So, are we wasting our time then?’ This was the jovial greeting from one farmer as I arrived to carry out a TB test shortly after the broadcast of Sir Brian May's BBC programme on bovine TB. Anticipating such reactions during my visits, I was prepared. ‘The test is far from perfect,’ I replied, ‘but wouldn't you agree that an imperfect test is better than no test at all?’ He readily agreed, and we got to work.
There are many cattle farmers across the country investing time, effort, money and emotional energy into carrying out testing for bovine TB – not to mention the vets and paraprofessionals who work in all weathers, handling cattle of every temperament and dealing with every conceivable kind of handling system. Unfortunately, Sir Brian May's programme risks eroding the trust between farmers and vets, increasing reluctance to carry out TB tests, and reducing confidence in the results.
Sir Brian stated that the intradermal comparative skin test detects only half of the infected animals. However, according to Animal and Plant Health Agency (APHA) figures, the test actually has a sensitivity range of 50–100%, with an average of 80%. Explaining concepts such as test sensitivity and specificity is challenging and something I approach with care in conversations with farmers. Characterising the test as ‘missing half the infected animals’, as Sir Brian did, oversimplifies the issue. A more accurate description is that up to half of the infected animals may be missed. This does not account for repeated testing; it is not the same half of the animals that test negative every time. Repeated short-interval testing (every 60 days) significantly increases sensitivity. I often use an analogy with testing for Johne's disease in dairy cows using quarterly milk serology, which farmers have found helpful in discussions about TB tests.
When the consequences of a positive test are severe – such as culling – we must prioritise specificity over sensitivity to avoid unnecessary culling. The programme's moving portrayal of farmers' distress at losing cows to TB highlights the importance of this balance.
There are ways to increase test sensitivity. In TB breakdown situations, test results can be reinterpreted at severe (rather than standard) interpretation, widening the net for potential reactors. Farmers can also access the interferon-gamma test, which offers higher sensitivity but lower specificity, as a tool to speed up the removal of infection from the herd. Many farmers in the high-risk area (HRA) are familiar with these tests, but in North Yorkshire, where I work, many farmers are TB-free and unaware of these supplementary methods. This risks farmers taking Sir Brian's claims at face value, such as the suggestion that they are not allowed to access other TB tests. In reality, validated TB tests are available to farmers where appropriate for their herds. Advocating for unvalidated testing risks undermining official protocols, raising unrealistic expectations, and wasting significant resources that could be better used to control TB effectively.
Another key fact to emphasise is that the TB testing strategy is working. The programme is misleading in this regard, as its statements about worsening TB in Devon refer to the situation in 2016–2019. However, as of 2024, TB is improving in Devon, with 8.7% fewer TB breakdown herds in the 12 months to March 2024 compared to the previous year. Cattle TB testing has played a vital role in this progress.
One area where farmers should consider test sensitivity more critically is in purchasing cattle. Sir Brian is correct in highlighting that cattle-to-cattle spread is a significant factor in bovine TB, particularly through cattle movements between farms. In the low-risk area (LRA) where I work, localised TB breakdowns almost always result from bringing cattle onto farms from the HRA. This practice often leads to frustration among neighbouring farmers, who then face radial testing zones. I am frequently asked, ‘Why can't we stop cattle purchases from high-risk or edge areas into the low-risk area?’
The answer is largely economic. Businesses in the HRA rely on selling cattle to regions where they can be reared more affordably, while businesses in the LRA depend on purchasing calves, stores, or finishing cattle from areas with higher production. Pre- and post-movement testing is an additional cost and routine task for these transactions. However, the risks of such purchases, including the limitations of testing in identifying all infected animals, are often overlooked. Vets and farmers could reduce these risks by avoiding high-risk purchases or rearing such animals in less risky ways, such as licensed finishing units. This is an excellent example of ‘controlling the controllable’ – a message that resonates with farmers facing what can seem like overwhelming challenges.
The TB Advisory Service offers advice on cattle purchases and more through a bespoke, free service for farms seeking to understand their controllable risks. Sir Brian May is not the only one committed to reducing TB; many unsung vets, industry bodies and researchers dedicate significant time and energy to this cause. I extend my gratitude to all those working tirelessly to improve our ability to control TB.
All of us – vets, farmers, scientists, consumers and animal welfare advocates – share the goal of reducing bovine TB. Sir Brian May is entitled to his opinion and the right to share it publicly. However, his criticisms of the skin test risk raising unhelpful doubts among farmers and complicating the work of those on the frontline.