Field evaluation of three blood-based assays for elk (Cervus canadensis) naturally infected with Mycobacterium bovis

Diagnosis of Mycobacterium bovis in wild populations is very challenging due to complications imposed by the use of traditional skin tests, poor sensitivity of gold standard tests which rely on culture of M. bovis from tissues and wide variations in severity of disease. Various combinations of a lymphocyte stimulation test (LST), fluorescence polarization assay (FPA) and the Cervid TB Stat-Pak were evaluated using two different validation approaches: a latent class analysis and classical statistical approach using culture as a gold standard. A validation subsample consisting of animals culled for population control and mortalities from capture provided an unbiased estimate of test performance for comparison. The sensitivity of the LST (0.83, 95% CI: [0.70-0.97] as a single test was similar to existing tuberculin skin tests, but the sensitivity of the FPA (0.40, 95% CI: [0.22-0.58]) and Cervid TB Stat-Pak (0.62, 95% CI: [0.41-0.83]) were lower in this population. Test performance of the LST and Cervid TB Stat-Pak in parallel was similar to the use of all three tests in parallel and inclusion of the FPA did not greatly enhance test performance. Prevalence of M. bovis in elk varied substantially between the high risk area of southern Manitoba (9.1%, 95% CI: [6.09-12.1%]) and lower risk areas outside this zone (0.76%, 95% CI: [0-2.26%]). Bayesian latent class analysis indicated lack of covariance between the two antibody tests (FPA and Cervid TB Stat-Pak) while the classical two-stage analysis indicated there was conditional dependence between the tests. All three tests when used in parallel resulted in 100% NPV using all three validation methods, indicating few elk were misclassified as false negative by post mortem culture. Similar to previous studies, this study found that combinations of blood tests that utilize cell mediated responses along with humoral antibody responses maximize the sensitivity of tests for diagnosis of M. bovis in wild cervid populations.