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 (SIT) 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.

New TB breakdown herds with lesion and/or culture-positive animals eligible for mandatory gamma testing must complete a minimum of two SITs with negative results before lifting of movement restrictions can be considered. Where the tests are de-coupled and the gamma test is completed before the first SIT, there are no implications for the number of further SITs 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 SIT. This is also the case for any herd with an ongoing breakdown which undergoes a gamma test where two further SITs 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 SIT and subsequently found to have visible lesions, then the SIT will only count as a check test and two further SITs will be required. When the gamma test is completed at the time of the first SIT and gamma test positive animals with visible lesions are identified, then this will trigger two further SITs irrespective of the skin test results. If at the time of the gamma test, only one more clear SIT is required to lift movement restrictions and gamma test positives are found and removed less than 60 days before the next SIT, the next skin test will only count as a check test. One or two further SITs will be required depending on whether visible lesions are identified in the gamma test positive animals.

How decoupling works in practice

The flow charts below show the course of events in three different scenarios:

  • Gamma test de-coupled and completed before the first SIT
  • Gamma test de-coupled and completed after the first SIT
  • Gamma test coupled i.e. completed at the same time as the first SIT

Keepers are advised to plan ahead. If gamma test positive animals with visible lesions of TB are found, or the herd is due a clearing test (a test that if clear, allows lifting of movement restrictions), check the dates of the herd’s next SIT. Aim to complete the next SIT within the window, ensuring that the test is started at least 60 days after removal of the last gamma test positive animal if your window allows. If the test is completed less than 60 days after removal of the last gamma test positive animal then the test will only count as a check test and further SITs will be required, meaning the herd may be under movement restrictions for longer.

Scenario 1

Scenario 2

Scenario 3