The TB Skin Test

In Northern Ireland, all herds are tested annually, as a minimum requirement, but some are tested more frequently if they are considered at increased risk.

On Day 1 of the test, two sites are clipped on the neck of the animal. The skin thickness at both sites is measured and recorded. Two types of tuberculin, one made from Purified Protein Derivative (PPD) from M. Bovis killed by heat and the other from PPD from Mycobacterium avium killed by heat, are injected under the outer layer of the skin of the neck (i.e., into the dermis) at the ‘bovine site’ and the ‘avian site’ respectively.

On Day 4 of the test, the skin reactions to the two types of tuberculin are measured and compared. When the bovine site reaction exceeds the avian site reaction by more than 4 mm, the animal is declared a reactor under Standard interpretation. When the bovine site reaction measures 1-4 mm more than the avian site reaction, the animal is declared an inconclusive under Standard interpretation.

Skin Tests can also be read under Severe or Super Severe interpretation on specific circumstances.

TB is a difficult disease to diagnose and no diagnostic test for it is perfect. The skin test may leave an infected animal behind or, more rarely, remove an animal that is not infected. However, the skin test is the best test available for screening live cattle.

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).

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 interpretation¹ and slightly higher at severe interpretation. A Northern Ireland specific study published in 2019 by O’Hagan et al.² estimated the sensitivity of the skin test at standard interpretation to be approximately 88%, although there was some bias towards a cohort of younger animals in the data used for this study. In practical terms this means that approximately 20% of TB-infected cattle can be missed by one round of skin testing using standard interpretation. Repeating the skin test increases the likelihood of detecting the infected animals in a herd. This is why an infected herd usually requires at least two consecutive negative skin tests before restrictions are lifted and are subject to more frequent testing for up to a year post de-restriction. Using severe interpretation also increases the sensitivity of the skin test. Further information on skin test sensitivity can be found at Tuberculin skin testing – Bovine TB | TB Hub.

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 has an excellent test specificity of 99.98%. This means that it is very rare that a non-infected animal will be wrongly classified as diseased.

Interferon Gamma test (IFNg)

This is a blood test approved since 2002 in the EU for use in conjunction with the TB Skin Test to maximise the probability of detecting TB infected animals in cattle herds affected by TB breakdowns. The IFNg is not an alternative to replace the skin test.

Animals which have been exposed to M. Bovis can respond to IFNg testing before they will respond to the TB skin test. IFNg testing will also sometimes identify TB-infected animals which do not respond to the skin test.

The IFNg test has a higher sensitivity (it will miss fewer TB-infected animals) but lower specificity (it may wrongly classify more non-infected animals as diseased) compared to the skin test. Due to the lower specificity, this test is currently only used in selected TB breakdown herds in NI.

Veterinary Service Animal Health Group has been using IFNg “Parallel” testing alongside the RH1 skin test in particular high risk breakdown situations since July 2004. Only animals over 6 months are eligible.

In the spring of 2021 policy was amended to permit the use of “standalone” IFNg testing in exceptional cases. This was intended to allow more rapid decision making in the case of explosive outbreaks when full or partial depopulation may warrant consideration, and in other exceptional circumstances.

Herds are selected based on defined criteria.

The test is currently offered under voluntary basis meaning the test is carried out under herd keepers’ agreement.

DAERA provides the personnel to perform the test and pays for the laboratory testing. There is no cost to the herd keeper.

Any animals disclosed as IFNg positive, regardless of skin test results, will be compulsory removed. This is a legal requirement under the Animal Health Law.

Compensation arrangements are the same as with skin reactors.

Currently, unless an IFNg positive is confirmed at postmortem examination the testing regime and disease status of the herd will not be affected.