This is one of the TB control enhancements in cattle introduced by Defra in April 2017 following a public consultation.
The primary diagnostic test used to screen cattle herds in Great Britain for TB is the tuberculin skin test. No diagnostic test is perfectly accurate and there is always a trade-off between sensitivity and specificity.
The ability of a test to correctly identify an infected animal as positive. The higher the sensitivity of the test, the lower the probability of incorrectly classifying an infected animal as uninfected (a false negative result).
The ability of a test to correctly identify an animal that is free from infection as negative. The higher the specificity , the lower the probability of incorrectly classifying an uninfected animal as infected (a false positive result).
The specificity of the skin test is high – approximately 99.98% at the standard interpretation. In practical terms, this means that you would expect approximately one false positive result for every 5,000 uninfected cattle tested using standard interpretation.
This very high specificity also gives us a high degree of confidence that the vast majority of the skin test reactors identified in the High Risk and Edge Areas of England are animals truly infected with TB, irrespective of whether or not lesions typical of the disease can be detected during post-mortem meat inspection in the slaughterhouse.
However, the main limitation of the tuberculin skin test is its sensitivity. Studies have shown that the skin test used in Great Britain is on average about 80% sensitive at standard interpretation, and slightly higher at severe interpretation.
In practical terms this means that on average 20-25% of TB-infected cattle can be missed by one round of skin testing using standard interpretation.
The interferon-gamma blood test is a supplementary diagnostic test used in specific scenarios to enhance the sensitivity of TB testing. This is a relatively expensive test and, therefore, its use has been targeted in TB breakdown herds outside of the High Risk Area (HRA) of England since October 2006.
It has also been deployed on a more limited and discretionary basis within the HRA, for example in herds affected by persistent (greater than 18 months duration) or extensive TB breakdowns.
The scientific evidence shows that, because of the limitations of the skin test, approximately 21% of TB breakdown herds in the HRA retain at least one infected animal after all skin testing has been completed and movement restrictions have been lifted. This substantial probability of residual herd infection contributes to the high rate of recurrence seen in the HRA, where nearly 60% of TB breakdowns occur in herds that have sustained a breakdown in the previous three years.
Making increased use of the gamma test alongside the skin test helps reduce the risk of leaving undetected infected cattle in a herd after movement restrictions have been lifted, thereby reducing the risk of repeat TB breakdowns and providing better protection for those that source cattle from other cattle keepers.
The control measure
The gamma test is applied as a mandatory test alongside the skin test to help resolve TB breakdowns with lesion and/or culture positive animals in the HRA where any of the following three criteria are met.
The APHA veterinary investigation concludes that the most likely bTB transmission route for the affected herd was contact with infected cattle (e.g. via cattle movements, residual cattle infection from a previous TB breakdown, or contact with a contiguous infected herd) and measures are in place to prevent further spread of the disease from this source.
The infected herd is located in one of the areas where at least two annual rounds of effective licensed badger culling have been completed.
There is clear evidence that repeat skin testing of the herd has failed to resolve a TB breakdown.
A substantial proportion of new TB breakdowns in the HRA (where approximately 90% of all new TB breakdowns are detected each year) are triggered by direct or indirect contact between badgers and cattle. And that is why we have not adopted a blanket approach to gamma testing across the HRA.
Selection of TB breakdowns herds under the three criteria
APHA identifies new TB breakdown herds where the APHA veterinary investigation concludes that the most likely TB transmission route was contact with infected cattle and where biosecurity controls are in place to prevent further spread of disease from this source. This includes TB breakdowns where the most likely source of infection is identified as purchased cattle, contact with contiguous cattle or residual cattle infection from a previous TB breakdown.
New TB breakdown herds in badger control areas that have completed at least two annual rounds of effective badger culling are selected by APHA. Such herds become eligible for mandatory gamma testing from 1 January following the official announcement that completion of two or more annual rounds of effective badger population control licensed by Natural England have been completed in a specific area. Those announcements normally take place in December and are published on GOV.UK.
APHA identifies TB breakdown herds that remain under restrictions despite repeat skin testing. In selecting which of these herds to test, APHA uses specific criteria such as the severity and duration of the breakdown and results of skin tests and post mortem/culture. These TB breakdowns are selected on a case by case basis with assessment by an APHA vet.
It is important to note that discretionary gamma testing in the HRA is already applied by APHA in certain situations, for example in extensive TB breakdowns with high numbers of reactors. This option is still available to APHA vets managing TB breakdowns that do not fulfill any of the three criteria detailed here.
By applying the gamma test alongside the skin test in certain TB breakdown herds in the HRA, we expect to:
- Maximise the probability of detecting infected animals in the herd. When used in isolation, the skin and gamma tests do not always identify every infected animal in a herd. The two tests identify different sub-populations of infected animals; for example the gamma test can identify infected animals at an earlier stage of infection, as well as infected animals that do not respond to the skin test.
- Shorten the duration of TB breakdowns as infected cattle are identified more quickly. Consequently, it is expected that the policy will help to lower the disease prevalence (i.e. the proportion of all registered herds affected by TB restrictions at any given time) and reduce the overall cost of breakdowns.
- Reduce the likelihood of residual cattle infection in herds that regain officially TB free (OTF) status and hence lower the rate of recurrent TB breakdowns. The risk of spreading TB to other herds via movements of infected cattle and to local wildlife following successful badger control operations will also be reduced.
The adoption of more sensitive TB testing regimes for infected cattle herds in badger culling areas will help maximise the expected disease control benefits derived from badger culling. To maximise these potential benefits, APHA deploys the gamma test in eligible TB breakdown herds at the earliest practical opportunity
Mandatory gamma testing is applied to supplement the skin test and help resolve TB breakdowns with lesion and/or culture positive animals in the HRA where any of the following three criteria are met:
- Criterion 1: The APHA veterinary investigation concludes that the most likely TB transmission route for the affected herd was contact with infected cattle (e.g. via cattle movements, residual cattle infection from a previous breakdown, or contact with a contiguous infected herd) and measures are in place to prevent further spread of the disease from this source.
- Criterion 2: The infected herd is located in one of the areas where at least two annual rounds of effective licensed badger culling have been completed.
- Criterion 3: There is clear evidence that repeat skin testing of the herd has failed to resolve a TB incident.
This control measure was introduced as part of the Government’s comprehensive 25-year strategy for eradicating bovine TB in England, alongside measures to address the disease risk posed by infected badgers. This measure enhances the sensitivity of testing in TB breakdown herds in the HRA, shortening the duration of breakdowns and reducing the rate of recurrence due to leaving undetected residual infection in herds. The use of more sensitive TB testing regimes for infected cattle herds in badger cull areas will help maximise the expected disease control benefits derived from badger culling.
TB breakdown herds in the HRA with lesion and/or culture positive animals that are eligible for the gamma test under any of the three criteria described above. If your herd is not eligible for gamma testing under the existing government policy, your private vet can still apply to APHA for private gamma testing at the herd owner’s expense.
APHA will contact you, explain why your herd is eligible and facilitate the testing.
No. Only animals over six months of age are sampled for gamma testing; younger animals are not tested. Also, specific, separate groups of cattle on the affected holding may be excluded from gamma testing if they are deemed to be at low risk of infection following a veterinary assessment by APHA.
For disease control reasons it is important that this test is carried out as soon as possible after the herd becomes eligible, so that infected animals can be identified and removed at the earliest opportunity. The sooner the blood test is deployed, the greater its potential benefits, such as reducing the time the herd is under TB restrictions. Where practical, gamma testing is completed before the first short interval test (SIT) in new TB breakdowns. APHA will discuss the timing of gamma testing with each herd owner before arranging the test.
When using any diagnostic test there is a balance to be had between the sensitivity (the probability of correct identification of infected individuals) and the specificity (probability of correct identification of uninfected individuals). On average, the gamma test has a specificity of 96.5% (3-4 false positives per 100 disease-free animals tested) compared with 99.98% for the skin test (1 false positive per 5,000-6,000 disease-free animals tested). However, the gamma test is more sensitive than the skin test (90% compared to 81%), i.e. it is less likely to miss infected animals. Under the policy APHA will only be using the gamma test in lesion and/or culture positive TB breakdown herds. In such herds any test positive animal is more likely to be infected and the increased risk (and cost) of taking out low numbers of false positive animals is outweighed by the need (and benefit) to identify and remove all infected animals.
Yes. If an animal has a positive result to either the skin test or gamma test then it is classified as a reactor and must be compulsorily slaughtered. It is important to note that the two different tests detect different sub-populations of TB-infected animals. This may be influenced by the different stage of infection. For example the gamma test tends to detect animals earlier on in the course of infection and therefore infected animals will not necessarily be positive to both tests. Furthermore, some animals may have temporarily depressed immune responses leading to weak skin test reactions. In that sense, using two tests is better than using just one.
No. Diagnostic tests for TB, particularly the gamma test, can detect animals at an early stage of infection and so lesions may not have developed in the animal yet, or they may be too small to identify at post mortem inspection. It is also important to note that post mortem inspection is not a very sensitive method of detecting TB infection, and lesions can also be missed at the slaughterhouse.
TB breakdown herds identified as eligible for mandatory gamma testing will initially undergo one blood test. Thereafter, the need for further blood tests is assessed by an APHA vet using specific criteria relating to the results of skin tests and post mortem/culture results. The number of gamma tests varies between TB breakdown herds according to their individual circumstances.
Only animals over six months of age are sampled for gamma testing. Following individual assessment by an APHA vet, certain herds (e.g. intensive fattening herds) may be eligible for exemption to mandatory gamma testing if they fulfill certain specific criteria.