Isolation of mycobacteria inducing cross-reactions in the complement fixation test for Johne’s disease

Eight bulls which gave positive reactions in the complement fixation test for Johne’s disease were identified in a group of 29 bulls which had previously given only negative reactions at repeated annual tests. One reacting bull was confirmed to be infected with Mycobacterium paratuberculosis at autopsy, but another reactor which was slaughtered had no visible lesions and mycobacteria were not recovered in culture. Faecal samples from seven further bulls (six reactors and one non-reactor) yielded mycobacteria which were not dependent on mycobactin for growth. When one of these strains was injected into two calves they developed positive reactions in the complement fixation test for Johne’s disease and to intradermal skin testing with purified protein derivatives of M avium, M paratuberculosis and M bovis. The strains did not satisfy the exact requirements for classification into any recognised mycobacterial species.