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Norovirus (NV) is the name of a genus of Caliciviridae proposed by the International Committee on Taxonomy of Viruses in 2002. It used to be called small round structured virus (SRSV) or Norwalk-like virus. NV is an RNA virus, being grouped into genogroup (G) I and II. GI has at least 14 and GII 17 different genotypes. A large amount of NV is excreted in stool and vomit and fecal excretion continues for about a week after disappearance of symptoms. It is transmitted by person-to-person infection directly or through fingers and also food poisoning is caused by contamination of food. The major symptoms are diarrhea, vomiting, nausea, and abdominal pain, which 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.

1. The Statistics of Food Poisoning in Japan: According to the Statistics of Food Poisoning in Japan in 2004, NV food poisoning outbreaks counted at 277, which was the second largest figure by etiological agent after Campyrobacter food poisoning. Cases of NV food poisoning numbered 12,537 accounting for 45% of all food poisoning cases. Since 2001, when bacterial food poisoning decreased, it has been standing the first rank (see IASR 24:309-310, 2003 and http://www.mhlw.go.jp/top.ics/syokuchu/index.html).

2. Reports of NV detection from outbreaks: Aside from the Statistics of Food Poisoning in Japan, Outbreak Reports from Infectious Agent Surveillance are made by prefectural and municipal public health institutes (PHIs) to the Infectious Disease Surveillance Center, National Institute of Infectious Diseases. In these reports, outbreaks due to person-to-person or unknown route of transmission of the agent are included. During December 2004-January 2005, outbreaks suspected of person-to-person infection increased suddenly (Fig. 1).

During September 2003-October 2005, the virus was detected from cases of food poisoning, gastroenteritis, and food handlers in 959 outbreaks, in 934 of which NV was detected by PCR (GII 744, GI 76 and GI+GII 73 outbreaks) (Table 1). In addition, sapovirus (SV) in 7 outbreaks and rotavirus (RV) in 14 outbreaks were solely detected and in some outbreaks multiple viruses were detected. NV GII-detected outbreaks started to increase earlier in the 2003/04 season from November, while in 2004/05, it increased markedly during December 2004-January 2005 and again in May (Fig. 2).

Scales of outbreaks: Numbers of cases per outbreak are tabulated for 803 outbreaks in which case numbers were reported (Fig. 3). In outbreaks suspected of person-to-person infection, 17-32 cases were the most frequently involved and in outbreaks suspected of foodborne infection, 9-16 cases.

Places of infection or consumption of incriminated food: The suspected places of outbreaks being suspected of person-to-person infection were homes for the aged (including elderly care facilities), primary schools, hospitals, nursery schools, and welfare facilities in this order of frequency (see Table 2 on p. 325). In 1/3 of the outbreaks occurring at homes for the aged, the route of infection was unknown. Outbreaks with a large number of cases are shown in Table 3. GII was detected from cases of all outbreaks.

Incriminated foodstuffs: Of 265 outbreaks suspected of foodborne, incriminated food was recorded in 74 (oyster in 30 and other shellfish in 6 outbreaks). Outbreaks in which NV was detected by PCR from food were only 16 (oyster in 6, well water in 2 outbreaks, and shijimi clams in soy sauce, tuna fish, and salad etc.); GII in 11, GI in 1 and GI+GII in 2 outbreaks. Methods for detection of NV from incriminated food materials are urgently needed. Besides, other outbreaks due to well water have occurred (see IASR 26:150-151, 2005 and p. 330 of this issue).

3. NV detection from children with gastroenteritis: Reports of NV detection from children with gastroenteritis increase from every year-end and outbreaks also increase simultaneously. In 2004 and 2005, reports of NV detection increased in May and June; in 2005 reports came out even in July and August (see Table 4 on p. 325 and p. 327 of this issue). In etiological survey for infectious gastroenteritis, the possible NV infection regardless of season must be kept in mind.

4. NV GII/4 prevailing during 2004/05 season: In Europe and US, outbreaks at elderly care facilities and schools occur frequently with NV GII/4 detected in 2002 with mutation in the polymerase-coding region of Lordsdale/93/UK type (2002 type) and 2004 type with further mutation of 2002 type. It has been confirmed that these 2002 and 2004 types were present simultaneously in Japan (see p. 325 of this issue).

Most outbreaks of NV infection occurring at elderly care facilities in Japan during the 2004/05 season were caused by GII/4 NV, as was the case in Europe and US. By analysis of the polymerase-coding region, the principally prevailing viruses were found to be 2004 type and somewhat dissimilar SaitamaU1/97-like virus (see p. 325-327&331 of this issue). In this GII/4 NV, mutation was seen not only in the polymerase-cording region but also in the capsid-coding region (see p. 325-327&331 of this issue).

There used to be few outbreaks due to NV GI, but in the 2004/05 season outbreaks due to GI/3, 8 (see p. 329 of this issue) in the Kyushu district and another due to GI/3 in Ehime Prefecture (see p. 327 of this issue) were reported and future trend seems noteworthy.

5. Conclusion: Correlation between increase in NV detection from domestic oysters and imported shellfish and increase in outbreaks of food poisoning has been seen and such has also been seen in the genotype of detected NV from shellfish and that detected in cases (see p. 335-337 of this issue). It is necessary to keep paying attention to thorough cooking of shellfish (1 min at 85Ž).

Since reports of outbreaks of NV infection at elderly care facilities increased suddenly during December 2004-January 2005, Ministry of Health, Labour and Welfare (MHLW) carried out investigation into the actual conditions and compiled the infection control manual in elderly care facilities, requesting report to health centers in compliance with the notice for a requirement on outbreak report in welfare facilities (notice by the Health and Welfare Bureau for the Elderly, MHLW on January 10, 2005) (see p. 332 of this issue).

When NV is not detected in outbreaks, tests for SV and RV are necessary (see p. 338-340 of this issue). Since outbreaks due to multiple viruses have occasionally been seen, usefulness of electron microscopy which allows simultaneous examination of these viruses have been stressed (see p.340 of this issue). To prepare for NV epidemics for not only winter but also other seasons, attention must be paid to infectious agent surveillance data and regular observation for health and strict enforcement of hand washing (see http://www.mhlw.go.jp/topics/syokuchu/kanren/yobou/040204-1.html).

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