We have had long-awaited news from APHA that they will begin their rollout of using the Polymerase Chain Reaction (PCR) test, in place of standard culture when confirming a TB diagnosis from lesions at slaughter. This should be a step forward in reducing the time taken for a decision to be made on any further disease restrictions.
What is a PCR test?
Each M. bovis bacterium contains unique DNA. PCR is a laboratory test which identifies tiny amounts of DNA present within M. bovis bacteria and amplifies it (by making millions of copies of the specific DNA sequence) to produce a quantity which is then detectable.
How will the new test be used by APHA?
From 30 March the PCR test will be used for tissue samples from carcases of:
- TB slaughterhouse cases in cattle and non-bovines i.e. animals routinely sent for private slaughter with suspicious lesions of TB identified
- Non-bovine animals such as camelids, goats, pigs, sheep and farmed deer that are removed as TB test reactors, direct contacts or clinical TB suspects
- Domestic pets (cats and dogs) and exotic species of animals (e.g. in zoos) submitted to APHA for laboratory investigation
How does the accuracy of the PCR test compare with microbiological culture?
In the test validation study, the M. bovis PCR test produced equivalent results to the traditional microbiological culture method.
How will the PCR test improve the management of TB incidents?
When a bovine slaughterhouse case is identified, the herd of origin’s officially TB free status is suspended pending the final test results from the laboratory. Animals can only move on or off the affected holding under a licence issued by APHA. Currently, tissue samples taken from the carcase are submitted for microbiological culture – that process can take up to 22 weeks to complete and during that time the herd remains under movement restrictions.
Rather than relying on culture results PCR testing will be used. The process will be much quicker – with test results typically available within three weeks.
If the test results are negative restrictions on the herd will be lifted thus limiting the negative impact of movement restrictions.
In most cases it will also eliminate the need for a check test – currently herd check tests (using the skin test) may be carried out pending the receipt of culture results.
The new PCR test will also allow rapid detection of M. bovis in tissue samples from carcases of non-bovine animals, to confirm or negate infection in TB test positive animals, suspected cases of TB identified at post-mortem examination in the laboratory, or during routine post-mortem meat inspection in the slaughterhouse.
Are there any limitations of the M. bovis PCR test?
Whole genome sequencing is not possible by this method, so further culture would be required if APHA wish to obtain useful information on the particular strain of TB, in order to help unravel the epidemiology of a breakdown.
Further information about the M. bovis PCR test is available on the TB Hub site:
https://tbhub.co.uk/tb-testing-cattle/pcr-test-for-detection-of-m-bovis-in-post-mortem-tissue-samples/
Paula Hunt – Synergy Farm Health, Tb Testing Veterinary Surgeon BVSc MRCVS
