To ask the Minister of Agriculture, Environment and Rural Affairs for his assessment of (i) the rate at which bovine TB skin tests provide false negative results; (ii) the impact this has on the spread of bovine TB; and (iii) how this will influence a change in his Department’s bovine TB eradication programme.

Tests for the identification of bovine TB (bTB) in cattle for use in Northern Ireland require the appropriate international approvals from the World Organisation for Animal Health and the European Union.

The Comparative Intradermal Tuberculin Test (skin test) and the interferon gamma blood test (IFNg) are currently the only two such tests approved in European legislation (Commission Delegated Regulation (EU) 2020/689).

Bovine TB is a difficult disease to diagnose and no diagnostic test for it is perfect. The effectiveness of any diagnostic test is related to the sensitivity and specificity of the test.

Sensitivity is the ability of a test to correctly identify an infected animal as positive, i.e. the higher the sensitivity of the test, the lower the probability of incorrectly classifying an infected animal as uninfected (a false negative result). The test can be interpreted at standard or severe interpretation.

Studies carried out in Great Britain have shown that skin test sensitivity lies between 52% and 100% with an average of about 80% sensitivity at standard interpretation and slightly higher at severe interpretation.

In practical terms this means that where infection is present in a herd, approximately 20% of TB-infected cattle can be missed by one round of skin testing using standard interpretation. Repeating the skin test and/or using severe interpretation increases the likelihood of detecting the infected animals in a herd.

Specificity is the ability of a test to correctly identify an animal that is free from infection as negative, i.e. the higher the specificity, the lower the probability of classifying an uninfected animal as infected (a false positive result). The skin test is regarded as having a test specificity of 99.98%. This means that it is very rare that a non-infected animal will be wrongly classified as infected. If severe interpretation is used to increase the sensitivity of the test, the specificity is lowered, i.e. more false positive animals will be taken. That is why severe interpretation is not usually used in routine surveillance testing.

The non-disclosure of infected animals is a recognised problem for eradication programmes across both the UK and Ireland. Non-disclosed infection poses a risk of spread within herds, and between herds, and recurrence of herd breakdowns. The risk of leaving residual infection in diseased herds can be mitigated to some extent by other methods including the use of severe interpretation, the removal of negative high risk animals, serial skin testing and the use of IFNg as an additional test.

Severe interpretation is applied to all breakdown tests where bTB has been confirmed or where there is more than one reactor animal, and herds are subject to more frequent testing for up to a year post-derestriction. My Department’s vets are encouraged to consider if other animals in the herd pose an increased risk even if they have skin tested negative.

Some breakdown herds will be offered the chance to use IFNg testing to try to disclose infection in the herd at an earlier stage than may be revealed by skin testing. IFNg testing will also disclose infected animals that the skin test may miss, but it is a supplement to the skin test rather than a replacement for the skin test.