Diagnostic tests for TB in cattle
In countries implementing bovine TB eradication programmes, different formats of the intradermal tuberculin test (commonly known as the skin test) are used as the primary method to screen cattle herds for TB. In general, the skin test performs well in identifying TB-infected cattle herds. At 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. This means that one round of testing is unlikely to detect every TB-infected animal that may be present in a herd.
Systematic and regular skin testing of cattle herds is likely to remain the cornerstone of the bovine TB eradication programmes in the UK for the foreseeable future. Nevertheless, additional diagnostic tests used on live cattle have been developed over the years to supplement the skin test in specific circumstances. The most successful and widely adopted of those additional TB tests is the interferon-gamma blood test developed in the late 1980s and early 1990s in Australia.
IDEXX antibody test
The IDEXX test is an antibody blood test that gained World Organisation for Animal Health (WOAH) 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, the Animal & Plant Health Agency (APHA) can deploy it in exceptional circumstances with prior consent from the herd owner.
It is a third-line test used in chronic TB breakdown herds that have been subjected to repeated skin testing and one or more rounds of interferon-gamma blood testing.
Enferplex antibody test
The Enferplex TB test is another blood test that can detect antibodies to the bovine TB bacterium in serum (and potentially milk samples) from infected cattle.
It gained WOAH approval as a supplementary TB test for cattle in May 2019, but, like the IDEXX test, it has not been officially recognised by the EU yet.
This test is currently not approved by Defra for statutory TB testing of cattle, although it can be used on a private basis in certain situations and subject to prior permission from APHA.
Other diagnostic tests for bovine TB
Apart from the interferon-gamma, IDEXX and Enferplex 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 non-validated 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 non-validated tests are PCR testing of faecal samples and phage testing.
PCR is a test in development by Warwick University to detect small amounts of DNA in environmental samples (e.g. faeces).
When can use of non-validated and/or non-statutory diagnostic tests for TB in cattle be considered?
Pending their full validation and official regulatory approval, the use of non-validated and/or non-statutory (non-Defra approved) tests can only be considered in exceptional circumstances and subject to prior permission from APHA on a herd-by-herd basis. 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 10(3) of the Tuberculosis in Animals (England) Order 2021 refers.
Defra has developed a protocol for use of non-validated and/or non-Defra approved 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 chronic/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 and non-statutory diagnostic tests for TB in cattle
- By definition, the performance characteristics of non-validated tests are not fully understood (e.g. the probability of false positive and false negative results);
- A non-validated or non-Defra approved test cannot be used to negate a positive result to any of the current statutory TB tests;
- Cattle keepers who are considering additional, voluntary TB testing of their animals should speak with their private vet to discuss the available options and contact APHA to find out whether their herd may be eligible;
- Communication and proactive engagement with APHA before any non-validated or non-Defra approved 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 if applicable;
- The fate (i.e. movement to slaughter or restriction for life) of the animals subject to non-validated or non-Defra approved 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 or non-Defra approved tests are deployed;
- Defra will not be liable to pay compensation for animals that give positive results to any privately funded non-validated or non-Defra approved tests initiated by a private vet.
- In addition to the IDEXX, Enferplex and non-validated tests, cattle farmers can also apply (through their vet) to APHA for permission to undertake skin and IFN-gamma tests on a private, voluntary basis.