The outbreak of acute gastroenteritis, which affected 4.1% of the population of a boarding school, was likely caused by norovirus and Bacillus cereus. 93.8% of the cases occurred
in 7 days from September 1 to 7, and the outbreak lasted almost two weeks. The epidemic curve showed that the onset time was concentrated, which strongly indicated that the outbreak was a continuous exposure pattern rather than a human-to-human contact transmission pattern. In addition, the lack of spatial aggregation of cases also indirectly supported food-borne transmission. At the same time, due to students' resistance, early isolation of student cases was not ideal. But after the implementation of measures such as closure of high-risk stalls, isolation of canteen employees with norovirus and Bacillus cereus positive, and thorough disinfection of canteen, the outbreak quickly subsided. This further confirmed the hypothesis that food supplied in the canteen was the root cause of the outbreak. Through retrospective cohort study, we also established daily cohort and calculated personnel exposure in each stall every day. Finally, we got multiple high-risk stalls, which provided
strong epidemiological evidence for food-borne transmission. Because the outbreak
was continuous exposure, this approach reduced the chance that cases might be misclassified and ensured that they were placed in a cohort every day as non-sick people.
Laboratory evidence suggested that the positive rate of norovirus was 71.4% (25/35) among the cases. Although this rate may be over-estimated due to
testing on only some selected cases, it was still much higher than the prevalence of norovirus reported in patients(17%) with acute gastroenteritis in developing countries,
supporting the causative role of norovirus in this outbreak. Moreover, our study is the first reported outbreak of genotype GI.6 norovirus in the Shunyi District of Beijing, China. Norovirus can be divided into at least 5 genogroups(GI–GV) and at least 35 genotypes. Human
disease is primarily caused by GI and GII norovirus. GII viruses are the most frequently
detected(89%), whereas GI viruses cause approximately 11% of all outbreaks [12-15].
Since 1999, the most prevalent genotype in mainland China has been GII.4, accounted for 64% of all detected genotypes. In the past decade, most reported norovirus outbreaks were also caused by GII.4, GI norovirus outbreaks
were relatively rare, and systematic description of the epidemiology and characteristics
of GI outbreaks was even rare. The outbreak was caused by norovirus GI.6. Compared with non-GI.6 outbreak, the GI.6
outbreak was characterized by food-borne transmission. When canteen employees served in these high-risk stalls, food might be contaminated by their faeces, or unhygienic practices by employees who expelled viruses and bacteria. Viruses and bacteria could also be excreted without symptoms, so improper handling by asymptomatic food handlers could also lead to outbreaks. Before the outbreak, in addition to food-borne infections such as eating high-risk foods (such as rice flour and fish
meal), canteen employees might also be infected through fecal-oral route, spilled vomit, or physical contact with contaminated environment or aerosols. Bacillus cereus had also been detected in the outbreak, which is believed to be an opportunistic pathogen that causes gastrointestinal symptoms associated with the
production of cereulide (emetic toxin) or enterotoxin (diarrheal syndrome). In this epidemic situation, between 10 and 1.6×105 CFU/g Bacillus cereus were detected in the incriminated foods, and diarrheal or emetic syndrome was often
associated with Bacillus cereus counts of 105 to 108 cells or spores. In addition, Bacillus cereus was detected in stool samples of
canteen employees and cases, and one of employees who tested positive sold Bacillus cereus-positive
foods. The obvious difference in clinical manifestations between Bacillus cereus (diarrheal-type) and noruvirus indicated that Bacillus cereus infection was likely to be a part of the cause of the outbreak.
Based on the experience of the epidemic situation, the following suggestions are put forward. Firstly, preventive measures such as cases for Isolation, health education, cleaning and disinfection of dwelling and dining place, identification and exclusion of symptomatic food handlers, even if the cause of the disease is not clear, should be carried out at an early stage, to avoid further development of the epidemic. Secondly, epidemiological investigations need to be carried out in a timely manner, especially in this outbreak caused by norovirus, transmission of the virus is diverse, and It is very difficult to detect the virus in contaminated food because of the
low titer of virus. So, to identify risk factors, It is all the more important to use epidemiological investigation. Finally, in order to quickly identify pathogens that cause for outbreaks, samples still need to be submitted at the same time for pathogen detection, and to implement targeted control measures.
There were also some limitations in this study. Firstly, it was a large-scale outbreak,
but the number of investigators was limited. Moreover, supervision department of food safety had more concerns about impact of the epidemic situation and worried about being held accountable, so they had poor collaboration with the Disease control department involved in the investigation. These causes had led to some cases not being verified and underestimating the incidence of the population. Secondly, only the existence
of high-risk stalls was investigated. Because the food handlers of the high-risk stalls were isolated or dismissed, the information on food manufacture and sale could not be obtained. This mad it impossible to analyze key control links in the processing and storage of food. Finally, some canteen employees might have failed to reveal any symptoms of gastroenteritis to the investigation team due to fear of adverse consequences, but this information was important to further trace the source.