Infection of a Dog with Mycobacterium avium Serotype II

Abstract

A 7-year-old male, black Labrador Retriever dog had a shifting leg lameness and reduced exercise tolerance for 2 to 3 days. Because of marked splenomegaly a splenectomy was done. The spleen was turgid with round edges and weighed about 4.5 kilograms. Tissue was mottled blue-grey and the cut surface bulged. Lymphoid follicles were not obvious. Ten days after surgery the dog was weak, anorectic and anaemic. The dog was killed and necropsied 3 hours later. The carcass was obese, mucous membranes were blanched, and the blood was watery. The liver was enlarged, mottled brown-red and firm, with sharp notched edges. On section, there were pale tracts about portal triads and darker centrilobular areas. Firm raised irregularly-shaped pink-yellow plaques, 1 to 3 centimeters in diameter, were on the anti-mesenteric serosal surface of the ileum and large intestine. The lesions extended through the serosa into the tunica muscularis and mucosa. Mesenteric, bronchial and peripheral lymph nodes were enlarged, firm, tan-yellow and had no corticomedullary differentiation. The renal capsules adhered to the surfaces of the kidneys, leaving focal pits and depressions when removed. The medulla was congested and pale linear streaks extended through the cortex into the medulla. Glomeruli were prominent. The lungs were congested. The left ventricle of the heart was dilated and the left atrioventricular valves were thickened. Bone marrow was grey-tan. Formalin fixed parts of liver, intestine and mesentery were processed for electron microscopy. With light microscopy there was massive infiltration of most body tissues, especially spleen, bone marrow, lymph nodes, palatine tonsils (fig. l), Peyer’s patches and liver, by macrophages with ovoid large open nuclei, a prominent single nucleolus and copious, finely granular to foamy, grey, cytoplasm. The cytoplasm contained many acid-fast bacilli. Macrophages and bacilli were found free in the efferent lymphatics and sinuses of the tonsils, in subpleural and perivascular lymphatics of the lung, under the endothelium of vessels, within sinusoids and in subcapsular, periacinar (fig. 2), and periportal areas of the liver. They were found occasionally along capillaries in the adrenal and in subcapsular accumulations in the kidneys. The plaques along the intestine contained masses of bacilliladen macrophages extending from the serosa through the tunica muscularis, submucosa and lamina propria. Many plasma cells were in the lamina propria. Peyer’s patches of the ileum and lymphoid follicles of the large intestine were most severely affected. Perivascular cuffing by mononuclear cells and lymphocytes was evident throughout the brain, the leptomeninges and spinal cord. A few glial nodules were randomly distributed throughout the cerebrum, usually associated with blood vessels. Only a few acid-fast bacilli were found free or within mononuclear cells in brain and cord. Electron microscopy showed many bacilli, from 1.1 to 1.3 X 0.2 to 0.5 micrometers, within cytoplasmic vacuoles in macrophages (fig. 3).