Clinical Exposures: Mycobacterium avium infection in a dog
Jan 01, 2006
Oral tetracycline was prescribed because subacute or chronic ehrlichiosis and Rocky Mountain spotted fever were differential diagnoses. The dog developed diarrhea five days after the tetracycline therapy was initiated, so tetracycline was discontinued and chloramphenicol therapy was begun. Two days later, the diarrhea had worsened. Serology results revealed a low antibody titer (1:64) for Rickettsia rickettsii antigens. Chloramphenicol was discontinued, and oral tetracycline therapy was reinstituted.
A wedge-section of prescapular lymph node was submitted for histopathologic examination, and the test results were consistent with mycobacteriosis. The dog's condition deteriorated quickly. The dog had developed rapidly progressive neurologic signs (stilted gait, hyperexcitability, hypermetria, muscle spasm followed by short episodes of ventroflexion and lateral recumbency). Because the prognosis was poor, the owner elected euthanasia.Necropsy and laboratory findings
Necropsy revealed that the dog was in good body condition. The tonsils (Figure 1) and peripheral and visceral lymph nodes were markedly enlarged and beige. On cut section, the nodes lacked corticomedullary differentiation. Mesenteric lymph nodes (Figure 2) were the largest, measuring up to 30 x 5 x 3 cm; mandibular lymph nodes measured 15 x 4 x 3 cm. The spleen was markedly enlarged and pink and contained numerous yellow-tan to off-white foci and nodules measuring 1 mm to 1 cm (Figure 3). Peyer's patches were prominent and white (Figure 4). The fat in the marrow of the long bones was replaced by pink tissue. The cranioventral lung lobes were wet, had a meaty consistency, and were mottled pink.
A histologic examination of the cerebrum, cerebellum, and brainstem revealed multifocal granulomatous meningoencephalitis of the cerebellum and brainstem. No lesions were observed in the cerebrum. The spinal cord was not examined because the owner requested a cosmetic necropsy. On histologic examination, the parenchyma of the bone marrow, lymph nodes, Peyer's patches, and spleen was replaced by diffuse granulomatous inflammation composed of sheets of large macrophages distended with acid-fast bacilli (Figure 5). The yellow-tan and off-white foci and nodules in the spleen were granulomas with necrotic centers surrounded by neutrophils, macrophages, and giant cells. Edema and mild multifocal granulomatous pneumonia had caused the gross changes in the lungs. Acid-fast stains were performed on impression smears of the spleen taken at necropsy and paraffin-embedded sections of lymph nodes because macrophages in all other organ lesions resembled those seen in the spleen and lymph nodes. Severe diffuse granulomatous enterocolitis affecting the jejunum, ileum, and colon had caused the recent onset of diarrhea.
Aseptically procured swabs of the spleen and lymph nodes were streaked onto Lowenstein-Jensen culture medium, and the isolate was submitted for identification.* The isolate, identified as Mycobacterium avium, was then forwarded to the National Veterinary Services Laboratories in Ames, Iowa, for serotype identification. The infectious agent was determined to be M. avium subspecies avium serotype 1.
Disseminated M. avium infections in dogs are rare.1 Basset hounds and miniature schnauzers appear to be predisposed to the infection, although other breeds are affected. Most reported cases have involved dogs 2 to 4 years of age.1
*The Commonwealth of Virginia, Department of General Services, Division of Consolidated Laboratory Services, Bureau of Microbiological Science Laboratory in Richmond, Va.