Epidemiology and outcomes of Group A streptococcal and non-Group A streptococcal septic arthritis in French children between 2010 and 2023, a national cohort analysis, and case-control study.

BACKGROUND: Invasive Group A streptococcal (GAS) infections in children are rising across Europe, yet GAS as a cause of septic arthritis (SA) remains poorly characterized. We aimed to describe the incidence trajectory of GAS SA in France, its clinical presentation relative to non-GAS SA, and predictors of sequelae. METHODS: A retrospective national multicenter cohort study with nested case-control elements in children aged 3 months to 16 years was undertaken. Cases were culture- or PCR-confirmed GAS SA from 15 tertiary pediatric orthopaedic centres in France; controls were non-GAS SA patients. Crude and age-standardized incidence rates were calculated for the period 2010-2023. Multivariable logistic regression with cluster-robust standard errors identified predictors of GAS aetiology and of functional sequelae at 30-day follow-up. FINDINGS: 137 GAS SA cases and 321 controls were included. GAS SA incidence rose steadily from 2010 onwards. The crude national incidence rate for 2021-2023 was 0.311 per 100,000 children under 15 years (95% CI 0.24-0.39), approximately 34 cases annually. GAS aetiology was independently associated with age 2-8 years, antalgic joint positioning (OR 3.94, 95% CI 1.61-9.67), elevated CRP (OR 1.14 per 10mg/L, 95% CI 1.09-1.18), and concurrent skin or soft tissue infection, (OR 4.68, 95% CI 1.29-16.98, p=0.02). Presenting with a SA in a large joint decreases the risk of GAS being the causative organism (OR 0.50, 95% CI 0.26-0.94, p=0.03). Sequelae occurred in 24.8% of GAS SA, comparable to S. aureus SA (41.7%). Predictors of sequelae included longer symptom duration (OR 1.08 per day, 95% CI 1.03-1.12), concurrent osteomyelitis (OR 4.47, 95% CI 2.16-9.27), reintervention (OR 3.64, 95% CI 1.79-7.42), concurrent skin infection (OR 3.80, 95% CI 1.46-9.88), and pre-hospital antibiotics (OR 2.37, 95% CI 1.29-4.33). INTERPRETATION: GAS SA is an emerging paediatric pathogen with a distinct clinical profile and sequelae burden comparable to S. aureus SA. Delayed treatment is a key modifiable predictor of poor outcomes; early recognition by orthopaedic surgeons, paediatricians, and primary care providers is essential.