A Rare Case of Septic Arthritis Secondary to Actinomyces europaeus Infection.

Actinomyces europaeus is an uncommon cause of human infection and a rare cause of septic arthritis. We report an 81-year-old woman with diabetes mellitus, chronic kidney disease, gout, and dementia who developed septic arthritis of the distal interphalangeal joint of the right third finger during hospitalization for Escherichia coli bacteremia. Computed tomography demonstrated erosive joint destruction with an adjacent abscess. Surgical incision, drainage, and arthrotomy revealed septic arthritis with associated abscess formation and degeneration of the terminal extensor tendon. Operative cultures grew A. europaeus together with multiple anaerobic organisms. The patient underwent surgical debridement, followed by prolonged oral doxycycline therapy and a short course of metronidazole for polymicrobial anaerobic coverage, resulting in complete clinical resolution. This case highlights an uncommon cause of native joint septic arthritis and emphasizes the importance of considering unusual pathogens in patients with pre-existing joint disease and atypical clinical presentations.