Tiny Joints, Big Clues: Neonatal Septic Polyarthritis as a Rare Presentation of Late-Onset Streptococcus agalactiae Sepsis.

Group B Streptococcus (GBS), or Streptococcus agalactiae, is a significant cause of neonatal morbidity and mortality, with late-onset disease (LOGBS) typically manifesting as bacteremia or meningitis. Osteoarticular infections, particularly septic polyarthritis, are exceedingly rare but potentially devastating complications of LOGBS. We report a rare case of LOGBS presenting as septic polyarthritis in a 28-day-old term male neonate. The infant presented with a 15-day history of progressive swelling in the left knee and right shoulder, accompanied by irritability and feeding difficulties. Ultrasonography revealed joint effusions, and aspiration yielded Gram-positive cocci in chains. Culture confirmed S. agalactiae, susceptible to ceftriaxone and vancomycin but resistant to multiple other antibiotics. Targeted antibiotic therapy was initiated, and surgical drainage of both joints was performed. The infant responded well, with complete clinical recovery. Immunophenotyping revealed reduced B-lymphocyte levels, suggesting a potential underlying immune vulnerability. This case underscores the importance of recognizing atypical manifestations of GBS in neonates. Prompt microbiological diagnosis using Gram stain and culture, combined with susceptibility-guided antimicrobial therapy and early surgical intervention, was instrumental in achieving a favorable outcome. The lymphocyte subset abnormality raises the possibility of an underlying immune dysfunction, warranting further follow-up. In regions without routine GBS screening, clinicians must maintain a high index of suspicion for uncommon presentations like septic polyarthritis. Early recognition, multidisciplinary management, and individualized therapy are key to optimizing outcomes in such rare neonatal infections.