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A naturally occurring outbreak of Mycobacterium avium-intracellulare infections in C57BL/6N mice.

Mycobacterium avium-intracellulare was isolated from breeder female C57BL/6N mice with unusual gross and microscopic pulmonary lesions. The C57BL/6N females were being used as the dams in a B6C3F1 production colony, the sires were C3H/HeN mice. A production colony of Fischer 344 rats was housed in the same room. Only the C57BL/6N mice had lesions attributable to the mycobacterium, and the organism was isolated solely from C57BL/6N mice. Infected mice showed no outward signs of disease. Lesions were characterized grossly by raised, tan, subpleural foci in the lungs and microscopically by aggregations of epithelioid cells, foamy macrophages, and variable numbers of lymphocytes. Microgranulomas also were present in the livers, spleens, and mesenteric lymph nodes.