Mycobacterium fortuitum Necrotizing Cellulitis after Dog Bite in a Pediatric Patient: a case report

Background Mycobacterium fortuitum ( M. fortuitum ), a rapidly growing nontuberculous mycobacterium (NTM), is an emerging pathogen in trauma-associated skin and soft tissue infections (SSTIs). Pediatric NTM infections are rare and often misdiagnosed due to atypical presentations and intrinsic β-lactam resistance. We report the first pediatric case of M. fortuitum necrotizing cellulitis following a dog bite in an 8-year-old child, highlighting diagnostic and management challenges. Case presentation : An 8-year-old child presented with necrotizing cellulitis following a dog bite, which was refractory to empiric β-lactam therapy. Histopathology revealed nodular granulomatous inflammation with epithelioid cells and purulent necrosis, and acid-fast bacilli were identified via Ziehl-Neelsen staining. Metagenomic next-generation sequencing (mNGS) rapidly identified M. fortuitum within 48 hours, confirmed by automated liquid culture (7-day growth) and targeted PCR. A multidisciplinary approach integrating broth microdilution-guided combination therapy (amikacin 0.5 g three times weekly, linezolid 0.3 g twice daily, and doxycycline 0.1 g twice daily) with weekly or biweekly surgical debridement achieved complete resolution of cutaneous lesions. Conclusions This case highlights three key points: (1) NTM infections should be considered in animal bite wounds unresponsive to β-lactams; (2) mNGS accelerates pathogen detection, enabling timely susceptibility-directed therapy; (3) Combined antimicrobial and surgical strategies are essential for successful treatment. Early multidisciplinary consultation is crucial to prevent chronic infection or systemic dissemination in complex SSTIs. Clinical trial number: Not applicable.