De-coupling of skin and interferon-gamma testing in TB breakdown herds in England

In England, mandatory interferon-gamma blood testing of eligible TB breakdown herds is deployed as soon as practicably possible after the detection of the first lesion- or culture-positive animal, in order to rapidly remove disease from the herd. This involves de-coupling the gamma test from the next short interval skin test and completing it as a stand-alone test as soon as possible after the affected herd qualifies for a supplementary blood test.

Rationale for de-coupling

Vet holding a sample in a cattle shed - TB Hub

Deploying the gamma test as soon as possible increases the chances of finding TB-infected animals. It is more sensitive than the skin test and identifies a different population of infected animals.

The gamma test can identify cattle at an earlier stage of infection than the skin test. This means that infected animals are removed sooner, with less time for transmission of infection to other cattle in the herd, and potentially wildlife.

Interferom gamma - Bovine TB

Deploying the gamma test early on in the breakdown potentially reduces the duration of movement restrictions and the chances of residual infection remaining in the herd when restrictions are lifted.

By de-coupling the skin and gamma tests, we are essentially removing TB-infected animals that were missed by the previous skin test earlier, thus maximising the probability that the herd goes clear at its next skin test.

Implications of de-coupling for TB breakdown herds

The gamma test cannot be deployed promptly in all cases, as there are logistical constraints both for APHA and for the farmer, and there can be situations where blood sampling for the gamma test is carried out shortly before a skin test. If the skin test identifies more reactors, further short interval testing will be required anyway. There is limited discretion on a case by case basis for the timing of the short interval test to be modified, subject to the satisfactory outcome of a veterinary risk assessment.

New TB breakdown herds with lesion and/or culture-positive animals eligible for mandatory gamma testing must complete a minimum of two short interval tests with negative results before lifting of restrictions can be considered. Where the tests are de-coupled, there are no implications for the number of further short interval tests required unless lesions are detected at slaughter of gamma test positive animals or if there is a positive culture  (however in most circumstances no samples would be submitted for culture from these animals). This applies even when such animals are removed less than 60 days before the first short interval test. This is also the case for any herd with an ongoing breakdown which undergoes a gamma test where two further short interval tests are required before the herd can be considered for lifting of restrictions.

If the tests are de-coupled and gamma test positive animals are removed less than 60 days before the first short interval test and subsequently found to have visible lesions, then the short interval test will only count as a check test and two further short interval tests will be required. When the gamma test is completed at the time of the first short interval test and gamma test positive animals with visible lesions are identified, then this will trigger two further short interval tests irrespective of the skin test results.

How decoupling works in practice

The examples below illustrate that completing a short interval test less than 60 days after removal of gamma test positive animals with visible lesions of TB does not lead to extra skin tests. There would be disease control benefits from earlier removal of such animals, though.  The only situation where this may not apply is in the very unlikely event that a gamma test is completed ahead of a potentially releasing short interval test. Scenarios A and B are examples of TB breakdowns where gamma test positives do not have lesions, and C and D are examples where gamma test positives have visible lesions.