Performance of immunohistochemistry vs real-time PCR methods for detecting mycobacterial infections of cattle screened by comparative tuberculin test

In addition to the fifty years since the test-and-slaughter program began in Iran and despite a significant reduction in the disease prevalence, positive tuberculosis cases are still being isolated from livestock farms across the country. Tests with 100% sensitivity and specificity are essential features for bovine tuberculosis diagnosis. The relationship between real-time PCR and immunohistochemistry (IHC) as two essential laboratory methods in the diagnosis of bacterial infections were aimed to evaluate single intradermal comparative tuberculin test (SICTT) results. One hundred thirty-eight cows in two groups were examined: Reactors (108 cows) and clean (as a control group; 30 cows). In the reactor group, 58(54.00%) cows were Mycobacterium bovis positive, 46(43.00%) were Mycobacterium avium subsp. paratuberculosis (MAP) positive, and 11(10.00%) were Mycobacterium tuberculosis positive. 32(55.00%) cows were co-infected with M. bovis and MAP and 5(4.55%) cows were co-infected with Mycobacterium tuberculosis and MAP in this group. Of 50 M. bovis negative cows of reactor group were 14(28.00%) MAP positive and 36(72.00%) negative, as well. Concurrent infection with all was observed in one reactor case. Comparing IHC and real-time PCR for the detection of bovine tuberculosis and Johne’s disease showed very good agreement (Kappa values 0.81 - 1.00). The results also provided further confirmation on IHC and real-time PCR as a sensitive and reliable diagnostic screening approach for detection of bovine tuberculosis. The use of one laboratory method to detect bovine tuberculosis is not sufficient alone.