Laboratory diagnosis of Johne's disease: a potential source of error.

Gastrointestinal tract tissues from cattle with suspected clinical Johne's disease (JD), in which acid-fast bacilli were not identified in mucosal smears, were examined histopathologically. Twenty-two per cent were positive and 4 per cent were suggestive of JD. Failure to identify mycobacteria in mucosal smears appeared to result from the presence of mild, often focal mucosal lesions which contained relatively few bacilli. Ten of the 22 histopathologically positive cases had complement fixation titres for JD but several false-positive titres also occurred.