Epidemiological investigation and analysis of an outbreak of foodborne disease caused by Clostridium perfringens in Nanjing

ObjectiveTo identify the suspicious food， etiologic agent， and risk factors of a foodborne disease outbreak in a middle school in Nanjing， and to examine issues identified during the investigation， thereby providing evidence to inform the prevention， control， and investigation of similar incidents in the future.MethodsField epidemiological investigations were conducted to identify the suspected meal and food items. On-site sanitary investigations were performed to determine the contamination points， and laboratory testing was undertaken to confirm the etiologic agent. Samples detected positive for Clostridium perfringens through PCR-based multi-pathogen screening were subjected to C. perfringens isolation. The obtained isolates were analyzed by whole-genome sequencing （WGS）， followed by downstream bioinformatic analyses including virulence gene profiling and phylogenetic analysis.ResultsTotally 56 suspected cases with diarrhea and abdominal pain were identified， among them a few cases exhibiting nausea and vomiting. The epidemic curve was consistent with a point source exposure pattern， with an incubation period of 2-14 h. Analytical epidemiological investigation identified the Chinese cabbage （“xiaobaicai”） served at dinner on August 14th as the suspected food item. C. perfringens was detected in 30 anal swab samples， one food sample， and one environmental sample. Specifically， the concentration level of C. perfringens in the dinner sample of Chinese cabbage on August 14th was 3.2×105 CFU/g. WGS revealed that isolates from 10 cases， 8 employees， and one food sample （Chinese cabbage） all belonged to the same ST734 Lineage 1 clade， and harbored the cpe virulence gene. Core-genome SNP analysis showed 0-2 SNP differences， indicating high genetic relatedness.ConclusionBased on on-site investigation and traceback analysis， this outbreak was attributed to the consumption of Chinese cabbage served for dinner in the school cafeteria， with C. perfringens as the etiologic agent. WGS proved valuable for precise tracing in foodborne disease outbreak investigation.