The histology and ultrastructure of the granulomatous lesions were studied in 18 pigs dosed orally with Mycobacterium intracellulare serotype 8. The pigs were killed 2, 4, 6, 8 and 12 weeks after dosing. Histologically, initial granulomatous lesions were seen in the tonsils 6 and 8 weeks after dosing. Initial or more advanced granulomatous lesions were observed in the tonsils and lymph nodes (mandibular, jejunal, ileocolic and superficial inguinal) 12 weeks after dosing. The initial granulomatous lesions appeared as slight proliferations of epithelioid cells and multinucleated giant cells. Advanced granulomatous lesions were nodular proliferations of epithelioid cells and multinucleated giant cells (epithelioid-cell nodule). In more advanced granulomatous lesions, the epithelioid-cell nodules increased in size and showed caseation necrosis, calcification and fibrosis. Ultrastructural examination revealed that the epithelioid-cell nodules consisted of mononuclear cells, epithelioid cells and multinucleated giant cells. In the caseous foci of the epithelioid-cell nodules, an electron-dense amorphous material with collagen fibres was deposited in the intercellular spaces. Although a small number of acid-fast organisms were observed histologically, they could not be detected ultrastructurally. The histological and ultrastructural findings of the present cases are compared with those of pigs infected with M. avium and with human lesions of tuberculosis and sarcoidosis.
