Lymphocytosis and Mycobacteriosis in a 15-Year-Old Mixed-Breed Dog

A 15-year-old mixed-breed dog with T-cell lymphoid neoplasia with persistent CD8⁺ lymphocytosis and peripheral lymphadenopathy despite chemotherapy was referred to the Auburn University Veterinary Teaching Hospital Oncology service approximately 7 months after diagnosis. Lymph node aspirates were performed and assessed by a board-certified clinical pathologist, which revealed granulomatous inflammation and intracellular, negative staining bacilli. Bacilli were confirmed Mycobacterium avium and other species within the M. avium complex by polymerase chain reaction (PCR) testing. Chemotherapy was ceased and antimicrobial therapy was initiated; however, the lymphocytosis continued to progress, and the dog began to decline, leading to euthanasia around 2 months after referral. Although T-cell lymphoid neoplasia and mycobacteriosis are rare comorbidities, this case highlights the importance of thorough cytological assessment of lymph node aspirates as part of a complete workup to rule out differential causes of lymphadenopathy. This may have allowed for earlier diagnosis and treatment of mycobacteriosis.