EVALUATION OF AN ENZYME-LINKED-IMMUNOSORBENT-ASSAY LICENSED BY THE USDA FOR USE IN CATTLE FOR DIAGNOSIS OF OVINE PARATUBERCULOSIS

A commercially available Mycobacterium phlei-absorbed enzyme-linked im munosorbent assay (ELISA) approved to detect antibodies to Mycobacteri um paratuberculosis in cattle was evaluated for its applicability in s heep. The potential for interference with ELISA results from cross-rea cting antibodies to Corynebacterium pseudotuberculosis was also invest igated. Serum samples were randomly selected from a collection of samp les obtained in 1986-1991 from 6 infected and 5 noninfected sheep floc ks varying in breed, age, and geographic origin. Tests were performed on sera from 27 paratuberculous sheep, confirmed by histopathology, ba cteriologic culture, and/or acid-fast staining of ileal mucosal smears , and on sera from 246 noninfected sheep. The optical density of each sample was expressed as a percentage of the optical density of a known positive sheep serum sample tested on the same plate. These values we re log-transformed to achieve normality of distribution, and sensitivi ty and specificity estimates were calculated based on 2 and 3 standard deviations above the mean of the percent positive value (PPV) of the noninfected sheep. A cutoff value of PPV greater than or equal to 55.7 4 resulted in an estimated sensitivity of 0.48 and a specificity of 0. 95. Sera from 10 noninfected sheep with PPV above the cutoff level of 55.74% were absorbed with heat-treated C. pseudotuberculosis organisms in addition to M. phlei antigens. Sera from 14 ELISA-positive paratub erculous sheep and 23 ELISA-negative noninfected sheep were similarly treated, and results were compared. Absorption with C. pseudotuberculo sis resulted in a significant decrease in PPV in all 3 groups of sheep sera, but a greater decrease was observed in the noninfected sheep wi th PPV above the cutoff level when compared with noninfected sheep wit h PPV below that level. Results of this study suggest that ELISA may b e of value in screening sheep flocks for paratuberculosis, but further experimentation is needed to optimize the sensitivity and specificity of the assay. Exposure to C. pseudotuberculosis may confound results obtained by M. phlei-absorbed ELISA for paratuberculosis.