Defra funded a study in 2015 to compare different tests for detecting M. bovis in badger faeces and to determine if any of them would be accurate enough to reliably identify TB-infected badger setts.
In the comparative study [1], test results on faeces collected from badger latrines in an intensively studied location in Gloucestershire, were compared with results of both blood tests and bacteriological culture to grow M. bovis from badgers from the same setts. Test results on faeces collected from uninfected, captive badgers were also assessed.
The headline finding from the study is that none of the six tests in the comparative study was sufficiently accurate to allow the Government to consider adopting a new badger control approach based on targeting only setts containing TB-infected badgers. The Warwick University PCR test performed the best but was not accurate enough to be a practical tool for widespread field use, although it is continuing to be used as a research tool on a small scale in order to further develop the test and its performance.
The study
False positive results (where TB-free setts are incorrectly identified as TB-infected) have been shown to be a problem with all the tests included in the comparative study. Even the Warwick University PCR test would be expected to incorrectly identify 25% of uninfected setts as infected using a testing programme based on testing ten samples per sett [2]. The researchers, based on other studies they have carried out, advise testing at least twenty samples per badger sett, which would produce a higher false positive rate (around 50% of setts incorrectly identified as positive).
Similarly, false negative results (where infected setts are missed by the test) would diminish the value of these tests for the purposes of targeted interventions.
Using the Warwick University PCR [3], testing 20 samples per sett in Gloucestershire during the summer months did identify approximately 80% of infected setts, but 1 in 5 infected setts were still missed. The test was not as good at detecting infected setts at other times of the year. Over the course of a year, testing 20 samples per sett resulted in missing around half the infected setts. Increasing the number of samples is a potential way to overcome these limitations; for example, testing 50 samples per sett during the summer led to identification of all positive setts in Gloucestershire. However, testing larger number of samples rapidly results in unacceptably high false positive rates for the test, making practical use very difficult.
TB-infected badger faeces contains intermittent low levels of TB bacteria. This means having to test multiple samples to find evidence of infection. A sett is considered to be TB-infected if one or more sample tests positive. Previous research studies with the Warwick University PCR test showed it required up to 50 samples per sett to find all known TB-infected setts, and the ability of the test to find TB-infected setts varied by season.
Collecting as many as 20-50 samples per sett is practically difficult and probably routinely possible only from larger setts. It is more difficult where there is a poorer understanding of locations of setts and associated latrines.
Conclusion
The Government’s current badger control policy is based on scientific evidence from the Randomised Badger Culling Trial and promotes area-based proactive culling of badgers in order to reduce the population densities. Concerns about ‘perturbation’, where culling only a small proportion of badgers is thought to increase TB transmission from badgers to cattle, support the current policy. The insufficient accuracy of the six tests in the comparative study does not support a change in policy, i.e. to consider adopting a new badger control approach based on targeting only setts containing TB-infected badgers.
The Government would need a test that was much more accurate than even the Warwick University PCR test before considering such an approach. Otherwise it could risk making the bovine TB problem worse. Private use of such tests is an option to inform biosecurity management decisions although anyone doing so should bear in mind the reliability of the tests and that Defra will not take official action on the basis of the results.
References
- SE3289 – Study to comparatively assess methods to detect M.bovis from badger faeces
- A sett is considered to be infected if one or more of the samples tested per sett produces a positive result (for example 1 in 10, 20 or 50 samples depending on the testing regime adopted). The Warwick PCR has false positive rate of around 2% – 3% (97-98% specificity) at an individual sample level, equivalent to around 1 in 40 uninfected samples incorrectly testing positive. In this way a very low level of false positives at an individual sample test level quickly leads to a test which results in a high level of false positives at a sett level.
- J Clin Microbiol. 2015 Jul;53(7):2316-23. doi: 10.1128/JCM.00762-15. Epub 2015 Jun 3
Performance of a Noninvasive Test for Detecting Mycobacterium bovis Shedding in European Badger (Meles meles) Populations. King HC, Murphy A, James P, Travis E, Porter D, Sawyer J, Cork J, Delahay RJ, Gaze W, Courtenay O, Wellington EM.