Evaluation of the specificity of the gamma-interferon test in Italian bovine tuberculosis-free herds

We investigated the specificity of the gamma-interferon test for bovine tub erculosis (TB) in 1557 cattle in 30 paratuberculosis-free and officially ce rtified TB-free dairy herds, located in three provinces of the Lombardy Reg ion in Northern Italy. The TB-free status of the herds under examination wa s further confirmed by the tuberculin skin test, by an antibody assay and b y Post mortem examination of animals culled from the herds during the study period. The specificity of the gamma-interferon tests after a single test and a double sampling scheme were 88.8% and 95.4%, respectively. After a si ngle test, 11.7% of dubious reactors were also detected, while most cattle (47.4%) were shown to be avian reactors, probably due to contamination from infected birds and/or forage. There was strong evidence that the specifici ty of the test could be related to the animals' interaction with environmen tal mycobacteria and/or ageing. To reduce the percentage of nonspecific bov ine reactors under alleged TB-free conditions, test procedures might involv e the use of more specific antigens and/or different reaction thresholds. ( C) 2000 Harcourt Publishers Ltd.