The tuberculin skin test can be interpreted at either standard or severe interpretation. Standard interpretation is the default used for all routine surveillance testing and other TB testing of herds that are not under TB restrictions (officially TB free herds). Severe interpretation is used in circumstances where bovine TB is strongly suspected or confirmed in a cattle herd, for example;

  • Testing animals in a TB breakdown herd
  • In England and Wales, testing animals traced from TB breakdown herds with lesion and/or culture positive animals
  • In Wales, testing animals traced from TB breakdown herds which meet criteria that indicate they are at an increased risk of infection

The main limitation of the skin test is its sensitivity, i.e. the probability that a TB-infected animal will be correctly identified as positive by the test.  Studies have shown that the average sensitivity for the skin test is about 80% at standard 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.

Severe interpretation of the skin test

Using severe interpretation involves lowering the cut-off point for an animal to be classified as a reactor.  What this means in practice is that some animals initially considered inconclusive reactors (IR) at standard interpretation will be re-classified as reactors under severe interpretation.  In all cases, including when test results are re-interpreted, cattle keepers will be notified of the results by APHA.

The effect of using severe interpretation is to increase the sensitivity of the skin test over the standard interpretation in order to;

  • Improve the detection of TB-infected animals in a herd
  • Speed up the resolution of a TB breakdown
  • Reduce the risk of leaving TB-infected animals behind when movement restrictions are lifted at the end of a TB breakdown

Application of the severe interpretation of the skin test is a common and longstanding APHA procedure in England, Scotland and Wales. It has been the normal approach for many years in all TB breakdown herds with lesion and/or culture positive animals.

Severe interpretation in England

Since 2014, severe interpretation has been used in all TB breakdown herds in the Edge Area of England regardless of whether post mortem and/or culture evidence of M. bovis infection is found.

In April 2016, the use of severe interpretation was further extended to all TB breakdown herds in the High Risk Area (HRA) and certain breakdown herds in the Low Risk Area (LRA) of England, regardless of whether they have lesion and/or culture positive animals. This is because studies have shown that the majority of such herds are truly infected with bovine TB.

In the HRA and Edge Areas of England, all TB breakdown herds (regardless of post mortem and culture results) require a minimum of two consecutive short interval tests read using severe interpretation before restrictions on the herd can be lifted.  This is applied at the beginning of the TB breakdown, i.e. the first two short interval tests after the test that found the reactor(s) – called the disclosing test.

The same testing regime is used in a small number of high-risk TB breakdown herds in the LRA of England in which all reactors have no visible lesions of TB and negative culture results.1

Severe interpretation in Wales

In Wales, severe interpretation is used in TB breakdown herds with lesion and/or culture positive animals and in those that meet certain criteria that indicate they are at an increased risk of infection, for example any in which more than one reactor has been found. Severe interpretation is applied to the first short interval test and also to the second if any IRs are found at the first test.

Severe interpretation in Scotland

In Scotland, severe interpretation is used in TB breakdown herds with lesion and/or culture positive animals. Severe interpretation is applied to the first short interval test, and the results of this test then dictate whether standard or severe interpretation is used at the next short interval test.

Re-interpretation of the skin test in England

In all TB breakdown herds with lesion and/or culture positive animals, the disclosing test results are re-interpreted using severe interpretation.  This means that animals that were IRs at the first test may become reactors following reinterpretation.  In all cases, cattle keepers will be notified of the results by APHA.

If lesion and/or culture positive animals are subsequently identified following an IR re-test in a herd with no previous post mortem or culture confirmation of M. bovis, the following will happen;

  • The IR test will be re-interpreted using severe interpretation
  • The test that disclosed the IRs in the first place will be re-interpreted using severe interpretation
  • Any tests that took place between these two tests will be re-interpreted using severe interpretation

This means that even if other animals tested clear at the most recent IR re-test, they may become reactors when the disclosing test (the test at which they were IRs) is re-interpreted using severe interpretation. Animals that become reactors upon re-interpretation are compulsorily slaughtered with compensation paid.

Re-interpretation of the skin test in Wales

Re-interpretation of the disclosing test using severe interpretation is carried out in TB breakdown herds with lesion and/or culture positive animals and in those that meet certain criteria that indicate that they are at an increased risk of infection.

If an IR re-test results in finding lesion and/or culture positive animals or in cases where the breakdown herd meets the criteria for increased risk of infection, the following will happen:

  • The IR re-test will be re-interpreted using severe interpretation
  • The test that disclosed the IRs in the first place will be re-interpreted using severe interpretation
  • Any tests that took place between these two tests will be re-interpreted using severe interpretation

This means that even if other animals tested clear at the most recent IR re-test, they may become reactors when the disclosing test (the test at which they were IRs) is re-interpreted using severe interpretation. Animals that become reactors upon re-interpretation are compulsorily slaughtered and compensation paid.  In all cases, cattle keepers will be notified of the results by APHA.

Re-interpretation of the skin test in Scotland

In Scotland, re-interpretation of the disclosing test using severe interpretation is carried out in TB breakdown herds with lesion and/or culture positive animals. This means that animals that were IRs at the disclosing test may become reactors.

A veterinary risk assessment is completed and if this indicates that any remaining cattle are at high risk of being infected, these animals will be removed as Direct Contacts (DCs) and compensation paid. For example, if there were other IRs at the disclosing test that have subsequently tested clear but which are identified as high risk, these animals can be removed as DCs.

 

Note

  1. TB breakdown herds in the LRA that:
  • had a TB breakdown with lesion and/or culture positive animals in the previous four years; and/or
  • are contiguous to another TB breakdown with lesion and/or culture positive animals, or to a TB-restricted holding containing non-bovine animals diagnosed with confirmed M. bovis infection