A large outbreak with almost 200 cases in China was caused by poor hygiene at the producer and inadequate food storage at schools, according to a recent study.

An outbreak of acute gastrointestinal illness occurred at two middle schools in a rural region of Chongqing in 2021. The source was rice noodles contaminated with Bacillus cereus.

In May 2021, an outbreak was reported in the two schools. More than 100 students from the schools had symptoms of vomiting and nausea, prompting the Chongqing Center for Disease Control and Prevention and the local CDC to investigate and implement control measures.

The first school has 55 staff and 536 students while the second school has 75 staff and 475 students. They are about 21 miles apart. The attack rate in the first school was significantly higher than that of the second with 150 compared to 48 cases and more males than females were sick.

A total of 198 cases were reported. The main symptoms were vomiting, stomach ache, and dizziness. The incubation period was 0.5 to 4.5 hours, with a median incubation period of 2.2 hours, according to the study published in the journal Plos One.  

All students and staff had breakfast every morning in both schools. Rice noodles were a common food locally. All cases had consumed rice noodles provided by the same food manufacturer. 

Problems at producer and schools

The producer was found to be operating without a valid license for food production. The facility had three workers, two of whom were responsible for food production, but neither had valid health certificates. Sanitation conditions at the site were poor, with dust and dirt observed, and rice stored on the ground and against walls.

Rice noodles produced on one morning in May were stored at the manufacturer without refrigeration or air conditioning and distributed to two schools on that afternoon. In the first school, rice noodles were stored at room temperature for 21 hours before cooking, while in the second school, they were kept at room temperature before being refrigerated. Local temperature ranged from 18 to 25 degrees C (64.4 to 77 degrees F).

Bacillus cereus was detected in 20 vomit samples, three anal swabs, and seven rice noodles samples.

Researchers said several factors may have contributed to the outbreak.

“Firstly, the laboratory results indicated that raw rice could have been contaminated with Bacillus cereus. The poor hygiene practices at the food manufacturer facilitated the presence and growth of Bacillus cereus during the rice noodle preparation and storage period. Secondly, inadequate temperature control played a role in promoting the growth of Bacillus cereus. Moreover, improper storage practices at two schools increased the risk of Bacillus cereus growth.”

Scientists also said several things need to be improved to prevent future outbreaks.

“First, the manufacturer must comply with the industrial standard for food production and storage, including ensuring that employees involved in the preparation of food have valid health certificates. Second, schools should strictly manage the foods offered to students and staff, including the selection, storage, and cooking processes. Furthermore, public health efforts should strengthen awareness of Bacillus cereus-related food poisoning.”