Ten people were sickened by Salmonella from chicken legs in a Chinese city after eating food ordered online in mid-2018, according to a new report.

Researchers said the investigation highlights the role of online food delivery platforms as a new mode of foodborne disease transmission. Collaboration between public health agencies and online food delivery platforms is essential for timely intervention and to limit the scale of outbreaks.

From late June to early July 2018, 10 cases of diarrheal disease were reported at two hospitals in the Nanshan District of Shenzhen, China. This outbreak was suspected to be foodborne and was notified to the Shenzhen Center for Disease Control and Prevention (Shenzhen CDC), according to the study published in Emerging Infectious Diseases.

All 10 patients were university students who had diarrhea and fever. Seven of them also reported nausea and vomiting. Cases were from six different colleges of the same university but lived in different dormitories and did not know each other.

Food prep to delivery
However, on one afternoon, all had eaten a food delivery of chicken legs with rice from the same restaurant near the university, ordered through an online delivery platform during a six-hour period from noon to 6 pm.

Foods were precooked an hour before anticipated orders and left at room temperature and then dispatched upon receipt of orders and delivered within one hour in ambient temperature at 29 degrees C (84 degrees F) using a linen storage bag.

The time from food preparation to delivery at ambient temperature was two hours which potentially enabled Salmonella Enteritidis to sufficiently multiply and cause illnesses.

A total of 21 samples were collected during laboratory and environmental investigations, including anal swab specimens from seven patients and 14 samples from the implicated restaurant (six from chicken legs, four from staff, two from kitchenware items, and two from other foods).

From these samples, nine were positive for Salmonella Enteritidis, which was isolated from five chicken legs and four patients.

A look at existing pulsed-field gel electrophoresis (PFGE) patterns within the Shenzhen CDC PulseNet local database showed five Salmonella Enteritidis isolates from sporadic cases within one month before the outbreak shared the same pattern. Routine surveillance identified five additional sporadic isolates with the same PFGE pattern within one month after the outbreak. However, no epidemiologic links were found between any of the 10 sporadic cases and the outbreak.

Whole genome sequencing (WGS) single-nucleotide polymorphism (SNP)-based cluster analysis showed all outbreak-associated isolates were genetically closely related to each other. A comparison between the 10 sporadic isolates and any of the outbreak-associated isolates indicated sporadic cases were not part of the outbreak.

A new way of spreading foodborne disease
The complex dynamics of online food delivery networks could result in a new means of spreading foodborne diseases and pose previously unknown effects on public health, said researchers.

“In contrast to traditional restaurant dining, online food delivery could send potentially contaminated food across wide geographic areas throughout a city within a short time to cause large-scale outbreaks,” according to the report. Online food delivery also poses additional food safety risks, including improper handling and storage temperature during transport.”

One new feature of the outbreak was patients ate food from the same source but were isolated from each other, compared with typical outbreaks that usually involve restaurants or catering events within a family or group settings. Additional cases might be more likely to be missed during the epidemiologic investigation.

“However, detailed information associated with food orders, such as the ordering and delivery time, the food items ordered, and the names and addresses of the merchant and the consumer, would all be electronically recorded over the online food delivery platform. Therefore, such remarkable level of detail would be highly valuable for prospective outbreak investigations,” researchers reported.

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