The TB skin test (the single intradermal comparative cervical tuberculin test or SICCT) is officially quoted as having ‘specificity’ of 99.98%. This means that false positive results only occur once in 5000 tests. Conversely the ‘sensitivity’ of the test is much lower, in the range of 50-80% at standard interpretation, meaning that up to 50% of infected cattle on a farm could be falsely negative or ‘clear’.

One study estimated that of recurrent breakdowns in 2003-2005, between a quarter and a half were due to infected cattle being missed by the skin test and remaining in the herd when restrictions were lifted. Although testing regimes are now more rigorous, farmers in TB-endemic areas are increasingly looking at test options to identify more residual infected cows. By more fully ‘clearing out’ the herd at a single test, the hope is that the infection doesn’t linger and spread slowly over the following months or years.

1. Stricter Interpretation of Skin Tests

Severe interpretation is used to test many restricted herds, and with the aim of missing fewer TB-infected cattle.
Many farmers are now beginning to work with their vet to re-interpret TB test charts for non-reactor cattle. The benefit is that this information is free and readily available, but the downside is that the exact sensitivity and specificity is unknown. However, experience is showing us that as we look back at old charts many cows with large bovine reactions go on to become reactors. Opting to send these cows to slaughter in response to large bovine reactors would have prevented the progression of the disease (and spreading of it) as the cows are removed from the herd earlier.
A Welsh government-funded project in Pembrokeshire has been created a traffic light system of ‘Risk Rate’ based on how many times cows have bovine lumps. Management decisions are then thought of in a similar way to Johnes control plans. This is something we can help produce for your herd to start the discussion.

2. Interferon Gamma Blood Test

The interferon gamma blood test is a lab based supplementary which is approved in the EU (and therefore UK) for use alongside the skin test to increase the overall sensitivity of testing. The test measures a cytokine protein which is produced by white blood cells in response to the TB bacteria. It is usually only used in known infected herds under TB restrictions. Typically only animals over six months are tested, as the developing immune system of younger animals can cause false positive results. When performed early in a breakdown, gamma testing can potentially reduce the duration of movement restrictions and the chance that residual infection remains in the herd.The ability to detect infected animals is higher, with a sensitivity of around 90%. Meaning 9 out of 10 infected animals would be detected with a single test, on average. However, this comes with the downside of lower specificity (96.5%), meaning a false positive occurs roughly in every 30 tests performed.
Usually, the test is performed by APHA blood testers, sometimes on the same day as a skin test, however we are part of a trial whereby vet practices will do the ‘government testing’. Since April 2016 farmers have also been able to apply for a voluntary ‘private’ gamma test, including herds under restriction where no statutory gamma test is planned. Tests cost in the region of £16.10, and must be approved in advance by APHA. Positive cattle would be subject to compulsory slaughter with standard compensation rates.

3. IDEXX Antibody Blood Test

Although the majority of the immune response to TB is through white blood cells, antibody responses can also be tested. The IDEXX test follows the skin test, typically by 10-30 days, as the proteins from the skin test ‘boost’ the immune response so it is more easily measured. APHA uses this as a third-line test, and a gamma test must have been performed on animals before it can be used.
Sensitivity is around 65% and specificity is around 98%.
In England, farmers can apply for a private test following gamma testing. In this scenario, positive animals must be slaughtered privately before movement restrictions can be lifted, and no compensation is paid. The test costs around £8.70.

4. Enferplex Antibody Blood Test

The Enferplex test has recently attracted a lot of media attention following its discussion in Brian May’s BBC documentary! Another antibody test, it has been approved by the World Organisation for Animal Health (WOAH) since 2019, but has not been approved by the EU or APHA. The test can be interpreted under two ‘severities’, with a quoted specificity of 98.4 – 99.7%.
As an unvalidated test, positive animals do not have to be compulsorily slaughtered and no compensation is given. They must be voluntarily slaughtered or restricted to the holding for life.

5. Faecal PCR Testing

This remains an unvalidated and experimental test which is only available in approved research scenarios. It can detect tiny levels of bovine TB DNA in muck samples.

6. Actiphage Blood Test

Another research-only unvalidated test, which uses a Mycobacterium-specific bacteriophage (a virus that infects bacteria) to split open the bacteria so that TB DNA could be measured. University of Nottingham it showed promising results, but funding for the use of the technology in TB testing has stalled. A small trial compared the test to skin testing and found detected 95% of cows which were skin test positive and had visible lesions, and it did not falsely detect TB in any skin-test negative cows.

Further information is available on APHA’s website and on TB Hub: https://tbhub.co.uk/tb-testing-cattle/