Invasive Streptococcus dysgalactiae subsp. equisimilis infections in a super-aged society: a Japanese nationwide observational study (2024-2025).

BACKGROUND: Streptococcus dysgalactiae subsp. equisimilis (SDSE), historically considered less virulent than Streptococcus pyogenes, has emerged as an important invasive pathogen, particularly in aging societies. Data on nationwide clinical and molecular epidemiology in super-aged populations remain limited. We aimed to identify admission abnormalities associated with the 28-day mortality for invasive SDSE (iSDSE) infections in Japanese adults and evaluate the molecular epidemiology of this emerging pathogen through a large-scale surveillance. METHODS: A prospective nationwide surveillance of iSDSE was conducted between March 2024 and March 2025 in 132 hospitals across Japan. Clinical, epidemiological, and laboratory data, including emm type distribution, were collected and analyzed. RESULTS: Overall, 278 patients with iSDSE infections were identified (median age, 83 years); one-quarter of the patients were residents of long-term care facilities. Nearly all patients had comorbidities. The 28-day mortality was 12.2% overall, which increased to 16.1% among patients aged ≥ 80 years. Fatal outcomes were strongly associated with bacteremic pneumonia and streptococcal toxic shock syndrome, whereas cellulitis showed a lower mortality burden. Intensive care unit admission, mechanical ventilation, and vasopressor use were more common in patients with fatal outcomes, reflecting disease severity. Variables associated with poor outcome included leukopenia, thrombocytopenia, and elevated levels of C-reactive protein, creatinine, and creatine kinase. Emm typing revealed stG6792 as the predominant lineage; however, a marked increase of stG840 was observed. No significant outcome differences were noted across emm types. CONCLUSIONS: These findings emphasize the combined impact of advanced age, comorbid conditions, and critical illness on iSDSE outcomes, highlight the clinical burden of iSDSE in older adults, and support continued monitoring in Japan.