Objective: To determine whether an interferon (IFN)-γ response sufficient to categorize cattle as positive for tuberculosis can be detected in blood collected at commencement of exsanguination at slaughter.
Animals: 15 Holstein cows.
Procedures: 12 cows were experimentally sensitized by SC injection with inactivated Mycobacterium bovis in mineral oil, which induced an immune response that mimicked natural infection with M bovis. Three nonsensitized control cows were injected SC with mineral oil alone. By 5 weeks after injection, only the 12 sensitized cows had positive results for tuberculosis with whole blood IFN-γ assay. At that time, all 15 cows were sent to slaughter and samples of blood were collected from each cow immediately before stunning and at commencement of exsanguination (within 90 seconds after stunning). A whole blood IFN-γ assay was performed on the samples. Conditional probability and paired t tests were used to analyze changes in the categorical test interpretation and qualitative IFN-γ production, respectively.
Results: All 12 sensitized cows had positive results for tuberculosis in samples obtained immediately before stunning, and 9 retained positive results for samples obtained at commencement of exsanguination. There was a significant decrease in the mean background-corrected IFN-γ ELISA optical density values for samples obtained at commencement of exsanguination.
Conclusions and clinical relevance: IFN-γ response sufficient to classify cattle as positive for tuberculosis could be detected in blood collected at commencement of exsanguination. These findings support further development and use of the IFN-γ assay on blood samples collected at exsanguination as part of a bovine tuberculosis surveillance program.