Use of ESAT-6 in the interferon-gamma test for diagnosis of bovine tuberculosis following skin testing

The whole blood interferon-gamma (IFN-gamma) test has proven to be a practi cal ancillary test for re-testing cattle for bovine tuberculosis 8-28 days following tuberculin skin testing. An improvement in the specificity of the IFN-gamma test could further reduce calling of false positive animals. The primary aim of this study was to evaluate a single mycobacterial antigen, ESAT-6 in the IFN-gamma test for use in skin test-positive cattle. These sk in test-positive cattle comprised 51 Mycobacterium bovis-infected animals f rom tuberculosis-infected herds and 85 non-infected animals from tuberculos is-free herds. The rest based on ESAT-6 had a higher specificity than the t est based on purified protein derivative (PPD) tuberculin, but this was off set by a small decrease in sensitivity. Use of a lower cut-off in the ESAT- 6-based test improved the sensitivity. while still maintaining a very high specificity. A secondary aim in the study was to assess the ESAT-6 and PPD- based tests for detecting bovine tuberculosis in skin test-negative animals from a persistently infected herd. The PPD-based test detected the majorit y of the lesioned or M. bovis-culture positive animals, while the ESAT-6-ba sed test detected a smaller proportion. The false negatives in the IFN-gamm a test from both the skin test-negative and positive groups were predominan tly M. bovis-culture positive animals with no visible lesions. The current study has shown that a defined specific antigen such as ESAT-6 can markedly improve the specificity of the IFN-gamma test for re-testing skin test-pos itive animals. An ESAT-6-based IFN-gamma test could he particularly useful to reduce the false positive rate, yet still maintain an acceptable level o f sensitivity. (C) 2001 Elsevier Science B.V. All rights reserved.