This is one of the TB cattle control enhancements being introduced by Defra in April 2017 following a public consultation completed last year.

Background

Tracing

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.

TB testing

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 explained below.

Sensitivity – the ability of a test to correctly identify an infected animal as positive, i.e. the higher the sensitivity of the test, the lower the probability of incorrectly classifying an infected animal as uninfected (a false negative result).

Specificity – the ability of a test to correctly identify an animal that is free from infection as negative, i.e. 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 (IR) 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 new measure

Initial skin tests of cattle in TB breakdown herds are read using severe interpretation, but cattle traced from these herds are normally 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.

The new 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 will also apply to animals traced to holdings in England from TB breakdown herds in Wales and Scotland.

Benefit

The likelihood of missing infected animals traced from TB breakdown herds will be reduced, as will 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.

Implementing the measure

Please note that 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 new measure requires all trace tests to be read at severe interpretation. If trace tested animals are included in a herd test and submitted on the same test chart, APHA will retrospectively apply severe interpretation to the individual traced animals when the test chart is reviewed.

It is important to note that 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 will be applied to the holding and normal TB breakdown procedures will be 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.

This new measure will be communicated to Official Veterinarians (OVs) to ensure they are aware of the policy change and the implications for reading TB tests and submitting test charts to APHA.

Timing of implementation

This measure will be applied to all traced animals identified in England from 1st April 2017.

Q&A

What is the policy change?

Severe interpretation of the skin test will be used for all spread tracings from TB breakdown herds to reduce the possibility of missing infected animals. This measure will also apply to animals traced to holdings in England from TB breakdown herds in Wales and Scotland.

Why is this control measure being introduced?

Initial skin tests of cattle on TB breakdown holdings are read using severe interpretation, but cattle traced from these herds are normally 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.

When will this control measure be introduced?

This measure will be applied to all trace tests in England from 1st April 2017.

Which tests will be affected by this control measure?

Severe interpretation of the skin test will be used for all spread tracings from TB breakdown herds with lesion/culture positive animals.

What happens if my vet tests a tracer animal at the same time as another test?

If a tracer animal is tested as part of a herd test read at standard interpretation, APHA will retrospectively apply severe interpretation to the tracer animal.

What happens if a tracer animal is disclosed as an IR at standard interpretation at a herd test, and is then subsequently re-interpreted at severe by APHA?

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.

What happens if an IR is disclosed at a test and is then subsequently identified as a tracer animal?

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 will be followed.