Septic Arthritis of the Left Ankle Caused by Brucella suis biovar 5: A Case Report.

We report a rare case of human brucellosis caused by Brucella suisbiovar 5 in a 75-year-old man from Nagano Prefecture, Japan. He had no history of immune disorders and presented with fatigue, left ankle pain, and swelling. Initial antimicrobial therapy for suspected cellulitis and tenosynovitis failed, leading to admission to our hospital. Imaging and synovial fluid analysis suggested septic arthritis with a periarticular abscess. Gram-negative rods were recovered from synovial fluid obtained via arthrocentesis of the left ankle joint. Although the isolate could not be identified using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry using a MALDI Biotyper system, it was successfully identified as B. suisbiovar 5 using species-specific PCR and nine-locus multilocus sequence typing (MLST). The patient was successfully treated with surgical debridement and prolonged antimicrobial therapy. This is the third documented human case of B. suisbiovar 5 infection in Japan, all originating from Nagano Prefecture, suggesting a localized cluster. This case highlights the diagnostic challenges of brucellosis, the limitations of routine identification methods, and the biosafety concerns for clinical laboratories. Enhanced awareness and surveillance are warranted in regions with emerging cases.