This guidance applies in England and Wales
The tuberculin skin test is the default test used when a cattle herd has a TB breakdown. The interferon-gamma blood test is also used in some cases to supplement the skin test.
The TB Skin Test
The tuberculin skin test is used to find cattle infected with Mycobacterium bovis (aka bovine TB). Two different interpretations of the skin test are used, depending on the disease situation. The sensitivity of a test is the probability that an infected animal (in this case with TB) will be correctly identified as infected by the test. Test developers can use different cut-off points to raise or lower the sensitivity of a test to achieve different things.
The specificity of a test is the likelihood that an uninfected animal will be correctly identified as negative to the test. The likelihood of false positives can be determined by the test specificity.
Standard interpretation has a very high specificity of 99.98% and is the default used for most routine surveillance TB testing. This specificity equates to a probability of 1 false positive result in approximately every 5,000 uninfected animals tested.
The sensitivity of the skin test is on average 81%. This means that in some situations infected animals can remain undetected by the test.
Severe interpretation is mainly used in TB incidents in situations where there is likely to be more infected animals in the herd than are being identified by standard interpretation.
Surveillance testing
How often Officially TB Free herds are tested depends on how many cases are found nearby, and whether the herd is in England, Scotland or Wales. Routine TB testing frequency can range across GB from 6 monthly to four yearly. Routine or whole herd surveillance testing is supplemented by additional risk-based area testing (known as radial and contiguous testing), herd post-breakdown testing and by individual animal trace, pre-movement and post-movement testing.
Short interval skin testing
Short interval tests include all animals in the herd and are carried out at minimum intervals of 60 days. Calves under 42 days of age are excluded from short interval testing, unless a risk is identified in this age group. The 60-day period is from the date the last reactor left your herd (and therefore the last possible day reactors could have infected other animals). If no reactors are found at a test, the next test is scheduled 60 days from the injection date of the previous test.
The minimum 60-day interval between skin tests is also necessary to reduce the risk of TB-infected animals becoming desensitised to the skin test injections– a phenomenon where their skin reactivity is depressed for some time afterwards, potentially leading to false-negative results. [1]
In the High Risk Area (HRA) and Edge Area of England, all TB breakdown herds must complete at least two consecutive short interval tests at severe interpretation with negative results before regaining their officially TB free (OTF) status. In the Low Risk Area (LRA) of England, TB breakdown herds must complete one or more short interval tests before the OTF status of the herd can be restored. The number of herd tests and their interpretation depends on certain criteria, and whether any typical lesions of TB are found and/or M. bovis is identified in tissue samples from reactors or slaughterhouse cases that triggered the breakdown.
All incidents in Wales require two clear tests for movement restrictions to be lifted. The test interpretation required (“standard” vs “severe”) can vary depending on reactor post-mortem results and whether enhanced measures are being used in the breakdown.
Interferon-gamma blood testing
The interferon-gamma test (or ‘gamma’ test for short) is a supplementary blood test used alongside the tuberculin skin test to maximise the probability of detecting TB-infected animals. It has been used in the UK since 2006.
All new TB breakdowns in the LRA and annual testing parts of the Edge Area in England with lesion and/or culture positive animals are eligible for mandatory gamma testing. In the six-monthly testing parts of the Edge Area and the HRA in England, mandatory gamma testing only applies to TB breakdown herds with lesion and/or culture positive animals that meet certain criteria.
All new TB breakdowns in Wales in the Low TB Area and the Intermediate TB Area North are eligible for mandatory gamma testing. Mandatory gamma testing is also used in herds with longer duration breakdowns (> 18months) or those that recur within 6 months of a clearing test (unless of purchased origin).
Animals less than six months old are excluded from gamma testing as their immune systems are still developing which can interfere with the test and lead to false positive results.
Levels of interferon gamma can be measured, and like the skin test, blood samples from TB-infected animals will release greater amounts in response to bovine tuberculin compared to avian tuberculin.
The sooner the gamma test is deployed, the greater its potential benefits such as reducing the length of time that the herd is under TB restrictions. Where practical, the gamma test is completed before the first short interval test in new TB breakdowns in England. APHA will discuss the timing of gamma testing with the herd owner before arranging the test. If the gamma test is carried out at the same time as the skin test, the blood sample is taken before the injections of tuberculins on day 1 of the test (TT1) or after the reading of the skin test on day 2 of the test (TT2).
There are four possible outcomes of the gamma test. Blood testing may affect the timing of the next short interval test, which will be confirmed by APHA.
If the animal is positive to the test, it is classed as a reactor (gamma reactor). It is compulsorily slaughtered and compensation is paid.
The animal has passed the test.
When a blood sample fails a quality control in the lab and is then re-tested and fails for a second time, the lab will request a new blood sample from the same animal (APHA will advise the herd owner accordingly). On occasions and following a veterinary risk assessment, animals with a re-sample result do not need to be tested again. An animal is only re-sampled once.
The blood sample was not tested by the lab and another blood sample will need to be collected from the animal. Reasons for this include the samples not meeting the required temperature range for testing.
IDEXX Antibody testing
The IDEXX antibody test is another blood test that is used in some TB breakdowns, usually the ones where there is most difficulty in clearing up infection in the herd. This test looks to identify a different immune response to the skin and gamma tests.
Check Tests
Check tests involve a tuberculin skin test of all animals in the herd; calves under 42 days old are normally exempted unless it is considered they are at high risk of infection in a particular herd. Herds with a TB breakdown are required to undergo a regime of skin tests known as “short interval tests” before movement restrictions can be lifted and the herd regains OTF status. Check tests do not usually count as short interval tests for the purpose of lifting the TB movement restrictions.
A skin test may be classified as a “check test” when:
- one or more TB reactors are found during a test that didn’t include the full herd (e.g., a routine herd test conducted in the LRA in England (RHT), a pre-movement test, etc.). A check test of the remaining animals in the affected herd is needed before short interval testing can begin,
- APHA has instructed a herd test pending laboratory results from a carcase that was found with TB-like lesions during routine post-mortem meat inspection in an abattoir (a “slaughterhouse case”,)
- there are discrepancies with British Cattle Movement Service records – e.g. the list of cattle ear tags that a TB tester holds does not match the ear tags of cattle that are presented to test,
- a skin herd test is performed less than 60 days after gamma blood test positive cattle or skin test reactors are removed,
- APHA hasn’t received confirmation that cleansing and disinfection have been completed at the end of a breakdown,
- an inconclusive reactor animal dies or is privately slaughtered before it can be retested.
Useful resources
- Factsheets on the skin test and interferon-gamma blood test
- More information about the tuberculin skin test
- Find out more about the interferon-gamma blood test