This is one of the TB cattle control enhancements introduced by Defra in April 2017 following a public consultation.
Spread tracing involves identifying and testing cattle that have moved off a TB breakdown holding that has had TB lesioned and/or culture positive animals. Any cattle that moved off before the application of TB restrictions are traced and, if still alive, TB tested to minimise the risk of disease spread to other herds.
The main test used in Great Britain for detecting TB in cattle is the single intradermal comparative cervical tuberculin test (SICCT), more commonly known as the tuberculin skin test. The skin test in cattle relies on detecting and measuring the immune response of the animal to tuberculin, a protein produced by Mycobacterium bovis, the bacterium that causes bovine TB. The skin test is comparative as the animal’s immune reaction to injections of both bovine and avian tuberculin is measured and compared. Cattle that are infected with Mycobacterium bovis tend to show a greater response to bovine tuberculin than avian tuberculin. Depending on the degree of reaction to the skin test and the interpretation of the test, the result is a pass (clear), fail (the animal is a reactor) or inconclusive (the animal is an inconclusive reactor and a re-test is required). No diagnostic test is perfectly accurate, and there is a trade-off between sensitivity and specificity.
The ability of a test to 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 (a false negative result).
The ability of a test to 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 (a false positive result).
Studies have shown that the tuberculin skin test used in Great Britain is on average about 80% sensitive at standard interpretation and slightly higher at severe interpretation. In practical terms this means that the skin test misses 20-25% of TB-infected cattle when standard interpretation is used.
Interpreting the test at severe interpretation entails lowering the positive cut-off point so that some animals normally classified as inconclusive reactors (IRs) at the standard interpretation become reactors. This increases the sensitivity of the test (the probability of correctly identifying truly infected animals as test positives) over the normal standard interpretation. Whilst the severe interpretation slightly increases the likelihood of false positive test results the consequences of missing infected animals from herds with known TB infection far outweigh the costs and other implications of false positive results.
The control measure
Initial skin tests of cattle in TB breakdown herds are read using severe interpretation, but cattle traced from these herds were previously tested using standard interpretation.
It is inconsistent to apply standard interpretation to cattle traced from a herd that is undergoing breakdown testing at severe interpretation.
The measure is therefore to use the severe interpretation of the test for all spread tracings in England from TB breakdown herds to reduce the possibility of missing infected animals. This measure also applies to animals traced to holdings in England from TB breakdown herds in Wales and Scotland.
Benefit of the measure
The likelihood of missing infected animals traced from TB breakdown herds is reduced, as is the risk of disease spread to other herds. This is particularly important for animals traced from TB breakdown herds to herds in the Low Risk Area (LRA), as the destination herd may go untested for up to four years.
Implementation
Trace tests are often carried out with other tests (for example herd tests) to save time, labour and veterinary costs. Most routine herd tests are read at standard interpretation and the measure requires all trace tests to be read at severe. If trace tested animals are included in a herd test and submitted on the same test chart, APHA retrospectively applies severe interpretation to the individual traced animals when the test chart is reviewed.
A proportion of animals that are IRs at standard interpretation will become reactors when severe interpretation is applied. If a traced animal is disclosed as an IR, cattle keepers are advised to ask the person carrying out the test whether it would become a reactor at re-interpretation. If a trace test discloses reactors or IRs that subsequently become reactors after re-interpretation, movement restrictions are applied to the holding and normal TB breakdown procedures applied. If the trace tested animals were tested as part of a herd test, the whole test is re-interpreted at severe interpretation in line with normal TB breakdown procedures.
Q&A
Severe interpretation of the skin test is used for all spread tracings from TB breakdown herds to reduce the possibility of missing infected animals. This measure also applies to animals traced to holdings in England from TB breakdown herds in Wales and Scotland.
Initial skin tests of cattle on TB breakdown holdings are read using severe interpretation, but cattle traced from these herds were previously tested using standard interpretation. It is inconsistent to apply the standard interpretation to cattle traced from a herd that is undergoing breakdown testing at severe interpretation. By using severe interpretation, the cut off point for a positive result is lowered, thereby increasing the likelihood of detecting infected animals.
Severe interpretation of the skin test is used for all spread tracings from TB breakdown herds with lesion/culture positive animals.
If a tracer animal is tested as part of a herd test read at standard interpretation, APHA retrospectively applies severe interpretation to the tracer animal.
A proportion of animals that are IRs at standard interpretation will become reactors when severe interpretation is applied. If a traced animal is disclosed as an IR, cattle keepers are advised to ask the person carrying out the test whether it would become a reactor at re-interpretation. If an IR becomes a reactor at re-interpretation, the animal will be compulsorily slaughtered, movement restrictions will be applied to the holding and normal TB breakdown procedures followed.
If an IR is disclosed at a skin test and then subsequently becomes eligible for a trace test, the following will apply depending on the timing. If the IR has already re-tested clear at the time that it is identified as a traced animal, it will not be retrospectively re-interpreted at severe. If the IR is pending re-test at the time that it is identified as a traced animal, the disclosing test (the test where it was first identified as an IR) will be retrospectively re-interpreted at severe for that individual animal. If the IR becomes a reactor after re-interpretation, it will be compulsorily slaughtered, the herd will be placed under movement restrictions and normal TB breakdown procedures followed.