A study reported that an outbreak of salmonella that mainly affected children in China was caused by contamination of kitchen-made mayonnaise used in egg sandwiches.
In September 2019, the Shenzhen and Dongguan Centers for Disease Control and Prevention were notified of a large group of suspected gastroenteritis primarily involving children who sought medical care in hospitals in the two cities.
A total of 254 cases were reported in Shenzhen and Dongguan, Guangdong Province, finds the study published in Frontiers in Microbiology.
Demographic data and exposure questionnaires were obtained for 121 patients. The age range of these patients was 2–61 years with most young children, but few faculty members and two staff relatives who had leftovers brought home.
Most people have fever, diarrhea, abdominal pain, and vomiting. A person with severe diarrhea, vomiting and fever was admitted to the intensive care unit but no one died. The distribution of cases over three days indicates an outbreak from the point of source with a single incubation period.
Children were picked up from half-day classes at noon and afternoon lunches and snacks were not provided. This snack in the nursery canteen on September 20 may have triggered the outbreak, the researchers said.
Epidemiological evidence indicated that all patients ate egg sandwiches provided as snacks for children and staff at a nursery in Dongguan, near Shenzhen.
Interviews found that mayonnaise containing raw eggs is spread directly on bread, made into sandwiches, and served without reheating.
Test results and tracking
During the investigation period, the nursery was suspended from operation; Waiting for a review of food preparation procedures in the canteen.
Salmonella Enteritidis has been isolated from case patients, food handlers, kitchen utensils and sandwiches with kitchen-made mayonnaise.
Of the 113 samples, 66 samples were positive for Salmonella Enteritidis, mainly from patients but also from food samples, food handlers and the mayonnaise mixing bowl.
The researchers said the outbreak highlights the importance of basic kitchen hygiene, and the challenges posed by food safety through the use of raw egg-based ingredients, especially in a nursery setting.
“This can be achieved by strengthening food safety training and supervision for food service providers and/or foster care providers, such as using pasteurized egg products or avoiding recipes that use raw eggs, which must be fully cooked. Hygiene measures included washing hands and using gloves before food handling, while raw and cooked foods are processed and stored separately to avoid cross-contamination.”
A tracking investigation confirmed that the eggs used to produce mayonnaise in the outbreak were purchased from Dalingshan Market in Dongguan and obtained from an egg distributor in Anshan, Liaoning Province, from Hebei chicken farm.
Due to a lack of cooperation from egg producers, distributors and wholesalers, the team was unable to obtain samples or isolates, so it was unable to determine the final transport path in the supply chain.
The lack of data exchange and communication channels is a common problem for foodborne investigations and surveillance in China, the scientists said.
They added that the overall findings highlight the advantages of complementing traditional epidemiological investigations with whole-genome sequencing analysis to provide definitive genetic evidence linking suspected food sources to infection.
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