Application of the direct leukocyte-migration agarose test in cattle from a Mycobacterium paratuberculosis-infected herd.

Using johnin as the specific antigen, the direct leukocyte-migration agarose test was applied to 10 cows from a Mycobacterium paratuberculosis-infected herd. Nine of the cows did not have signs of paratuberculosis; 1 cow was clinically affected. The cows were slaughtered and, on the basis of histopathologic and cultural examinations of the intestines, were classified as infected or noninfected. Six of the healthy cows from which the bacterium was not isolated and the clinically affected cow reacted with migration indices lower than those of 3 healthy animals that were found to harbor the bacterium. It is suggested that the long incubation period of paratuberculosis could to some extent be due to immunologic tolerance.