Septic arthritis caused by <i>Streptococcus equi</i> subspecies <i>zooepidemicus</i> : a case report

ABSTRACT Background Streptococcus equi subspecies zooepidemicus (SESZ) is an uncommon zoonotic pathogen in humans but can cause severe joint infections. Reported cases are often linked to exposure to horses, dairy products, or raw pork, though infections may occur without an identifiable animal source. Case Summary An 84-year-old man with multiple comorbidities presented with acute left shoulder pain, fever, and chills. Synovial fluid aspiration yielded turbid fluid containing calcium pyrophosphate dihydrate crystals, while Gram stain was negative. Despite empiric broad-spectrum antibiotics, his symptoms persisted, requiring surgical irrigation and debridement. Cultures from the joint aspirate and drain revealed β-hemolytic, mucoid colonies of gram-positive cocci (Lancefield group C), identified as SESZ by Vitek 2, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, and whole-genome sequencing. Interestingly, the isolate had an atypical biochemical phenotype, i.e., it could not ferment sorbitol. Antimicrobial therapy was changed to intravenous ceftriaxone according to susceptibility testing, resulting in subsequent clinical improvement. Conclusion Although rare, SESZ should be considered in the differential diagnosis of septic arthritis, particularly in elderly patients, even in the absence of clear zoonotic exposure. Timely recognition, adequate surgical drainage, and prolonged targeted antimicrobial therapy are essential for favorable outcomes.