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Note: This note is intended to act as a guide to raise awareness of the planned changes to TB breakdown testing. It does not constitute legal advice, nor is it a substitute for obtaining legal advice.
Q1 What will change and when?
A1. From 6 April 2016 all new breakdown herds in the HRA, regardless of post-mortem or laboratory culture results, will require two consecutive short interval (60-day) herd tests with negative results read under ‘severe’ interpretation. This will be applied at the beginning of the breakdown, i.e. the first two tests after the herd is placed under TB movement restrictions.
The results of the test that disclosed the initial reactor(s) in that herd will also be re-assessed by APHA under the ‘severe’ interpretation. Thereafter, any further tests that might be necessary until the herd movement restrictions are lifted will be read under ‘standard’ interpretation, provided that no post-mortem or laboratory evidence of TB has been found in that herd.
Q2 Why are you changing the current approach?
A2. To minimise the risk of lifting restrictions on TB-infected herds prematurely, i.e. before all infected cattle have been identified and removed. There is a high TB recurrence rate for breakdown herds in the High Risk Area (HRA) of England. This is due in part to the imperfect sensitivity of the skin test. By completing a minimum of two herd tests instead of one, and applying a more rigorous interpretation in those tests we increase the sensitivity of the TB testing protocol and reduce the risk of leaving infected animals behind.
This has been the normal approach for many years in all TB breakdown herds with lesion or culture-positive animals and for all TB breakdowns in the Edge Area of England since 2014. We are now extending this to lesion- and culture-negative TB breakdown herds in the HRA as well, because studies have shown that in the higher TB prevalence areas of GB the majority of such herds do represent true infections with the bovine TB bacterium.
Q3 What is the difference between ‘severe’ and ‘standard’ interpretation?
A3. Severe interpretation means lowering the positive cut-off point of the tuberculin skin test. As a consequence, some animals that would be normally deemed ‘inconclusive’ at the standard (routine) interpretation of the test are classified as reactors on the severe interpretation and removed. The use of a more rigorous interpretation of the skin test in specific circumstances is provided for in the EU bovine TB legislation.
Q4 Why is the same approach not being taken in the Low Risk Area (LRA) of England too?
A4. TB breakdown herds in the LRA with neither post-mortem nor laboratory evidence of disease will continue to require one short interval skin test with negative results at standard interpretation before restrictions are lifted. Although the specificity of the tuberculin skin test is very high (only one false positive result expected for every 5,000 to 6,000 TB free cattle tested), in the very low TB prevalence areas of England there is a higher probability that lesion and culture-negative reactors are not true cases of TB.
However, herds in the LRA that suffer a TB breakdown and are contiguous to another TB breakdown herd with post-mortem evidence of TB, or had a history of any type of TB breakdown in the previous four years, or are permanently subject to annual TB surveillance testing by virtue of their higher risk business pattern, will also require two consecutive short interval tests with negative results at severe interpretation.