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Huge reservoir of undetected bTB infection found
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Exclusive, by Adele Waters

The scale of undetected bovine TB (bTB) has been revealed through a new analysis of official data from Defra.

It shows that tens of thousands of cattle could be acting as a reservoir for bTB in England.

Data obtained from Defra via a parliamentary question show that some 5000 additional infected cattle were identified (from 249 confirmed herd breakdowns) after interferon gamma testing was rolled out across some of the high-risk area (HRA) in England last year. These were infected cattle that were missed by the standard bTB skin test.

Gamma testing, a supplementary blood test that alongside the skin test maximises the probability of detecting bTB, was introduced in 2014 to low-risk and edge areas for confirmed herd breakdowns. In 2017 the government rolled this out to the HRA (when 4 per cent underwent the new test) and the following year this was stepped up to 13.25 per cent of the HRA.

Extrapolation of the figures shows that if all 1879 confirmed breakdowns in the HRA had been gamma tested, this could, potentially, have identified around 37,000 additional infected cattle (see box).

The analysis conducted by the Animal Welfare Group (AWG), a loose coalition of vets and scientists, said the final tally of potentially infected cattle in the HRA in 2018 would need to include the 20,000 cattle already detected by the skin test that year so the total number of infected cattle could have been as high as 57,000.

Jan Bayley, researcher for the group and who obtained the figures, said: ‘We have known for some time that the skin test is missing many infected cattle, but even we were staggered by the sheer scale of hidden infection lying undetected in these herds.

‘This problem is not confined to the HRA. Data published recently confirm that counties in the edge area are equally demonstrating large numbers of additional infected cattle being identified with gamma testing. These figures show that we have a massive problem on our hands if we are to clear infected cattle from herds.’

The AWG believes that the 249 herds are a representative sample because their profile was typical of all infected herds across the HRA (102 were from persistently infected herds and more than 140 were from areas that had undergone two years of badger culling). The group also considers that the figure is unlikely to include many false positives, as Defra has already confirmed that a test-positive animal in the HRA is more likely to be infected due to the high level of endemic infection present.

These figures are an important contribution to understanding the scale of the problem

Responding to the figures, Alastair MacMillan, former veterinary adviser to Defra and expert in disease control in farm animals, said: ‘It has become well established that there is a large level of undisclosed TB infection remaining in herds after routine skin testing and this poses a major barrier to controlling the disease. These figures are an important contribution to understanding the scale of the problem.’

It is expected that greater awareness about the scale of this underlying cattle infectivity will put pressure on Defra to change its management of bTB – for example by recommending the use of different bTB tests – when it publishes its response to a review of its 25 year bTB strategy (Godfray review) shortly.

Sheila Crispin, fellow and past president of the RCVS, said: ‘This is going to need major collaboration between Defra, farmers and vets to address the massive scale of, as yet unidentified, infection. Indeed it may even require a fundamental re-think as to how the disease is controlled, but the first step is for everyone involved to recognise the sheer scale of the problem.’

The BVA, which is currently undertaking an in-depth, comprehensive review of its position relating to bTB and is considering all aspects of disease control (including cattle testing) urged caution.

James Russell, BVA junior vice president, said: ‘The primary data presented here are of interest, but we would urge caution over extrapolating the figures without careful modelling. We will analyse the primary data further before we make our recommendations.

‘It is vital that we utilise every tool in the toolbox in our efforts to curb and eradicate bTB. We continue to support a comprehensive and evidence-based approach to tackling this devastating disease.’ •

How was the analysis done?

  • 249 confirmed breakdowns in the high-risk area (HRA) were gamma tested in 2018 out of a total of 1879 confirmed breakdowns in the HRA.

  • This represents 13.25 per cent of confirmed breakdowns in the HRA.

  • Gamma testing identified 4898 infected cattle (in addition to those identified by skin testing).

  • By extrapolation, if all 1879 confirmed breakdowns in the HRA had been gamma tested this, potentially, could have identified, in total around a further 37,000 infected cattle, which had not been detected by the skin test.

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