Objective To understand the pathogenic factors and sources of infection of an outbreak of unexplained diarrhea in a school in Yulin, Guangxi Zhuang Autonomous Region in 2020, and provide suggestions for the prevention and control of such epidemic. Methods　Based on the case definition developed, case finding, case investigation and food hygiene investigation were carried out, and samples were collected for laboratory testing, pathogen isolation and identification. Pulse field gel electrophoresis (PFGE) and genome sequencing technology were used for strain homology analysis, and a retrospective cohort study was conducted to identify risk factors. Software SPSS19.0 was used for statistical analysis.

Results A total of 28 diarrhea cases were found, in whom 26 were students and 2 were family members of the students. The main clinical symptoms were fever (96.43%), diarrhea (78.57%), abdominal pain (60.71%), vomiting (35.71%). The average incubation period was 15 hours. Food hygiene investigation found that 2 workers without health certificates joined in the packaging of finished sandwiches after finishing egg white separation. The retrospective cohort study showed a significant difference in attack rate between sandwich eaters and non-sandwich eaters (RR=32.27, 95%CI: 4.34–239.94).14 anal swab specimens, 2 blood specimens, and 1 food specimen were collected from the biological specimens of the case, and a total of 17 strains of Salmonella enteritis were detected; The results of PFGE and genome sequencing showed that the 17 strains were highly homologous.

Conclusion This foodborne diarrhea outbreak was caused by cross contamination of S. enteritis in the sandwich preparation process. It is suggested to strengthen the food hygiene supervision and management in schools and food production enterprises to prevent foodborne diseases.