Late-Onset Group B Streptococcal Septic Arthritis of the Shoulder and Proximal Humeral Osteomyelitis Presenting With Reduced Spontaneous Arm Movement in an Afebrile Infant

We report a case of a one-month-old girl with late-onset group B Streptococcus (GBS) septic arthritis of the shoulder and proximal humeral osteomyelitis who presented with persistent reduction in spontaneous movement of the right upper extremity without fever, swelling, erythema, or warmth.Initial radiography showed no fracture, dislocation, or bone destruction.Because the reduced arm movement persisted, magnetic resonance imaging (MRI) performed approximately three weeks after symptom onset revealed findings suggestive of right shoulder septic arthritis and proximal humeral osteomyelitis.Blood and joint fluid cultures yielded Streptococcus agalactiae, identified as serotype III, confirming GBS bacteremia and septic arthritis, whereas proximal humeral osteomyelitis was diagnosed based on MRI findings.She underwent surgical irrigation and debridement followed by six weeks of antimicrobial therapy.Her right upper-limb function gradually improved, and no functional asymmetry was observed at two years and one month of age, although delayed development of the proximal humeral epiphyseal ossification center had been noted.This case emphasizes that late-onset GBS osteoarticular infection should be considered in young infants when unilateral reduction in limb movement persists for several weeks, even in the absence of fever or overt local inflammatory signs.