Bovine TB eradication failure: the imperfect world of testing cattle
- Badger Trust Staff Team
- Oct 9, 2024
- 10 min read
Failings with the current system for cattle bTB testing, and how better cattle testing could actually be the solution to saving badgers.
Bovine tuberculosis (bTB) testing is crucial for managing and controlling the disease in cattle, but the main test currently used in England is not sensitive enough and is missing up to half of infected cattle.
Every time an infected cow is wrongly identified as being not-infectious, a badger is blamed for bringing bTB into a herd.
We urgently need a policy that addresses this failing in cattle bTB testing.
There are four main failings with the current system for testing bTB in cattle.
The main test used is highly inaccurate — missing up to half of infected cattle.
The testing frequency and schedule is not sufficient to stop disease spread across the country.
The main test used cannot differentiate between infected and vaccinated cattle.
And, private vets and farmers are not allowed to choose when to test for bTB, and what test to use without express permission!
But there is hope. Better, more accurate tests do exist that would allow more robust combination testing practices to help remove bTB from our cattle herds. This would improve cattle welfare, increase farmer well-being, and save the lives of many tens of thousands of badgers that are being wrongly accused of being a significant contributor to the spread of bTB.
The world of disease testing can be very complicated. Here, we have tried to break down and simplify the complex system of cattle testing options for bTB in cattle. We show how the failings of the current testing policy in cattle should be urgently addressed rather than using badgers as a scapegoat to prop up a failing policy.
Read on to see how better cattle testing could actually be the solution to saving badgers!
bTB Test options - is there actually a choice?
Testing of bTB in live cattle can be done via skin tests, culture, blood tests, urine, faeces, or tracheal aspirate methods, although none of these tests allows for perfect determination of M. bovis infection status of live cattle [1]. The tests routinely chosen for cattle are based on ease of obtaining a sample, cost, time of processing the sample, and, ideally, the accuracy of the test's detection.
There are currently only two tests that are validated for use to routinely look for bTB in cattle in England; the Single Intradermal Comparative Cervical Tuberculin test (SICCT) and the supplementary interferon-gamma (IFN- γ) blood test [2]. A validated test for testing for bTB in cattle is one that has been approved by the World Organisation for Animal Health (WOAH/ previously OIE).
Choosing to test your cattle can even be illegal!
As we will see, the reliance on the SICCT test as the standalone test for routine cattle testing is not scientifically supported [3, 4].
Other non-validated tests are available to detect bTB, but are only allowed to be used under express permission from APHA. If these non-validated tests are used without permission, you can actually be breaking the law.
This tight hold that the government has over controlling if and when farmers are even allowed to look for the disease suggests a lack of willingness to measure, and therefore, deal with, a larger scale of bTB-infected cattle than currently accurately identified.
1. The SICCT test
The Single Intradermal Comparative Cervical Tuberculin Test, or SICCT test as an easier way of saying it, is the most widely used bTB test for cattle in England. The percentage of positive individuals identified by the SICCT test can be as low as 49% at standard test interpretation [3,4], meaning one in two to one in five (50% - 20%) infected animals could be missed each time a herd is tested. This means that a substantial portion of bTB-infected cattle are misdiagnosed with false negatives and continue to be treated as bTB-free cattle when in fact, they are not.
The SICCT Test is conducted much like the human tuberculin skin test done in humans. First, two sites on the animal’s neck are shaved and the skin thickness measured. Then, two types of tuberculin are injected into the neck — the Lelystad bovine and avian Purified Protein Derivative (PPD). The avian tuberculin is thought to make the bovine test more reliable. Four days later the skin is measured again. Based on the size of the lump the cow is graded as negative, inconclusive, or positive for bTB.
As you can see, using this method is also highly susceptible to human error and is very subjective. Whether a result is deemed positive depends on the severity of the testing protocol in place, which may be higher in different risk areas or if the herd has had a previous breakdown. There has been further evidence that the judgement of what constitutes a positive result is affected by the gender of the vet administering the test [5,6].
The effectiveness of the SICCT test is further compromised by a variety of factors, including but not limited to:
It can also be highly stressful for the animals involved in multiple catches, and can put farmers and vets at risk of injury.
An additional flaw with the SICCT test is the inability to detect between infected and vaccinated cattle (also known as the DIVA test). This failing is why cattle in England are not currently vaccinated, even though recent studies have shown how effective the bTB cattle vaccine is.
2. Interferon-gamma blood test (IFN-γ)
The IFN-γ is a blood test used to detect the Interferon-gamma hormone, which is raised in infected cattle. Whilst this test is very good at detecting infected cattle missed by the SICCT test, in fact it is over 90% sensitive, and 96.6% specific, it requires samples to be kept in temperature-controlled conditions for laboratory analysis[9], is more costly to perform, and can produce a higher number of false positives than the SICCT, making it politically unacceptable as a primary diagnostic test[10].
It is used instead, as a supplementary test to the SICCT test in specific herd breakdowns.
Definitions:
Specificity: Test specificity is the percentage of people who test negative for a disease when they do not have it. It's a measure of a test's ability to correctly identify people without a disease.
Sensitivity: Sensitivity refers to a test's ability to designate an individual with disease as positive. A highly sensitive test means that there are few false negative results, and thus fewer cases of disease are missed.
3. The IDEXX Elisa
The IDEXX ELISA is an antibody assay but is only allowed in England for discretionary use in exceptional situations where there are chronic or persistent herd breakdowns after the SICCT and IFN-γ have been used repeatedly.
For a herd in England to be eligible for the interferon-gamma blood test, a vet must advise its application on the basis that the herd has suffered a severe breakdown, has experienced persistent or recurrent bTB problems, or where the bTB breakdown is experienced in a herd located in a low bTB incidence area. Should a farmer wish to use the IDEXX antibody test, the farmer would need to do so privately and only where permission has been obtained from APHA [11].
“Unexplained infections in cattle are routinely presumed to have come from badgers, without any evidence. Effectively ignoring the limitations of the SICCT and IFN-γ cattle tests and calling it ‘unexplained’.”
Non-Validated tests
Some tests are not yet validated by the EU or by the WOAH, meaning that they are not yet rolled out by the government for routine use. This does not mean that the tests are flawed or inaccurate, and indeed some of these tests are already used at the discretion of APHA or the Welsh government.
The benefit of using non-validated or non-DEFRA-approved tests is that it allows farmers to identify high risk animals missed by other tests and thereby manage their herd, e.g. in isolated groups until the end of lactation.
Two tests that have the potential to dramatically improve the accuracy of detecting bTB in cattle are the Actiphage blood test and the Enferplex antibody test.
1. Actiphage
Actiphage is one such test that can be used at the discretion of APHA. This blood test can detect M.bovis bacterium within six hours and is thought to have a sensitivity of 95% and a specificity of 100%. Farmers have to pay for it privately from PBD Biotech if they want to use it after acquiring permission from APHA. However, we have heard that due to policy issues, Actiphage is no longer being offered in the bTB market.
2. Enferplex antibody test
The Enferplex antibody test, although approved by WOAH, has not yet passed EU regulations or been approved by DEFRA. The Enferplex has a diagnostic specificity of 98.4 - 99.7% using high sensitivity and high specificity settings of the test, respectively[12]. Results from this test to date appear to undermine the accuracy of both the SICCT and the combination SICCT/IFN-γ test even further. The Enferplex can also differentiate between vaccinated and infected cattle.
All animals that test positive with tests not currently validated, however, do not have to be compulsorily slaughtered, and compensation is not given[13,14]. Test-positive animals are either voluntarily slaughtered or restricted to the holding for life. Testing from non-validated tests does not contribute to the lifting of movement restrictions.
Highlighting the Policy Gap
On the BBC documentary ‘Brian May: The Badgers, The Farmers and Me’ released on Friday, 23rd August 2024, we saw how vet Dick Sibley conducted an experiment on the accuracy of the different tests. Dick Sibley tested 42 High Risk cattle using four different tests for bTB. Not a single cow showed positive on all four tests. This discrepancy between results highlights the importance of using more than one test type to accurately identify bTB.
What Dick Sibley further showed, was that faecal testing for bTB could be an important, and underutilised test for detecting, and managing, bTB in cattle.
Cattle Testing Timeframes
The additional challenge with these tests is the timeframe in which they are currently used. Up to 80% of cattle go untested for bTB in their lifetime. This is because cattle testing in England can be as infrequent as every four years in low risk areas. Movement between herds may result in animals either avoiding tests or cattle being slaughtered before the next testing regime. Cattle in England are also exempt from routine bTB testing under various conditions, including moving to exempt agricultural shows or from low-risk areas of England into low-risk areas of Wales, as well as cattle housed in artificial insemination facilities, and when travelling for veterinary treatment [15].
Unless testing frequency is increased nationally, and the best combination tests used as standard, bTB will continue to spread.
The Future’s Testing
The accuracy of these tests has revealed devastating implications for the number of infected cattle going unnoticed[16]. Thousands of infected cattle may have been missed by the widely used SICCT testing regime, allowing infected cattle to freely take part in the 2.8 million cattle movements that occur annually.
More accurate testing would reduce stress and improve farmers’ welfare by reducing the number of tests required to get an accurate response, enabling farmers to act quickly in the event of a breakdown or to be able to trade without reservations. This would simultaneously improve cattle welfare.
Removing the barrier of making vets gain express permission to test cattle for bTB would also allow vets greater freedom over when and what tests to use.
If the costs currently attributed to badger culling were redirected to the implementation of annual combination testing as advised in veterinary journals, then the government could better support farmers to protect their herds from the main cause of bTB transmission, cattle-cattle infection. If sufficient funds, money could include both cattle testing and subsidising farm labour for testing.
Wildlife issues are a distraction from the real issues of tackling bTB in cattle and the badger cull has failed to control the disease in cattle.
References
[1] De la Rua-Domenech, R., Goodchild, A.T., Vordermeier, H.M., Hewinson, R.G., Christiansen, K.H. and Clifton- Hadley R.S., 2006. Ante mortem diagnosis of tuberculosis in cattle: A review of the tuberculin tests, γ-interferon assay and other ancillary diagnostic techniques. Research in Veterinary Science, 81: 190–210
[2] APHA, n.d. Exceptional private use of non-validated or non-DEFRA approved tests for TB on cattle in England. Available from: http://apha.defra.gov.uk/vet-gateway/tb/non-valid-tb-testing/index.htm [Date Accessed 30/10/2023]
[3] Godfray, H.C., Blacquière, T., Field, L.M., Hails, R.S., Petrokofsky, G., Potts, S.G., Raine, N.E., Vanbergen, A.J. and McLean, A.R., 2014. A restatement of the natural science evidence base concerning neonicotinoid insecticides and insect pollinators. Proceedings of the Royal Society B: Biological Sciences, 281(1786): 20140558.
[4] Nunez-Garcia, J., Downs, S.H., Parry, J.E., Abernethy, D.A., Broughan, J.M., Cameron, A.R., Cook, A.J., De La Rua- Domenech, R., Goodchild, A.V., Gunn, J. and More, S.J., 2018. Meta-analyses of the sensitivity and specificity of ante- mortem and post-mortem diagnostic tests for bovine tuberculosis in the UK and Ireland. Preventive Veterinary Medicine, 153: 94-107
[5] Enticott, G., 2012. Regulating animal health, gender and quality control: A study of veterinary surgeons in Great Britain. Journal of Rural Studies, 28(4): 559-567
[6] Enticott, G., 2012. The local universality of veterinary expertise and the geography of animal disease. Transactions of the Institute of British Geographers, 37: 75–88
[7] Claridge, J., Diggle, P., McCann, C.M., Mulcahy, G., Flynn, R., McNair, J., Strain, S., Welsh, M., Baylis, M. and Williams, D.J., 2012) Fasciola hepatica is associated with the failure to detect bovine tuberculosis in dairy cattle. Nature Communications, 3(1): 1-8
[8] Amos, W., Brooks-Pollock, E., Blackwell, R., Driscoll, E., Nelson-Flower, M. and Conlan, A J. 2013. Genetic predisposition to pass the standard SICCT test for bovine tuberculosis in British cattle. PLoS One 8(3): e58245.
[9] Drewe, J.A., Tomlinson, A.J., Walker, N.J. and Delahay, R.J., 2010. Diagnostic accuracy and optimal use of three tests for tuberculosis in live badgers. PLoS One, 5(6): e11196.
[10] Strain, S.A., McNair, J., McDowell, S.W. and Branch, B., 2011. Bovine tuberculosis: a review of diagnostic tests for M. bovis infection in badgers. Agri-Food and Biosciences Institute.
[11] DEFRA, 2015. Bovine TB: get your cattle tested in England. Last updated: 12 October 2022. Available from: https:// www.gov.uk/guidance/bovine-tb-getting-your-cattle-tested-in-england#contents; [Date Accessed: 12.06.23]
[12] O’Brien, A., Clarke, J., Hayton, A., Adler, A., Cutler, K., Shaw, D.J., Whelan, C., Watt, N.J. and Harkiss, G.D., 2023. Diagnostic accuracy of the Enferplex Bovine Tuberculosis antibody test in cattle sera. Scientific Reports, 13(1): 1875.
[13] Robertson, A. 2020. TB knowledge exchange. Available from: https://www. tbknowledgeexchange.co.uk/
[14] TB Knowledge Exchange, 2020. Development of the Actiphage test for bovine TB. Available from: https://www.tbknowledgeexchange.co.uk/wp-content/uploads/Phage_factsheet_13.02.2020_TB_hub.pdf
[15] APHA, 2022. Pre- and post-movement testing of cattle in Great Britain. APHA [online]. Available from: https:// www.gov.uk/government/publications/bovine-tb-pre-movement-and-post-movement-testing-in-great-britain/cattle- movements-exemptions-from-pre-movement-or-post-movement-bovine-tb-testing [Date Accessed: 07.03.2023]
[16] Macdonald, D. and Newman, C., 2022. The Badgers of Wytham Woods: a model for behaviour, ecology, and evolution. Oxford University Press, New York.
