Pathological evaluation of paratuberculosis in naturally infected cattle.

Thirty-two of 51 cattle infected with Mycobacterium paratuberculosis had chronic enteritis, chronic lymphangitis or mesenteric lymphadenopathy, or all three, at slaughter. Granulomatous inflammatory lesions were mild to advanced and predominantly involved the distal small intestine. Rectal involvement was seen only in five cattle. Fourteen had microgranulomas in the liver. There were three cytological forms of macrophages: histiocytic, polygonal and epithelioid. The latter two types had engulfed moderate numbers of acid-fast bacilli. The histiocytic macrophages usually were packed with acid-fast bacilli. Except in the liver and occasionally its nodes, remote lesions of paratuberculosis were not found in other organs. One animal had endocardial and aortic calcifications. Most cattle with signs of diarrhea had globule leukocytes in or around myenteric ganglion cells. The thymus of 3- to 8-year-old cattle with clinical signs frequently had mild to advanced involution. The thymus of similarly aged infected animals without clinical signs, and of paratuberculosis-negative animals, had not involuted.