Disseminated Mycobacterium Avium Serotype 1 Infection in a Seven-Month-Old Cat

Abstract

A 7-month-old, neutered, male, domestic shorthair cat was presented for examination to a practicing veterinarian after 2 weeks of anorexia and unspecified treatment for “chronic respiratory disease.” Physical and hematologic examinations revealed generalized lymphadenopathy, fever (39.7 C), and a corrected total blood leukocyte count of 17,329/μ1 with 27 nucleated red blood cells/100 leukocytes. Blood packed cell volume was 17%, and plasma contained 6.5 g/d1 total protein. After the animal had shown nonspecific signs for an additional week, smears made from aspirated bone marrow and a popliteal lymph node collected by biopsy and fixed in formalin were referred to the College of Veterinary Medicine at Iowa State University (ISU) for microscopic evaluation, and the cat was returned to its owner. Following a histologic diagnosis of disseminated mycobacterial disease, the cat was rehospitalized at the referring clinic for further observation and treatment. No cutaneous lesions were found. After consultation with the owner, the cat was euthanatized, necropsy was performed by the referring veterinarian, and tissue specimens were collected and submitted to ISU for further laboratory examination. Samples of lungs, liver, spleen, intestine, and tracheobronchial and mesenteric lymph nodes were received both frozen and in formalin. The remaining popliteal lymph node was received frozen, and bone marrow from a femur was received fixed in formalin. Blood had been collected in serum and anticoagulant (ethylenediaminetetraacetic acid) tubes prior to euthanasia. This report describes disseminated Mycobacterium avium serotype 1 infection of a young cat in the absence of cutaneous lesions or evidence of antecedent immunosuppressive disease or treatment. At necropsy, mandibular, ileocecal, and tracheobronchial lymph nodes were reportedly “very enlarged,” and the lungs contained nodular densities throughout. The fixed ileocecal and tracheobronchial lymph nodes and lung nodules measured 1.0, 1.5, and 0.5 cm in diameter, respectively. Bone marrow smears revealed adequate megakaryocytes, myeloid hyperplasia, and erythroid hypoplasia. No atypical cells were noted, but fine, elongate, refractile forms, unstained by Wright’s stain and frequently aggregated in clusters, were seen in the cytoplasm of mononuclear cells and large multinucleate cells (Fig. 1). Acid-fast staining by the Ziehl-Neelsen method (ZN) showed abundant, 4-6 μm, slender, acidfast (beaded) organisms in the same cells. Peripheral blood smears and whole blood revealed anemia (hemoglobin 5.6 g/dl packed cell volume 18%, red blood cell count 2,850,000/ μl), leukopenia (total corrected leukocyte count 4,700/μ1), and lymphopenia (1,175/μ1). Serum contained 6.6 g/d1 of total