About Bovine TB
Testing & movements
This information note presents the results of interferon-gamma (IFN-γ) blood testing of TB breakdown herds in badger culling areas in England in 2017. Data covers April to December 2017 inclusive, and further evaluations will follow as more data becomes available.
Since April 2017 Defra’s policy has been to deploy mandatory IFN-γ testing in all new TB breakdown herds with lesion and/or culture positive animals in areas that have completed two annual rounds of effective licensed badger culling.
In 2017 this policy affected new TB breakdown herds in three cull areas in Dorset, West Somerset and West Gloucestershire. As of 2017, the Area-3 Dorset cull area which began culling in 2015 had completed three annual rounds (‘seasons’) of intensive culling. Area-1 Gloucestershire and Area-2 Somerset began culling in 2013 and completed four seasons of intensive culling, followed by supplementary badger culling in 2017.
The IFN-γ test is used in parallel with the skin test in new TB breakdown herds with lesion and/or culture positive animals to maximise the chances of finding all infected animals in the herd. In these situations, the risk of taking out low numbers of false positives is outweighed by the need to identify and remove all infected animals from the herd as quickly as possible.
Studies in Great Britain, Northern Ireland and the Republic of Ireland consistently show that IFNγ-positive animals have a high risk of becoming skin test positive and developing visible lesions if not removed, when compared with IFNγ test-negative animals in the same herds.1
Deploying the IFNγ test at the beginning of a new TB breakdown aims to:
Defra’s decision to roll out the IFN-γ test after completion of the second season of badger culling represents a balance between allowing culling enough time to reduce the reservoir of infection in the local badger population, whilst not unnecessarily delaying the benefits of supplementing the skin test with IFN-γ testing in TB breakdown herds.
Cattle herds in areas that have undergone badger culling have a reduced risk of contracting bovine TB from badgers, and so the risk from cattle to cattle transmission becomes relatively more important by comparison with other routes of infection.
We also know that recurrent TB breakdowns and residual M. bovis infection in cattle herds that regain OTF status after a breakdown are key features of the epidemiology of bovine TB in the High Risk Area (HRA) of England, irrespective of the role played by badgers.1,2
It is therefore necessary to remove infected animals from cattle herds as quickly as possible after the onset of culling, to prevent transmission of infection back into the remaining badger population. This was the veterinary ideal originally proposed in Defra’s public consultation document published in autumn 2016.
Additionally, attention to on-farm biosecurity and informed purchasing is essential as part of a holistic approach to prevent reinfection of cattle herds and protect the benefits accrued from badger culling and IFN-γ testing.
Intensive badger culling is carried out in England during a defined period in summer and autumn and the greatest reductions in local badger populations are generally achieved during the first season of culling. For the purposes of selecting TB breakdowns for mandatory IFN-γ testing, two seasons of effective culling constitutes a period of 15 months. For example, new TB breakdown herds with lesion and/or culture positive animals in the seven badger culling areas that started culling in late summer of 2016 will become eligible for mandatory IFN-γ testing from January 2018 after completion of two seasons of culling.
The evidence to date suggests that M. bovis does not persist in the environment long enough to warrant delaying deployment of the IFN-γ test in badger culling areas beyond the current policy of 15 months after the onset of culling.
For more information on persistence of M. bovis in the environment, see the pages on the TB Hub.
Between 1st January and 31st December 2017, the proportion of cattle blood samples with positive IFN-γ test results was 5.6% overall for all testing scenarios in England. In more than 98% of cases the test was applied with the skin test in herds with lesion and/or culture positive animals.
During the first nine months of implementation of the policy (April 2017 – November), a total of 6,090 samples from herds with new TB breakdowns with lesion and/or culture positive animals located in badger cull areas have been processed for IFN-γ testing by APHA. The overall proportion of those samples with a positive test result was 6.7% (see table 1 below).
Table 1 – IFN-γ test results in the three badger culling areas where the new TB breakdown testing policy has been in force since April 2017
The smaller number of herds tested in Area-1 Gloucestershire and Area-2 Somerset compared with Area-3 Dorset is probably a reflection of the reduction in the number of TB breakdowns with lesion and/or culture positive animals identified in those counties in 2016 and 2017 (the declining trend in Gloucestershire goes back to 2012).4,5
The observed percentage of test-positive samples is within the range of values that Defra expected based on experience of using the IFN-γ test in similar situations in the Edge Area (4.2% test positivity in 2017) and in some persistently infected herds in the HRA (9.8% test positivity in 2017) on a more sporadic, discretionary basis.
Tables 2, 3 and 4 show the proportion of samples with a positive IFN-γ test result for each herd that has been sampled in the three badger culling areas over the months June to November 2017 inclusive. Note that there were no new TB breakdowns with lesion and/or culture positive animals in any of the culling areas in April and May, hence there were no IFN-γ submissions for these months. Submissions for each herd collected in the same month, or over two consecutive months, represent one ‘round’ of IFN-γ testing for which the proportion of samples with a positive IFN-γ test result has been calculated. Large herds may be sampled in two or more submissions.
Table 2 – IFN-γ test results for individual breakdown herds sampled in badger culling Area-3 Dorset since the rollout of the new testing policy in April 2017
Table 3 – IFN-γ test results for individual breakdown herds sampled in badger culling Area-1 Gloucestershire since the rollout of the new testing policy in April 2017
Table 4 – IFN-γ test results for individual breakdown herds sampled in badger culling Area-2 Somerset since the rollout of the new testing policy in April 2017
There is nothing unusual or unexpected about the IFNγ test results seen so far in TB breakdown herds within badger cull areas. So far, the overall percentage of IFNγ test positive samples in TB breakdown herds sampled in the first three badger culling areas has been 6.7%.
This is 2.5% higher than the equivalent percentage of 4.2% in new TB breakdowns with lesion and/or culture positive animals sampled in the Edge Area during 2017.
Whilst three herds yielded IFN-γ positivity percentages greater than that for HRA herds quoted above (11.1%, 13.1% and 13.2% respectively), most herds fell between the Edge and HRA test positivity rates for 2017, and five herds fell below the Edge Area positivity rate. There is always likely to be ‘outliers’ to expectations, and herds that undergo IFN-γ testing are exposed to different levels of M. bovis infection.
However, the number of samples tested from TB breakdown herds in badger culling areas is still relatively small, and the policy has only been in place for nine months. Therefore, more data is required to draw firm conclusions as to whether TB breakdown herds in badger cull areas yield a significantly higher proportion of IFN-γ positives than those in the HRA that are not undergoing badger culling (or where culling has taken place for longer or shorter periods).
Disease control policy makers and epidemiologists have to look at the bigger picture and focus on the population at large for planning, impact assessment and modelling purposes. Defra will continue to monitor the IFN-γ testing policy in the badger culling areas and results will be interpreted in the context of the wider bovine TB eradication strategy.
Gormley E, Doyle MB, et al. (2006) Diagnosis of Mycobacterium bovis infection in cattle by use of the gamma-interferon (Bovigam) assay. Vet Microbiol 112:171–179
Collins JD MM, McGill K, Kelly A, Fitzsimons T (2000) A longitudinal study of cattle found positive to the Interferon g assay for Mycobacterium bovis infection, International Symposium on Veterinary Epidemiology and Economics (ISVEE) 9, Colorado (USA)
Lahuerta-Marín A, Gallagher M, et al. (2015). Should they stay, or should they go? Relative future risk of bovine tuberculosis for interferon-gamma test-positive cattle left on farms. Veterinary Research 46:90
Coad M, Downs SH, et al. (2008). Blood-based assays to detect Mycobacterium bovis-infected cattle missed by tuberculin skin testing. The Veterinary Record, 162