Granuloma development in cattle after intratonsilar inoculation with Mycobacterium bovis.

OBJECTIVE: To examine the temporal development of tuberculous lesions in cattle inoculated with Mycobacterium bovis. ANIMALS: 15 mature crossbred cows obtained from a herd with no history of M bovis infection. PROCEDURE: Inoculation of cattle was done by intratonsilar instillation of 1.48 X 10(5) to 5.4 X 10(7) colony-forming units of M bovis strain 2045T. At 3 to 4 hours, 4 weeks, 6 weeks, and 8 weeks after inoculation, tissues were examined for gross and microscopic lesions and processed for isolation of M bovis. RESULTS: Retropharyngeal lymph nodes from cattle examined 4 weeks after inoculation contained microgranulomas consisting of aggregates of macrophages with few neutrophils. Retropharyngeal lymph nodes from all cattle examined 6 and 8 weeks after inoculation contained multiple, large, coalescing granulomas consisting of central areas of necrosis with mild fibrosis, numerous macrophages, lymphocytes, plasma cells, multinucleated giant cells, and neutrophils. Three of 8 cattle examined 6 or 8 weeks after inoculation had lesions in nonretropharyngeal sites with morphologic characteristics similar to that seen in retropharyngeal lymph node granulomas from cattle examined 4 weeks after inoculation. CONCLUSION: Granulomas can develop in draining lymph nodes of cattle in as little as 4 weeks after inoculation via intratonsilar instillation of M bovis. Intralesional morphologic changes between 4 and 6 weeks after inoculation indicate an increase in cellular chemotaxis and differentiation. Dissemination of bacteria to distant sites most likely was by lymphatic and hematogenous routes after establishment of the primary infection in retropharyngeal lymph nodes.