IDEXX laboratories (Maine, USA) have developed a diagnostic blood test for TB in cattle known as the IDEXX antibody test. As the name suggests, it detects specific antibodies produced by the animal in response to infection with Mycobacterium bovis, the bacterium that causes bovine TB. Antibodies are an important part of the host’s immune response to foreign material such as bacteria and viruses. While cellular immune responses tend to predominate in TB-infected cattle, circulating antibodies against M. bovis may also appear, and can be exploited using antibody tests.
The IDEXX antibody test is not yet recognised as an official test for TB in cattle in EU legislation, however it is approved by the World Organisation for Animal Health (WOAH) to supplement the tuberculin skin test in cattle. This means that although it cannot be used as a statutory TB test in Great Britain, the Animal & Plant Health Agency (APHA) can use it in exceptional circumstances with prior consent from the herd owner. In Wales, the IDEXX antibody test is considered a relevant test under TB legislation, which means that APHA does not need permission from the herd owner to use the test or to remove test-positive animals.
It is a third line test used on cattle in chronic TB breakdown herds where repeated tuberculin skin testing and use of the interferon-gamma blood test has already occurred.
The IDEXX antibody test is also one of the three blood tests officially approved for voluntary and statutory TB testing of South American camelids in Great Britain.
How accurate is it?
No diagnostic test is perfectly accurate and there is a trade-off between sensitivity and specificity.
The probability that a test will correctly identify an infected animal as positive. The higher the sensitivity of the test, the lower the probability of incorrectly classifying an infected animal as uninfected i.e. a false negative result.
The probability that a test will correctly identify an animal that is free from infection as negative. The higher the specificity, the lower the probability of incorrectly classifying an uninfected animal as infected i.e. a false positive result.
WOAH data suggest an overall moderate test sensitivity of 65% and a specificity of 98% for the IDEXX antibody test in cattle. To maximise the sensitivity of the test, a prior tuberculin skin test is required to boost M. bovis-specific antibody levels in TB-infected cattle. For this reason, APHA recommends that blood samples for the IDEXX antibody test are taken within 10 to 30 days of a prior skin test. Cattle of any age can be tested.
Data from Great Britain show that antibody tests are less sensitive overall compared with the tuberculin skin test and interferon-gamma blood test. However they can be useful for identifying small numbers of infected cattle that are skin and interferon-gamma test negative; approximately 3-5% of cattle from TB breakdown herds with lesion and/or culture positive animals in Defra study SE3263 “Validation of new serology tests for bovine tuberculosis”.
When is it used?
Use of the IDEXX antibody test by APHA may be considered in multiple-reactor herds with confirmed chronic infection where, despite repeated skin testing and use of the interferon-gamma blood test, the APHA case vet suspects ongoing cattle to cattle transmission. In these cases, residual infection in the herd may be due to infected cattle which produce little or no response to either the skin and interferon-gamma tests, but produce high levels of M. bovis-specific antibody which can be detected by the IDEXX test. The test is used by APHA in exceptional circumstances, and eligibility is determined on a case-by-case basis.
In England only, the IDEXX antibody test is available for private use at the herd owner’s expense, however prior permission from APHA must be obtained by the private vet.
Actions prior to use of the test
APHA-instructed tests
In England, before the IDEXX antibody test can be used, the herd (or management group) in question must have undergone interferon-gamma blood testing if this is required under the current Defra policy.
The herd owner must agree to surrender any test-positive animals for slaughter, with compensation paid by Defra.
In Wales, use of the IDEXX antibody test does not require consent from the herd owner and test-positive animals are removed with compensation paid by the Welsh Government.
Privately-funded tests
In England only, herd owners wishing to use the IDEXX antibody test privately should first discuss this with their private vet who will need to seek 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 10(3) of the Tuberculosis in Animals (England) Order 2021 refers. The private vet must also obtain informed written consent from the herd owner for use of the test.
Before the IDEXX antibody test can be used privately in a TB breakdown herd, the herd (or management group) in question must have undergone interferon-gamma blood testing if this is required under the current Defra policy. For privately-funded IDEXX antibody tests, Defra will not pay compensation for any test-positive animals slaughtered.
Action on receipt of test results
APHA-instructed tests
Where the IDEXX antibody test is instructed by APHA and carried out according to Defra policy, compensation will be payable for the removal of any test-positive animals as normal.
Privately-funded tests
In England only, for privately-funded IDEXX antibody tests, all results must be shared with APHA by the private vet. Any test-positive animals will need to be slaughtered before APHA can lift movement restrictions from the herd and restore its officially TB free (OTF) status. Defra will not remove and pay compensation for any test-positive animals where the test was privately-funded. The removal and slaughter costs are the responsibility of the herd owner.