First report on a foodborne outbreak of Streptococcus dysgalactiae Subsp. equisimilis in China

Streptococcus dysgalactiae subsp. equisimilis (SDSE), a Gram-positive, beta-hemolytic group G Streptococcus, possesses virulence factors such as M protein, streptolysin O, streptolysin S, streptokinase, and hyaluronidase. This study reports a foodborne outbreak caused by SDSE infection recently occurred in China. In April 2019, an acute tonsillitis outbreak involving 155 patients occurred in a company in Changzhou, Jiangsu Province, China. Epidemiological surveys and laboratory investigations were conducted to identify the cause. All patients were clinically diagnosed with acute suppurative tonsillitis or acute pharyngitis. SDSE was identified as the causative pathogen, isolated form 81.4% (35/43) of patients, 12.5% (1/8) of canteen staff, and 33.3% (1/3) of tested food samples (fried meat). Multilocus sequence typing (MLST) and emm typing confirmed all isolates as ST98-stG480.0, suggesting contamination of fried meat by an SDSE-carrying canteen staff member as the source of the outbreak. The mean onset time was 45.8 h, with an SDSE prevalence rate of 27.2%. This study is the first to report a foodborne outbreak caused by SDSE in Changzhou city since 2003. The identified strain, ST98-stG480.0, was exclusively associated with respiratory infections and exhibited no gastrointestinal symptoms. These findings underscore the importance of enhanced surveillance and in-depth investigation into the characteristics and public health implications of this strain.