Norovirus infections in symptomatic and asymptomatic food handlers in Japan

Abstract

Noroviruses are the leading cause of outbreaks of gastroenteritis in the world. At present, 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 were examined for norovirus by real-time reverse transcription-PCR; the results were then confirmed by sequence analysis. Norovirus was detected in 449 of 2,376 (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 mean viral load (1 log unit higher) than norovirus GI, i.e., 3.81 x 10(8) versus 2.79 x 10(7) copies/g of stool. Among norovirus GI strains, GI/4 had the highest mean viral load, whereas among GII strains, GII/4 had the highest mean viral load (2.02 x 10(8) and 7.96 x 10(9) copies/g of stool, respectively). Importantly, we found that asymptomatic individuals had mean viral loads similar to those of symptomatic individuals, which may account for the increased number of infections and the predominance of an asymptomatic transmission route.

FIG. 1.

Map of Japan showing the prefectures/places where the specimens were collected.

FIG. 2.

Phylogenetic tree of the norovirus GI sequences detected in this study (italicized specimen numbers). Norovirus nucleotide sequences were constructed with the partial N-terminal capsid region (14) by using the norovirus Hawaii GII sequence as an outgroup. We simplified the tree to include only unique sequences; that is, when two or more sequences in the same outbreak had 100% nucleotide similarity, we named a single consensus sequence, and if a sequence had one or more nucleotide mismatches with others in the same outbreak, we gave the sequence a distinct name. The numbers on the branches are the bootstrap values for the clusters. Bootstrap values of 950 or higher were considered statistically significant for the grouping (14).

FIG. 3.

Phylogenetic tree of the norovirus GII sequences detected in this study (italicized specimen numbers). Norovirus nucleotide sequences were constructed with the partial N-terminal capsid region (14) by using the Norwalk virus GI sequence as an outgroup.

FIG. 4.

Number of outbreaks caused by each genotype. Only outbreaks due to a single genotype are included. For example, norovirus GII-associated outbreak 8 had two GII/4 sequences and was therefore considered to be a GII/4-associated outbreak, but the GII-associated outbreak 9 had GII/4 and GII/5 sequences, so the cause of the outbreak was unknown, and it was excluded from the figure.

FIG. 5.

Amino acid alignment of the partial N-terminal-region capsid sequences of the norovirus GII strains. The predicted highly conserved MKM capsid start sequence is boxed. Asterisks indicate conserved amino acids among these capsid sequences.

FIG. 6.

Viral loads for GI and GII noroviruses. Dotted lines represent the mean for each genogroup.