Haemophilus parainfluenzae Infection Presenting as Prosthetic Joint Infection of the Shoulder, Vertebral Osteomyelitis, and Iliopsoas Abscess During Tocilizumab Therapy: A Case Report.

We describe the first case of prosthetic joint infection of the shoulder (PJIS), vertebral osteomyelitis, and iliopsoas abscess (IPA) caused by Haemophilus parainfluenzae (HPI). The patient was a 76-year-old male with a history of rheumatoid arthritis (RA) treated with tocilizumab (TCZ) and multiple prior surgeries for left prosthetic shoulder arthroplasty due to left degenerative shoulder osteoarthritis. He was admitted for shoulder and lower back pain. Imaging studies revealed dislocation of the left prosthetic shoulder joint and fluid accumulation suggestive of a dorsal hematoma, as well as a compression fracture at the T11 vertebra. Conservative treatment failed to alleviate the shoulder pain, and a humeral head replacement was subsequently performed. Intraoperatively, a purulent collection was observed in the area initially presumed to be a hematoma. On day 10 after admission, lumbar MRI revealed vertebral osteomyelitis at the L1/2 level and a left-sided IPA. The same day, surgical drainage via iliac fossa incision was performed, and a drain was placed. HPI was isolated from both the shoulder joint and lumbar specimens. The patient underwent two joint aspirations for recurrent shoulder swelling and received a combination of antibiotics, which were later de-escalated to levofloxacin and trimethoprim/sulfamethoxazole, and finally switched to oral amoxicillin-clavulanate before discharge. This case underscores the critical need for heightened vigilance and close monitoring for PJI, particularly during immunosuppressive therapy with agents such as TCZ and other biologics. Further accumulation of cases is needed to clarify the relationship between TCZ and HPI infections.