Quadruple co-infection with Mycobacterium tuberculosis complex and M. kansasii in a chronically immunosuppressed cat.

An 8-year-old spayed female American Shorthair cat was evaluated for refractory perioral nodules and respiratory distress persisting for 9 months. The patient had a history of feline atopy and chronic gingivostomatitis managed with long-term immunosuppressive therapy, including cyclosporine A and prednisolone. Pre-surgical computed tomography (CT) revealed a 2.1 cm nasolabial mass without osteolysis, later diagnosed as a mycobacterial infection. This unusual infection was tentatively attributed to pharmacological suppression of cell-mediated immunity, potentially impairing both granuloma formation and inflammatory bone resorption. Fine-needle aspiration (FNA) and Ziehl-Neelsen (ZN) staining demonstrated a high burden of acid-fast bacilli (AFB). However, conventional cultures, including inoculation on 3% Ogawa medium, failed due to rapid overgrowth of Klebsiella pneumoniae (K. pneumoniae). Post-mortem multi-target real-time PCR and whole genome sequencing (WGS), together with internal transcribed spacer (ITS) sequencing and phenotypic antimicrobial testing, identified a fatal co-infection comprising Mycobacterium tuberculosis complex (MTBC), Mycobacterium kansasii, K. pneumoniae, and multidrug-resistant Pseudomonas aeruginosa. This report describes a rare feline case of a fatal polymicrobial infection involving both MTBC and non-tuberculous mycobacteria (NTM) in conjunction with opportunistic gram-negative pathogens. The findings suggest that chronic iatrogenic immunosuppression may alter or obscure classical diagnostic features of mycobacteriosis and facilitate complex co-infections. This case highlights the importance of high-sensitivity molecular diagnostics and suggests that immunocompromised companion animals may, in rare instances, serve as cautious indicators of possible household mycobacterial exposure within a One Health framework.