Norovirus infections in symptomatic and asymptomatic food-handlers in Japan

Chubu Food & Environmental Safety Center, Shizoka, Japan; Department of Virology II, National Institute of Infectious Diseases, Tokyo, Japan

^* To whom correspondence should be addressed. Email: ghansman{at}nih.go.jp.

	Abstract

Noroviruses are the leading cause of outbreaks of gastroenteritis in the world. At present, the norovirus genogroup II/genotype 4 (GII/4) strains are the most prevalent in many countries. In this study we investigated 55 outbreaks and 35 sporadic cases of norovirus-associated gastroenteritis in food-handlers in food catering settings, between 10 November 2005 and 9 December 2006, in Japan. Stool specimens were collected from both symptomatic and asymptomatic individuals and examined for norovirus by real-time RT-PCR and then confirmed by sequence analysis. Norovirus was detected in 449 of 2376 (19%) specimens. Four genogroup I (GI) genotypes and 12 GII genotypes, including one new GII genotype were detected. The GII/4 sequences were predominant, accounting for 19 of 55 (35%) outbreaks and 16 of 35 (46%) sporadic cases. Our results also showed that a large number of asymptomatic food-handlers were infected with norovirus GII/4 strains. Norovirus GII had a slightly higher median viral load (one log higher) when compared to norovirus GI, i.e., 3.81 x 10⁸ versus 2.79 x 10⁷ copies/g of stool. For norovirus GI, the GI/4 had highest median viral load, whereas for GII, the GII/4 had the highest median viral load (2.02 x 10⁸ and 7.96 x 10⁹ copies/g of stool, respectively). Importantly, our results found that the asymptomatic individuals had similar median viral loads to the symptomatic individuals, which may account for the increased number of infections and predominance via an asymptomatic transmission route.