Submitted to: Journal of Veterinary Diagnostic Investigation
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/1/2005
Publication Date: 3/1/2006
Citation: Robbe Austerman, S., Stabel, J.R., Palmer, M.V. 2006. Evaluation of the gamma interferon ELISA in sheep subclinically infected with Mycobacterium avium subspecies paratuberculosis using a whole cell sonicate or a johnin purified-protein derivative. Journal of Veterinary Diagnostic Investigation. 18(2):189-194. Interpretive Summary: The aim of this study was to evaluate the gamma interferon ELISA for Johne’s disease in sheep. Tissue culture was used to determine whether they were infected or non-infected, but a separate population of Johne’s disease free sheep was also used to compare against the tissue culture negative sheep from infected flocks. This study also evaluated the accuracy of 2 different antigen preparations used in the test, a whole cell sonicate and a johnin PPD. The test was more accurate using sheep from Johne’s free flocks as known negative sheep rather than sheep that were just tissue culture negative. This may have implications in the usefulness of the test. The johnin PPD was more accurate than the whole cell sonicate.
Technical Abstract: The aim of this study was to evaluate the gamma interferon ELISA for paratuberculosis in sheep using receiver-operating characteristic analysis. Tissue culture was used as the reference test, but with two different negative populations: sheep from the same infected flocks but with tissue culture negative results, and then with sheep from flocks without paratuberculosis. This study also evaluated the accuracy of 2 different antigen preparations, a whole cell sonicate and a johnin PPD. Changing the negative sheep used in the analysis affected overall accuracy of the ELISA. The area under the curve was 0.809 (95% CI=0.726 - 0.881) using tissue culture negative sheep and 0.897 (0.862-0.925) using sheep from non-infected populations for the Johnin PPD and was 0.683 (0.574-0.787) and 0.831 (0.764-0.889) for the whole cell sonicate. Using the Johnin PPD, the cut point that classified the most sheep correctly was an optical density reading of 0.25 for a sensitivity of 66.7% (95% confidence interval 47.2-82.7) and a specificity of 93.5% (85.5-97.9) using tissue culture negative sheep and 98.3% (96.4-99.4) using sheep from non-infected populations. Using the whole cell sonicate, the cut point that classified the most sheep correctly was 0.20 for a sensitivity of 40.7% (19.4 - 57.6) and a specificity of 88.7% (77.0-95.7) using tissue culture negative sheep and 97.6% (93.04-99.5) using sheep from non-infected populations. The johnin PPD was more accurate than the whole cell sonicate, p value=0.034.