Under EU regulations, the tuberculin skin test is (and is likely to continue to be) the primary diagnostic test for TB in live cattle in the field. We foresee only a minor, if any, role for non-immunological assays in the screening of cattle populations for TB.
DEFRA has been funding work using the polymerase chain reaction (PCR) technique to develop tests for Mycobacterium bovis (M. bovis) since 1999. This has included work to develop a reliable and rapid bovine TB screening test that can detect the presence of M. bovis DMA in infected cattle tissues. Work to date shows the value of the application of PCR techniques in certain situations, for example in cattle tissue samples from suspect cases of TB disclosed at routine slaughter, where the speed of the result is of importance.
The use of automated PCR machines has been trialled by the Veterinary Laboratories Agency for use in routine detection of M. bovis in a range of bovine tissue samples in the laboratory. A review of this and its incorporation into routine laboratory diagnostic techniques is currently being planned. However, PCR is not yet as sensitive, specific or reliable as conventional bacterial culture in detecting TB.
DEFRA is investing £1.3 million on work on PCR over the next 3 years that will allow us to tell the difference between M. bovis and similar species from environmental samples. This is not likely to be available as a field test in the short term.
No reverse transcription stage is required for the detection of M. bovis organisms by PCR, as DMA (not RNA) is the constitutive nucleic acid in the genome of mycobacteria.