Why the vets trust in TB test for cattle
IN response to Dishonesty on TB is diabolical (Star, Sept 18).
As one of the few local vets actively involved in TB testing cattle in this area, I must totally refute the statement by Mr Harris of Carmel that 90 per cent of cattle slaughtered for TB do not have the disease. This is absolutely and totally untrue.
The testing method employed has been in continuous use for more than 50 years and in its initial stages of use enabled us to reduce the incidence of TB in cattle to less than 4 per cent of the UK national herds. It is widely acknowledged by all interested parties that this test shows an accuracy of 99.9 per cent. This means that of every thousand cattle indicated by this test to have TB only one may be taken as a false positive. Moreover it has to and successfully does differentiate between cattle showing an immune response to avian tuberculosis which is of no zoonotic (human health) or financial interest and mammalian tuberculosis which has a massive implication in both these areas. Simply explained, cattle are injected on day one with both types of purified avian and bovine tuberculin at a site on the cow's neck where a positive immune response produces a swelling at each site, which is examined and measured three days later. The relative increases in the swellings are compared on a table from which a diagnosis of infection with bovine tuberculosis is made.
Due to the sensitivity of the test and its response to immune stimulation, Mr Harris is correct that many cattle slaughtered as a result of an intra-dermal skin test reaction do not show visible signs of the condition at post-mortem. All cattle in Wales are tested for TB annually and on infected farms the testing interval is aimed to be as close to 60 days as possible. This very proactive testing protocol means nearly all cattle taken from infected farms do not have time to develop clinical signs or detectable post-mortem lesions.
I can assure Mr Harris that as a vet directly involved in diagnosing and removing infected cattle that I would and could not do this if I believed the test I was using was only 10 per cent accurate.
Richard M Gibson