Diagnostic tests for TB in cattle

In countries implementing bovine TB eradication programmes, variants of the intradermal tuberculin test (commonly known as the skin test) are used to routinely screen cattle herds for TB. The skin test is also approved by the EU and the OIE (World Organisation for Animal Health) for international trade of cattle. In general, the skin test performs well in identifying TB-infected cattle herds. At the standard interpretation, the comparative format of the skin test used in the UK and Ireland has a very high specificity (i.e. false positive results are highly unlikely), but only a good to moderate sensitivity, meaning that one round of testing is unlikely to detect all of the TB-infected animals present in a herd. Further information about the skin test can be found on the TB hub.

Systematic and regular skin testing of cattle herds is likely to remain the cornerstone of TB control in cattle for the foreseeable future. Nevertheless, additional diagnostic tests used on live cattle have been developed over the years to supplement the primary skin test in specific circumstances. The most successful and widely accepted of those additional TB tests is the interferon-gamma blood test developed in the late 1980s and early 1990s in Australia. More information about the characteristics and current uses of the interferon-gamma blood test in Great Britain can be found on the TB hub.

The IDEXX test is an antibody blood test that gained OIE approval as a supplementary TB test for cattle in 2012, but it has not been officially recognised by the EU yet. This means that although it cannot be used as a statutory TB test in Great Britain, APHA can use it in exceptional circumstances with prior consent from the herd owner. It is a third line test used in chronic TB breakdown herds where repeated tuberculin skin testing and use of the interferon-gamma blood test has already occurred. Further information about the IDEXX antibody test can be found on the TB hub.

Other diagnostic tests for bovine TB

Apart from the interferon-gamma and IDEXX blood tests, other laboratory tests for TB have been developed for use in cattle, but their application has been confined to the experimental setting or to very specific field situations. In fact, no laboratory test has been universally adopted as a reliable and cost-effective alternative to the tuberculin skin test.

The vast majority of these novel diagnostic tests for bovine TB have not been validated yet. This means that their performance characteristics (such as sensitivity, specificity, predictive values, repeatability of results, etc.) have not been fully evaluated in the cattle population of interest. As such, these tests are not approved for official use in cattle, but they may be considered for exceptional private use in cattle herds with chronic TB breakdowns. Examples of these tests are the Enferplex TB serological test, PCR testing of faecal samples and phage testing:

  • The Enferplex test produced by Enfer Laboratories, Ireland is a blood test for detection of specific antibodies to M. bovis
  • PCR (polymerase chain reaction) is a test in development by Warwick University to detect small amounts of DNA in environmental samples. Further information on the PCR test can be found on the TB hub
  • The Actiphage test is a test in development by Nottingham University to detect M. tuberculosis complex bacteria in blood from TB-infected cattle. It uses a Mycobacterium-specific bacteriophage (virus that infects bacteria)

When can use of non-validated diagnostic tests for TB in cattle be considered?

Pending their full validation and official regulatory approval, the use of non-validated tests can only be considered in exceptional circumstances and subject to prior permission from APHA. Failure to seek written consent from APHA for private TB testing of cattle constitutes an offence under the Animal Health Act 1981, of which Article 13(3) of the Tuberculosis (England) Order 2014 refers.

Defra has developed a protocol for use of non-validated diagnostic tests for bovine TB in England, which is available for private vets on the APHA vet gateway.

This protocol applies to cattle herds affected by persistent TB breakdowns with lesion and/or culture positive cattle where repeat skin and interferon-gamma blood testing has not resolved the infection.

Key factors to consider with non-validated diagnostic tests for TB in cattle

  • By definition, the performance characteristics of such tests are not fully understood (e.g. the probability of false positive and false negative results);
  • A non-validated test cannot be used to negate a positive result to any of the current statutory TB tests;
  • Cattle keepers should speak with their private vet to discuss possible options for additional TB testing if they are interested in finding out whether their herd may be eligible;
  • Communication and proactive engagement with APHA before any non-validated tests are deployed is essential. APHA also expects private vets to engage with the test manufacturer, so that testing will contribute to the evidence needed for the candidate test to be validated;
  • The fate (i.e. movement to slaughter or restriction for life) of the animals subject to non-validated tests will be determined by the outcome of these tests. A formal written agreement is required between the private vet and the herd owner before any non-validated tests are deployed;
  • Defra will not be liable to pay compensation for animals that give positive results to any privately funded non-validated tests initiated by a private vet.