A Salmonella outbreak linked to egg fried rice in China affected more than 220 people this past year, according to researchers.

Scientists evaluated the incident using epidemiological surveys, routine laboratory testing methods, and whole genome sequencing (WGS) and findings were published in the journal Frontiers in Microbiology.

The outbreak occurred in Beijing over three days  in March 2021 with 225 of 324 diners who ate at a canteen showing gastrointestinal symptoms. All patients developed diarrhea and high fever, accompanied by abdominal pain, nausea, and vomiting.

Twenty strains of Salmonella enteritidis were recovered, including 19 from patient samples, and one from remaining egg fried rice. Epidemiological investigations and lab tests confirmed the outbreak was related to undercooked egg fried rice.

Patients included 223 males and two females, and the average age was slightly older than 24. The shortest incubation period was one hour, the longest was 38 and a half and the average was just over 14 hours.

Cleaning and cooking issues
Scientists did a food hygiene survey, focusing on the food preparation process in the canteen, and collected samples from patients, food and the environment.

This survey showed that it only took 1 minute and 50 seconds for the egg fried rice to go from frying the raw materials to the finished product. This was placed in a basin, which contained residual raw egg liquid and had not been cleaned.

Four of 69 food samples were positive for Salmonella, including two from egg fried rice, one from spicy chicken, and one from preserved egg lean meat porridge. Seven of 13 eggs were weakly positive for Salmonella while leftover rice was negative.

A total of 20 strains were isolated, 16 from base staff, three from canteen service staff, and one from egg fried rice.

An epidemiological investigation found that when the egg fried rice was processed for dinner, the shell of the egg was not cleaned before use, and the cooking time of the egg fried rice was too short. Also, the processed egg fried rice was placed in a basin without cleaning, which contained raw materials.

Resistance gene analysis showed that the 20 outbreak isolates were multi-drug resistant strains.

Results show it is necessary to strengthen the education of food processors and consumers about the risk of cross-contamination of raw eggs and food, and to improve hygiene measures to prevent illness caused by Salmonella enteritidis, said researchers.

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