De-coupling

In England, mandatory interferon-gamma (IFNγ) 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 IFNγ test from the next short interval skin test and completing the former as a stand-alone test as soon as possible after the affected herd qualifies for a supplementary blood test.

Rationale

De-coupling has the following advantages in terms of disease control;

  • Deploying the IFNγ test as soon as possible increases the chances of finding infected animals. The IFNγ test is more sensitive than the skin test and identifies a different population of infected animals
  • The IFNγ test can identify cattle at an earlier stage of infection than the skin test. Therefore, infected animals are removed sooner, with less time for transmission of infection to other cattle in the herd and (potentially) wildlife, thereby reducing the magnitude of the incident
  • Deploying the IFNγ 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 at the end of the breakdown
  • By de-coupling the skin and IFNγ tests, we are essentially removing 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 IFNγ 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 IFNγ 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 IFNγ herd testing are required to 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 IFNγ test positive animals (or if there is a positive culture, but in most circumstances no samples would be submitted for culture from these animals) 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 an IFNγ 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 IFNγ 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 count as a check test and two further short interval tests will be required.

When the IFNγ test is completed at the time of the first short interval test and IFNγ test positive animals with visible lesions are identified, then this will trigger two further short interval tests irrespective of the skin test results.

The examples below illustrate that completing a short interval test less than 60 days after removal of IFNγ 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 if the very unlikely event that an IFNγ test is completed ahead of a potentially releasing short interval test.

A and B are examples of TB breakdowns where IFNγ test positives do not have lesions, and C and D are examples where IFNγ-test positives have visible lesions.

tb table de coupling