A Salmonella outbreak that mainly sickened children in China was caused by contaminated kitchen-made mayonnaise used in egg sandwiches, according to a study.

In September 2019, the Shenzhen and Dongguan Centers for Disease Control and Prevention were notified of a large cluster of suspected gastroenteritis involving primarily children who sought medical care at hospitals in the two cities.

A total of 254 cases were reported in Shenzhen and Dongguan, Guangdong province, found the study published in the journal Frontiers in Microbiology.

Demographic data and exposure questionnaires were obtained for 121 patients. The age range of those sick was 2 to 61 years old with most young children but a few teaching staff and a couple of relatives of staff who consumed leftovers brought home.

Most people had fever, diarrhea, abdominal pain, and vomiting. One person with severe diarrhea, vomiting, and a fever was admitted to the intensive care unit but nobody died. The distribution of cases over three days suggested a point-source outbreak with a single incubation period.

Children from half-day classes were picked up at noon and not given lunch and afternoon snacks. Researchers said this snack in the nursery canteen on September 20 may have led to the outbreak.

Epidemiological evidence indicated that all patients had consumed egg sandwiches served as snacks to children and staff at a nursery in Dongguan, located near Shenzhen. 

Interviews found that mayonnaise containing raw eggs was spread directly on to bread, made into sandwiches, and served without heating.

Testing and traceback findings
During the investigation period, the nursery was suspended from operation; pending a review of food preparation procedures in the canteen.

Salmonella Enteritidis was isolated from case-patients, food handlers, kitchenware, and sandwiches with kitchen-made mayonnaise.

From 113 samples, 66 were positive for Salmonella Enteritidis, mainly from patients but also from food samples, food handlers, and the mayonnaise mixing bowl. 

Researchers said the outbreak highlights the importance of basic kitchen hygiene, and the food safety challenges posed by using raw egg-based ingredients, especially in a nursery setting.

“This can be achieved by strengthening the food safety training and supervision for food service providers and/or caterers at nurseries, such as the use of pasteurized egg products or avoid recipes using raw eggs, which should be fully cooked. Hygiene measures included handwashing and the use of gloves before handling food, whereas raw and cooked foods are processed and stored separately to avoid cross-contamination.”

A traceback investigation confirmed that eggs used for mayonnaise production in the outbreak were purchased from the Dalingshan market, Dongguan and sourced from an egg distributor in Anshan, Liaoning Province, from a Hebei chicken farm.

Due to a lack of cooperation from the egg producer, distributors, and wholesalers, the team was unable to obtain samples or isolates so could not establish the definitive transmission pathway in the supply chain.

Scientists said the lack of data sharing and communication channels is a common problem of foodborne investigations and surveillance in China. 

They added the overall findings highlight the advantages of complementing traditional epidemiological investigations with whole genome sequencing analysis to present definitive genomic evidence linking suspected food sources to infections. 

(To sign up for a free subscription to Food Safety News, click here.)