Systemic mycobacteriosis caused by 'Mycobacterium avium subspecies hominissuis' in a 14-month-old Japanese black beef steer.

A 14-month-old Japanese black beef steer presented with severe chronic diarrhea and emaciation and was euthanized. Postmortem examination showed thickened and corrugated intestinal mucosa and enlarged granulomatous mesenteric lymph nodes with caseating necrosis. Numerous epithelioid cells and multinucleated giant cells infiltrated in the lamina propria and the submucosal tissue of the intestines. These cells were also observed in the systemic organs. Many acid-fast bacilli were detected in the cytoplasm of these cells and were identified as 'Mycobacterium avium subsp. hominissuis' (Mah) on the basis of the results of molecular examinations and immunohistochemistry. These findings indicate that Mah can cause systemic mycobacteriosis, and this unique infection needs to be distinguished from Johne's disease and tuberculosis in cattle.