Atypical presentation of disseminated mycobacteriosis due to <i>Mycobacterium avium</i> in an aged cat

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.