Characteristics of Mycobacterium bovis infected herds tested with the interferon-gamma assay

The IFN-gamma (interferon gamma) assay is used in Ireland as an ancillary diagnostic test to the single intradermal comparative tuberculin test (SICTT) to maximise the detection of Mycobacterium bovis infected animals (bTB) in cattle herds. Understanding the relationships between herd and animal risk factors and IFN-gamma test results is critical to enable the development and evaluation of policy measures on how best to use the test. In this study, we set out to characterise Irish herds with IFN-gamma test positive animals in terms of herd size, number of SICTT reactors and number of IFN-gamma positive tests, and to evaluate the IFN-gamma test in terms of the test cut-off values. The results showed that larger herds with more SICTT reactors were likely to have more IFN-gamma positives in the herd, and herds with an IFN-gamma test positive animal that was also positive for bTB lesions at post-mortem had higher numbers of IFN-gamma positive animals in the herd. Raising the cut-off values for the IFN-gamma test only marginally decreased the combined sensitivity of the IFN-gamma and the SICTT for diagnosis of bTB lesioned animals. The analysis has provided valuable information on the performance of the IFN-gamma test as it is used under current bTB infection levels in Ireland.