on to every other area where there is bovine TB.

Critical to any strategy to eradicate TB is a need to develop bespoke approaches that address the drivers of infection in those areas. The scientific literature is large and very clear on this, the three drivers of ongoing bovine TB within UK and Ireland are cattle to cattle (for example through the movement of cattle), badgers to cattle and cattle to badgers. The balance between these will vary across landscapes and farm types. In order for a comprehensive approach to the control and ultimately the eradication of this infection from cattle and badgers will require the addressing of all the risks specific to those areas. To only address one of them will inevitably lead to the ongoing presence of this infection across the province with enormous consequential costs to the public purse and to private industry.

Sean T Hogan, Chair, TB Eradication Partnership