The Topic of This Month Vol. 28, No. 10 (No. 332)

Norovirus epidemic in Japan during 2006/07 season

(IASR 28: 277-278, October 2007)

Norovirus (NVjis an RNA virus, which is classified into genogroups (G) I-V. Principally GI and GII cause human infection.  At least 15 and 18 genotypes are known for GI and GII, respectively.  A large amount of NV is excreted in stool and vomit and fecal excretion continues for a long period of time after disappearance of symptoms (see p. 286 of this issue).   Food poisoning is caused by ingestion of food contaminated with NV and person-to-person infection may occur indirectly through fingers.  Such major symptoms diarrhea, vomiting, nausea, and abdominal pain last usually for 1-3 days.  For aged persons and infants with strong dehydration, such symptomatic treatment as transfusion is applied.  Care must be taken for suffocation due to deglutition of vomit and for aspiration pneumonia.

1. Occurrence of infectious gastroenteritis under the National Epidemiological Surveillance of Infectious Diseases (NESID): Cases of infectious gastroenteritis reported by approximately 3,000 pediatric sentinel clinics in the 2006/07 season started to markedly increase from week 42 of 2006, approximately 4 weeks earlier than average year, reaching 22.8 cases per sentinel in week 50, being the highest peak since the start of surveillance in 1981.  The accumulated total number of reported cases in the 4th quarter of 2006 reached 166.8 per sentinel, 1.6-fold larger number of the preceding year (see Fig. 1 and

2. NV detection from children with gastroenteritis: Reports of NV detection from children with gastroenteritis increase from every late fall and outbreaks also increase simultaneously.  In 2004 and 2005, reports of NV detection increased in May and June (see IASR 26: 323-324, 2005).   Then during 2005/06-2006/07 seasons, detection of NV has been reported every month (see Table 1 and  In etiological survey for infectious gastroenteritis, the possible NV infection regardless of season must be kept in mind.  In 2005/06-2006/07 seasons, cases of NV infection complicated with acute encephalopathy were reported (see p. 292 of this issue).

3. NV detection from outbreaks: Prefectural and municipal public health institutes (PHIs) sent Outbreak Reports from Infectious Agent Surveillance to the Infectious Disease Surveillance Center (IDSC), National Institute of Infectious Diseases.  These reports include summary information of each incident of food poisoning due to infection through food, symptomatic complaints (including those suspected of food poisoning but not decided), and outbreaks of gastroenteritis due to person-to-person or unknown route of transmission.  In November 2006, reports of outbreaks of NV infection increased suddenly (Fig. 2).  During September 2006-August 2007, incidents in which NV was detected from cases of food poisoning and gastroenteritis and/or personnel participating in cooking counted at 1,227, 2.6 times larger number of the average of the preceding three seasons (475.3 incidents).

Of these, outbreaks of infectious gastroenteritis accounted for the majority, being 899 incidents, food poisoning 218 incidents, symptomatic complaints 106 incidents, and unknown 4 incidents.  NV GII was detected by PCR in 1,196 incidents (including 11 incidents in which both GI and GII were detected).  GII/4 was detected in 442 (95%) of 465 incidents in which GII genotype was identified by nucleotide sequencing.   Besides, GII/2 was detected in 9 incidents, GII/13 in 6 incidents, GII/5 in 3 incidents, GII/3 in 2 incidents, GII/9 in 2 incidents, and GII/6 in one incident.  GI only was detected in 16 incidents and genogroup was unknown in 15 incidents.

Route of infection: The suspected routes of infection of 1,227 incidents, in which NV was detected, were person-to-person in 752 incidents, foodborne in 239 incidents, and unknown in 236 incidents.  Incidents suspected of person-to-person infection have markedly increased from 2005/06 season (Table 2).

Place of infection or consumption of incriminated food: The suspected places where NV infection occurred were homes for the aged (including elderly care facilities), hospitals, welfare facilities, nursery schools, restaurants, hotels, and primary schools in this order of frequency (Table 3).  Most incidents at homes for the aged were suspected to be caused by person-to-person infection.  Increase from preceding seasons is outstanding in incidents occurring at homes for the aged, hospitals, welfare facilities, and nursery schools (Table 3).

Scale of outbreaks: Of 960 incidents, in which number of cases was reported, 102 incidents each involved 2-8 cases, 175 incidents 9-16 cases, 582 incidents 17-64 cases, 96 incidents 65-256 cases and 5 incidents ≥ 257 cases.  Incidents involving many cases are listed in Table 4.  NV GII was detected in all incidents and GII/4 was identified in all incidents genotyped.

4. NV GII/4 prevailing during 2006/07 season: Outbreaks due to GII/4 variants having new mutation in the capsid-coding region of NV (E2006a and E2006b, first detected in December 2005 in UK and Spain, respectively) occurred frequently in European countries in 2006 (CDR Weekly 16, No. 25, 2006 and Eurosurveillance Weekly 11, 14 December, 2006).  Genetic analyses of NV detected from specimens collected by 11 PHIs confirmed the presence of the E2006b variant in May 2006 in Japan and caused a large-scale epidemic in 2006/07 season (see p. 279 of this issue).

5. The Statistics of Food Poisoning in Japan: In the statistics of food poisoning compiled by the Ministry of Health, Labour and Welfare (MHLW), the number of NV food poisoning outbreaks in the 2006/07 season was 1.7-fold larger and that of NV food poisoning cases 2.7-fold larger than the average of the preceding 4 seasons; however, incidents due to oysters or shellfish were small in number, suggesting an increase due to food contaminated secondarily by food handlers (see p. 282 of this issue).

6. Conclusion: To prevent NV infections including food poisoning and other settings, it is important that attention must be paid to surveillance data about infectious gastroenteritis cases and NV detection and regular observation for health and strict enforcement of hand washing.  Since incidents of food poisoning due to secondary contamination of food by asymptomatic food handlers have been seen (see p. 285 of this issue), thoroughgoing of fundamental sanitary control of food-handling facilities is desired.  Since outbreaks of person-to-person infection has frequently occurred among many people after exposure to NV due to inadequate treatment of vomit, special care must be taken for treatment of not only feces but also vomit.  The MHLW revised Q&A concerning NV in March 2007 (, and IDSC publishes the latest information on demand (

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