Experimental infection of weaner sheep with S strain Mycobacterium avium subsp. paratuberculosis

We sought to determine whether infection of recently weaned 12-16-week-old Merino lambs with an Australian S strain M. a. paratuberculosis, at doses consistent with natural exposure, could be detected in the first few months post-inoculation. Such detection would facilitate the use of weaner sheep as sentinel animals for the presence of infectious doses of M. a. paratuberculosis on pastures. In controlled pen trials, oral doses of approximately 10(7)-10(8) viable organisms were demonstrated to be infective, whereas doses below 10(4) organisms failed to produce detectable infection. Mycobacterium avium subsp. paratuberculosis (M. a. paratuberculosis) was isolated from intestinal and/or lymphoid tissues collected at necropsy 7 or 14 weeks after first infection, but there were no associated gross or microscopic lesions. Skin testing with intradermal Johnin detected all three infected lambs at 13 weeks post-infection, and one of the three infected lambs at 6 weeks post-infection, with 100% specificity. Results for whole blood IFN-gamma assay showed some correlation with infection status but lacked specificity. One infected lamb gave a positive result in an ELISA for antibodies to M. a. paratuberculosis, 14 weeks post-infection and 1 week after skin testing. This was the first demonstration of experimental infection with S strain M. a. paratuberculosis in Australian Merino sheep at doses likely to be representative of natural infection. Culture from tissues in the first few months post-exposure could facilitate the use of naive weaner sheep as tracer animals to detect heavy contamination of pastures with M. a. paratuberculosis, but low-level contamination may not be detected in such a system.