Early lesion formation in cattle experimentally infected with Mycobacterium bovis.

Early lesion formation was examined in 13 calves inoculated intranasally with 2 x 10(7) colony-forming units of Mycobacterium bovis and killed either singly or in pairs at intervals of < or = 7 days from post-inoculation day (pid) 3 to pid 42. Immunological examinations were carried out before and after infection, and sequential necropsies were performed. M. bovis was recovered as early as pid 3, from the upper respiratory tract mucosae, retropharyngeal lymph nodes and caudal lung lobe. Gross tuberculous lesions were detected in both the upper respiratory tract mucosae and in the lungs of the calves killed from pid 14 onwards. Lesions were also present in the lymph nodes draining these areas. On histological examination, neutrophils appeared to play a key role in the earliest stages of lesion formation, and lesion mineralization was observed for the first time at pid 35. The contemporaneous development of lesions and cellular immunity, as demonstrated by in-vitro lymphocyte proliferation and interferon-gamma assay responses, provided further evidence of the role of immunopathogenic mechanisms in the development of bovine tuberculosis.