Abstract
Objectives Although noroviruses (NV) sometimes cause large outbreaks of gastroenteritis, the source of infection is often unclear. However, evidence of environmental contamination has been reported. We have encountered two norovirus outbreaks suggesting the environmental contamination was the source. This study presents results for descriptive epidemiology of these two outbreaks.
Methods Two outbreaks of norovirus gastroenteritis occurred in March and December 2009 at the same elementary school. In the first outbreak, whether the school children or the teaching staff had consumed school lunch and whether they had entered the mid area of the second floor in the S school building were studied as risk factors. In the second outbreak, the focus was on whether they had entered the second or third floors in the T school building. We compared the cumulative attack rates with and without these risk factors. To detect the norovirus gene, real–time polymerase chain reactions were performed with stool specimens. When NV–positive, sequence analyses, homology tests and phylogenetical analyses were further conducted.
Results The norovirus genotype GII/9 was detected from stool specimens of the patients in the first outbreak, and genotype GI/4 in the second outbreak. Homology testing showed the tested viral genes to have 100% identity in each outbreak. In the first, gastroenteritis cases were concentrated in the school children who entered the mid area of the second floor in the S school building regardless of their consumption of school lunch, though the cumulative attack rate in the class that consumed the school lunch was higher than that in the class that did not. In the second outbreak, the patients were concentrated partly on the third, and particularly, on the second floor in the T school building, despite the fact that all the children shared the same lunch menus.
Conclusion The two outbreaks occurred independently with different genotypes. The distribution of the gastroenteritis cases in the school buildings suggests environmental contamination as an infection source of norovirus in the background of these two outbreaks. Furthermore, the spatial and temporal patterns of the gastroenteritis cases suggest that there might have been multiple sources on the different floors in the second outbreak. These results indicate that understanding widespread environmental contamination of norovirus is critical to prevention of further outbreak of norovirus gastroenteritis.
