Existence of multiple genotypes associated with acute gastroenteritis during 6-year survey of norovirus infection in Japan.

Norovirus (NoV) is recognized as one of the most common causative agent of diarrheal disease in young children worldwide. The current study was undertaken to determine the distribution of NoV genotypes in Japan. A total of 3,864 fecal specimens from children with acute gastroenteritis in five regions (Tokyo, Maizuru, Saga, Sapporo, and Osaka) of Japan from July 1995 to June 2001 were collected and then tested for the presence of NoV by RT-PCR. Three hundred sixty four were found to be positive for NoV, accounting for 11%. The highest prevalence of NoV infection was in November, December, and January as the early winter months in Japan. NoV was subjected to be further characterized to sequencing analysis. All NoVs belonged to two different genogroups I and II and these represented 3% and 97%, respectively. This finding indicated that NoV genogroup II was the dominant group causing acute gastroenteritis in Japan. Interestingly, NoV strains were classified into 16 distinct genotypes including genogroup II genotype 9 that was firstly identified in Japan. Of these, NoV genogroup II genotypes 3 and 4 dominated over other genotypes and became the leading strains in Japanese pediatric population. In conclusion, diarrhea due to NoV infection is still a health burden in Japan. This report also stresses the great genetic diversity as well as the importance of NoV causing the diarrhea in Japan.