Salmonella contamination of eggs and egg shells has been identified as a public health problem worldwide. Here, we reported an outbreak of severe gastrointestinal symptoms caused by Salmonella enterica serovar Enteritidis (S. enteritidis) in China. We evaluated the outbreak by using epidemiological surveys, routine laboratory testing methods, and whole genome sequencing (WGS). This outbreak occurred in a canteen in Beijing, during March 9–11, 2021, 225 of the 324 diners who have eaten at the canteen showed gastrointestinal symptoms. The outbreak had characteristical epidemiological and clinical features. It caused a very high attack rate (69.4%) in a short incubation time. All patients developed diarrhea and high fever, accompanied by abdominal pain (62.3%), nausea (50.4%), and vomiting (62.7%). The average frequency of diarrhea was 12.4 times/day, and the highest frequency of diarrhea was as high as 50 times/day. The average fever temperature was 39.4°C, and the highest fever temperature was 42°C. Twenty strains of S. enteritidis were recovered, including 19 from the patients samples, and one from remained egg fried rice. Antibiotic susceptibility test showed that the 20 outbreak strains all had the same resistance pattern. PFGE results demonstrated that all 20 strains bore completely identical bands. Phylogenetic analysis based on WGS revealed that all 20 outbreak strains were tightly clustered together. So the pathogenic source of this food poisoning incident may was contaminated egg fried rice. Resistance gene analysis showed that the outbreak strains are all multi-drug resistant strains. Virulence gene analysis indicated that these outbreak strains carried a large number of virulence genes, including 2 types of Salmonella pathogenicity islands (SPI-1 and SPI-2). Other important virulence genes were also carried by the outbreak strains, such as pefABCD, rck and shdA. And the shdA gene was not in other strains located in the same evolutionary branch as the outbreak strain. We speculated that this is a significant reason for the serious symptoms of gastroenteritis in this outbreak. This outbreak caused by S. enteritidis suggested government should strengthen monitoring of the prevalence of outbreak clone strains, and take measures to mitigate the public health threat posed by contaminated eggs.
Introduction
World Health Organization (WHO) estimated the global burden of foodborne diseases, the results showed that almost 1 in 10 people fall ill every year from eating contaminated food and 420,000 die as a result (Dewey-Mattia et al., 2018). Salmonellosis is one of the most frequently reported foodborne diseases worldwide. In particular, disease caused by non-typhoid Salmonella is a global public health problem, whether in a high-income country or a low-income country (Feasey et al., 2016). Each year, approximately 40,000 Salmonella infections are reported to the United States Centers for Disease Control and Prevention (CDCs) (Vaughn et al., 2020). Salmonella enterica serovar Enteritidis (S. enteritidis) is the predominant Salmonella serotype accounting for between 40 and 60% of laboratory-confirmed illnesses of salmonellosis in recent years (Quick et al., 2015). Salmonella enteritidis typically cause a self-limiting gastroenteritis with the symptoms of diarrhea, fever, abdominal cramps, and dehydration (Jiang et al., 2020). Salmonellosis is mainly caused by eating eggs and egg products contaminated with S. enteritidis (90%) and has become a serious health problem. It has been attributed to this serovar’s unusual ability to colonize ovarian tissue of hens and to be able to present within the contents of intact shell eggs (Chousalkar et al., 2018).
Here we reported a severe gastroenteritis outbreak of S. enteritidis linked to contaminated egg fried rice. There were 225 cases of diarrhea and fever in a short period of time in a canteen in Beijing within 3 days. Epidemiological investigations and laboratory tests confirmed that the outbreak was caused by S. enteritidis and was related to the undercooked egg fried rice. At present, such a large-scale outbreak with severe clinical symptoms of S. enteritidis caused by undercooked eggs is rarely reported (Li et al., 2020). Therefore, we reported the outbreak and examined its molecular characteristics using whole genome sequencing (WGS).
