Granuloma development in cattle after intratonsilar inoculation with Mycobacterium bovis

Objective-To examine the temporal development of tuberculous lesions in cat tle 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 wer e examined for gross and microscopic lesions and processed for isolation of M bovis. Results-Retropharyngeal lymph nodes from cattle examined 4 weeks after inoc ulation contained microgranulomas consisting of aggregates of macrophages w ith few neutrophils. Retropharyngeal lymph nodes from all cattle examined 6 and 8 weeks after inoculation contained multiple, large, coalescing granul omas consisting of central areas of necrosis with mild fibrosis, numerous m acrophages, lymphocytes, plasma cells, multinucleated giant coils, and neut rophils. Three of 8 cattle examined 6 or 8 weeks after inoculation had lesi ons 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 l ittle as 4 weeks after inoculation via intratonsilar instillation of M bovi s. Intralesional morphologic changes between 4 and 6 weeks after inoculatio n indicate an increase in cellular chemotaxis and differentiation. Dissemin ation of bacteria to distant sites most likely was by lymphatic and hematog enous routes after establishment of the primary infection in retropharyngea l lymph nodes.