Atypical presentation of disseminated mycobacteriosis due to Mycobacterium avium in an aged cat

Abstract

In cats, mycobacteriosis tends to present in a syndromic manner, with cases either being due to tuberculosis (TB) (in countries where TB is endemic), one of the "leprosy-like" diseases affecting the skin and subcutis, panniculitis caused by infection of subcutaneous tissues generally with rapidly growing Mycobacteria spp. or widely disseminated granulomatous disease, which is usually caused by members of the Mycobacterium avium-intracellulare complex (MAC). Disseminated MAC disease is rare, but when it occurs, usually develops in immunocompromised hosts with defective cell-mediated immunity. This report describes a case of widely disseminated mycobacteriosis in a 10-year-old American Shorthair cat with an atypical multi-organ distribution including rarely documented thyroid gland involvement. The cat presented for a chronic history of inappetence and weight loss. Abdominal ultrasonography revealed a large mass on the left kidney, and an aspirate (FNA) from this mass showed abundant negative-staining bacilli which were confirmed to be acid-fast with Ziehl-Neelsen (ZN) staining. This was consistent with a mycobacterial aetiology. Necropsy revealed mycobacterial granulomas and/or granulomatous inflammation in the kidneys, thyroid gland, liver, spleen, lungs and left mandibular lymph node, with abundant intralesional acid-fast bacilli in all these tissues. Polymerase chain reaction (PCR) and culture on samples of all affected tissues were positive for M. avium. Collectively, the findings are consistent with disseminated mycobacteriosis due to M. avium with atypical distribution of lesions. Very likely, the cat had underlying immunodeficiency of undetermined cause, exacerbated by the administration of depot corticosteroid.

Keywords: Mycobacterium avium; cats; mycobacteria.

Figure 1

(A) Smears made from a fine needle aspirate (FNA) of the renal mass in a cat with disseminated M. avium infection stained with Modified Wrights stain. Black arrow shows a macrophage containing abundant intracytoplasmic clear staining bacilli. A few neutrophils are also present. (B) A replicate smear of the same FNA stained with a Ziehl–Neelsen (ZN) stain to highlight acid‐fast organisms. Image shows abundant positively stained intrahistiocytic acid‐fast bacilli.

Figure 2

(A) The mandibular lymph node in a cat with disseminated M. avium infection. Black arrow shows the markedly enlarged left mandibular lymph node. (B) The cut surface of the markedly enlarged left mandibular lymph node in a cat with disseminated mycobacteriosis. Black arrow indicates the cut surface. (C) Left and right lobes of the thyroid gland in a cat with disseminated mycobacteriosis. Each thyroid lobe has a moderately demarcated, slightly firm, pale‐yellow nodule (indicated by the black arrows). (D) Abdominal organs in situ in a cat with disseminated mycobacteriosis. Black arrow indicates the left kidney with a large mass. (E). Kidneys after removal at necropsy in a cat with disseminated mycobacteriosis. Black arrow indicates the cut surface of the large mass associated with the left kidney. This mass extends into and effaces the underlying cortex and medulla. (F) High power photomicrograph of the left renal granuloma stained with a ZN stain to highlight acid‐fast bacilli in a cat with disseminated mycobacteriosis. Image shows abundant positively stained intrahistiocytic acid‐fast bacilli in the lesion.

Figure 3

(A) Low power photomicrograph of a renal blood vessel stained with a ZN stain to highlight acid‐fast organisms in a cat with disseminated mycobacteriosis. Black arrow shows a cluster of acid‐fast bacilli within the vascular lumen. (B) Higher power photomicrograph of the same section. Black arrow shows a cluster of acid‐fast bacilli within the vascular lumen.