Norovirus was behind a foodborne outbreak in 2018 caused by eating take-out food in China, according to a study.

The outbreak involved 157 probable cases and 20 people had laboratory-confirmed norovirus infections in two weeks in March 2018.

Food delivery services have become common in China in recent years but the hygienic condition of delivered food is not easy to control. Contamination can occur at any stage, from preparation to distribution, and food handlers must play an important role, said researchers.

The outbreak was managed by restrictions on off-campus dining and implementation of disinfection and case isolation measures.

Food delivery link
The study, published in the journal BMC Public Health, reported students at the school in Guangzhou developed a sudden onset of symptoms of diarrhea and vomiting.

Guangzhou is a city in Guangdong Province, southern China. There were at least two outbreaks in four schools from 2015 to 2017 in the city, according to the National Public Health Emergency Event Surveillance System.

A case control study was carried out using online standardized questionnaires. The control group was randomly selected healthy students of the same sex and age as cases but without gastrointestinal symptoms.

The proportion of students who had eaten delivery food three days before onset of disease in the case group was 2.69 times that of the control group.

Having take-out food three days earlier, and exposure to similar cases 72 hours before onset and cases in the same dormitory were risk factors.

Vomiting was the most common symptom, followed by diarrhea, abdominal pain and fever. All cases were mild and no hospitalization, severe illness, or deaths occurred. All cases were students, aged 17 to 23 years old, including 47 males and 110 females.

Norovirus detection
A total 379 samples were collected, including rectal swab samples from cases, of food handlers in the off-campus restaurant and canteen staff in the school and 32 environmental swabs, 21 from the canteen in the school and 11 from the off-campus restaurant.

Rectal swab samples from employees and environmental swabs from the off-campus restaurant offering delivery service to students were collected. Of 169 rectal swabs, 10 were positive for norovirus, and two of 11 environmental swab samples were positive. All samples were negative for other viruses and bacteria.

The strains detected in five samples (two confirmed cases, a food handler, and two environmental swab samples) were all norovirus genogroup GII.3 reference strains.

Detection of norovirus in environmental swab samples from the off-campus restaurant and rectal swabs from food handlers presented a chain of evidence, according to the study.

Researchers said poor handling practices among infected food handlers are a common infection source in foodborne outbreaks in Guangzhou.

Foodborne transmission made up 40 percent of norovirus outbreaks in the city from 2016 to 2018, among which contamination by kitchen workers accounted for a large proportion, they added.

Food handling practices carry potential risk of acute gastroenteritis outbreaks due to inadequate surveillance and supervision, said researchers.

“Greater attention should be paid to the monitoring and supervision of food handlers in off campus restaurants to reduce the incidence of norovirus-related acute gastroenteritis associated with delivery food.”

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