Fatal equine gas gangrene associated with Clostridium septicum in the Brazilian Amazon: clinicopathological and molecular characterization of a 20-case series.

Clostridial gas gangrene is an acute necrotizing infection of skeletal muscle characterized by rapid tissue destruction and severe systemic toxemia in horses; however, systematically documented case series from tropical settings remain scarce. Here, we describe a clinicopathological and molecularly confirmed series of 20 fatal equine gas gangrene cases diagnosed in the eastern Brazilian Amazon (Pará State) between 2018 and 2026. Horses presented with abrupt onset of severe lameness, rapidly progressive swelling of a limb or muscle group, and palpable subcutaneous crepitus, frequently following penetrating trauma, intramuscular injections, or other breaches of tissue integrity. Despite antimicrobial and anti-inflammatory treatment, all animals died or were euthanized due to rapid deterioration and poor prognosis. Necropsy consistently revealed extensive hemorrhagic myonecrosis with serosanguineous exudation and intramuscular/subcutaneous gas accumulation. Histology showed widespread coagulative to floccular myonecrosis, marked interstitial edema and hemorrhage, frequent vascular thrombosis, and abundant intralesional bacillary structures morphologically compatible with clostridia. A multiplex PCR assay targeting major histotoxic clostridia detected Clostridium septicum DNA in affected skeletal muscle from all cases, with no amplification of other targets. This case series provides clinically relevant documentation of fatal C. septicum-associated myonecrosis in horses raised under tropical conditions and reinforces the importance of early recognition, strict aseptic practices during intramuscular procedures, and integration of pathology with molecular diagnostics for definitive etiological confirmation.