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

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Abstract
The aim of the study reported here was to estimate the sensitivity and specificity of the gamma interferon (IFN-γ) ELISA for paratuberculosis in sheep using receiver-operating characteristic analysis. Bacteriologic culture of tissues was used to define the reference positive population (n = 33). Two reference negative populations were used: culture-negative sheep from infected flocks (n = 77), and sheep from noninfected flocks (n = 358). We also evaluated the accuracy of 2 Mycobacterium avium subspecies paratuberculosis (MAP) antigen preparations, a whole-cell sonicate (MpS) and a johnin purified-protein derivative (PPDj). The source of the reference negative sheep used in the analysis affected overall accuracy of the IFN-γ ELISA. The area under the curve was 0.683 (95% confidence interval 0.574–0.787), using culture-negative sheep from infected flocks, was 0.831 (0.764–0.889), using sheep from noninfected flocks for the MpS, and was 0.809 (0.726–0.881) and 0.897 (0.862–0.925) for the PPDj, respectively. Using the MpS, the cut point that classified the most sheep correctly was an optical density reading of 0.20, for sensitivity of 40.7% (19.4–57.6) and specificity of 88.7% (77.0–95.7) or 97.6% (93.04–99.5), depending on the reference negative population used. Using the PPDj, the cut point that classified the most sheep correctly was 0.25 for sensitivity of 66.7% (47.2–82.7) and specificity of 93.5% (85.5–97.9) or 98.3% (96.4–99.4), respectively. The PPDj was more accurate at identifying MAP-infected sheep than was the MpS (P = 0.034).

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