The results suggest that the vehicle of transmission in this outbreak was dried radish salad prepared by the food handler A and served to students of the elementary school on June 12. This finding is not surprising, given that approximately 50% of all NV outbreaks in the United States are linked to ill food-handlers (13
). Ozawa et al. reported that the frequency of NV detection was 19% in outbreak facilities in Japan and that 73% of symptomatic food handlers and 7% of asymptomatic food handlers were positive for NV (14
). They suggested that asymptomatic infections are common and contribute to the spread of the infection in areas of outbreak.
In the case of dried radish salad, there was a process of washing, steeping in water, dipping in red pepper sauce with the food handler A's hands. Additionally, there is a possibility that the food handler A did not have the dried radish salad. This means that food handler A might not be infected through the dried radish salad but through other routes. This suggests that the salad had been contaminated with NV through food handler A's hands. Considering the incubation period and the fact that the food handlers A, B did not participate in cooking cucumber-crown daisy salad, the chance is very low that the cucumber-crown daisy salad was a vehicle of transmission. Considering the fact that food handler D participated in cooking cucumber-crown daisy salad and dried radish salad simultaneously, it is possible that she contaminated both menus and did not excrete NV on the day of stool examination. However, additional evidence was not available.
Also, in case of fresh cabbage mix and young radish kimchi, finished products were used for the school meals only by unpacking the food. Therefore, there was no possibility that the products were contaminated by food handlers. Considering no occurrence of food poisoning in other schools supplied by the same company, Both of them could hardly be the vehicle of transmission.
It is known that the period that NV can be detected is usually from 2 days after the symptoms start to 2 weeks later after the recovery. However, Elise et al. reported that the period that the virus RNAs are detected in the stools of the patients turned out to be 28.7 days (13.5 days-44.5 days) on an average (15
). Also, Atmar et al. reported the similar results (16
). This reveals that the excretion of NV from food handler A can last longer than 2 weeks. Therefore, food handler A can be infected by NV before June 12 (dried radish salad was provided on June 12) and positive for NV until June 26.
Students stated that they usually drank water that they brought from home. They said they did not drink water from the water purifiers. Few students drank water from the water purifier near their classrooms. However, there is no chance that water purifiers might have been the source of infection, because the NV cases ranged across the grades. It is not plausible that all the purifiers installed on every floor were contaminated at once.
Food handlers used the water from the water supplies in the kitchen when making cucumber-crown daisy salad and dried radish salad. Drainage pipes outside the kitchen had been worn out and buried near the water supply pipes at intervals of about 50 cm. NV can survive below the residual chlorine concentration rate 10 ppm (17
). However, there was no possibility of cross contamination between the water supply pipes and the drainage pipes. It is because pathogenic E. coli
that suggest fecal contamination were not detected in the water from the water supplies in the kitchen. In terms of water supplies, the risk of cross contamination increases when it rains, but it did not rain within 1 week before the outbreak. Even though there was no replacement of the drainage pipes, additional outbreak has not occurred for a year since the outbreak.
In conclusion, NV is considered to be the cause of this outbreak. The findings in this report are subject to at least three limitations. First, it was impossible to detect NV in preserved foods and food ingredients due to technical limitations. In order to confirm the vehicle of transmission, it is necessary to detect NV in the suspected foods. Second, it appears that certain answers of questionnaires are inaccurate because the majority of respondents are children. The statistical result that four menus were significant simultaneously shows this possibility. Third, it was impossible to decide whether food handlers A, B were infected through the outbreak or not, because they ate the canteen foods. Despite these limitations, this outbreak in Incheon provides insights and lessons that the excretion of NV from asymptomatic food handlers may be an infection source of NV outbreaks, especially in a place where underground water is not used. For a better risk management, more focus should be put on asymptomatic food handlers.