The Topic of This Month Vol. 31, No. 11 (No. 369)

Norovirus epidemic in Japan during 2006/07-2009/10 seasons
(IASR 31: 312-314, November 2010)

Norovirus (NoV) is an RNA virus and a major cause of gastroenteritis around the world.  NoVs can be classified into five genogroups (GI-GV).  Human NoV infection is mainly caused by GI and GII, for which there are 15 and 19 genotypes, respectively.  A large amount of NoV can be excreted in stool and vomit, and virus can persist in the infected individual for long periods (weeks to months) even after the disappearance of symptoms (see p. 319 of this issue).  NoVs are more often than found in contaminated food and as such are a leading cause of food-related infections.  Person-to-person infection is frequent because only a few virus particles are needed (less than 10) for infection, and may occur indirectly through fingers.

1. Infectious gastroenteritis cases reported under National Epidemiological Surveillance of Infectious Diseases (NESID): Under the NESID, NoV cases are not reported as such, but as “infectious gastroenteritis”, a group of infections with manifestation of vomit and diarrhea caused by bacteria or viruses.  It is a category V infectious disease under the Law Concerning the Prevention of Infectious Diseases and Medical Care for Patients of Infectious Diseases.

Infectious gastroenteritis reported from 3,000 sentinel pediatric clinics generally increases sharply towards the end of the year (  In the 2006/07 season, from week 36 of 2006 (September) to week 35 of 2007 (August), reports began to increase from week 42 of 2006, approximately 4 weeks earlier than average years, and reached a peak in week 50, when it attained 22.81 cases per sentinel, the highest figure since the surveillance started in 1981.  In the 2009/10 season, reports began to rise late and peaked in week 4 of 2010 (14.32 cases per sentinel) (Fig. 1).

2. On NoV that caused recent epidemics: Prefectural and municipal public health institutes (PHIs) are requested to report individual cases of isolated/detected pathogens to Infectious Disease Surveillance Center (IDSC), National Institute of Infectious Diseases (NIID).  The peak of NoV detection coincides with the peak of infectious gastroenteritis (Fig. 1).  In the 2006/07-2009/10 seasons, GII was predominant, but GI was also reported (

From 2006, PHIs started to report the results of NoV genotyping.  Among the NoVs detected from infectious gastroenteritis patients aged 0-15 years, GII/4 was detected in 85% of the cases in the 2006/07 season (Table 1).  Although GII/4 continued to be dominant in the following years, the second most frequent genotype was GII/3 in 2007/08, GII/6 in 2008/09 and GII/2 in 2009/10.  In the 2009/10 season, in particular, GII/2 was detected in 36% of the cases; its frequency was close to that of GII/4 (42%).  Compared with GII/4, GII/2 was mostly found in the 3-19 years of age group (Fig. 2).  A similar phenomenon was also observed with the new influenza AH3 variant, which had higher level of infection in this age group (IASR 20: 289-290, 1999).

In the 2008/09 season, a genetically distinct new variant of NoV GII/4, 2008a strain, was detected using RT-PCR and sequence analysis.  In order to rapidly detect the new GII/4 variant a new immunochromatographic diagnostic kit is currently under development, which should have an increased sensitivity and specificity compared to the previous immunochromatographic diagnostic kit (see p. 316 of this issue).

In North America, Europe and Australia where GII is dominant (as in Japan), there has been an increase of GII/12 infections (discussed at the 4th International Conference on Caliciviruses).  Both in and outside of Japan, GII/4 outbreaks appear to be on a decline, and global-level genotype changes may occur among prevalent NoVs.

3. NoV Outbreaks: PHIs are requested to inform IDSC, NIID, of outbreak summary of food poisoning, food-related health complaints, and gastroenteritis with person-to-person infection or those with unknown transmission routes.  In the 2006/07 season, NoV outbreaks were reported in large numbers in November (Fig. 3).  In the 2007/08 and 2008/09 seasons, the peak was in December and January in the 2009/10 season.

The number of outbreaks, in which NoV was detected from gastroenteritis patients (including food poisoning cases) or persons engaged in food preparation, was 563-849 in 2007/08-2009/10.  This was lower than the preceding 2006/07 season by 40-60%.  In the 2006/07 season, GII/4 was responsible for 90% of the NoV outbreaks, while in the 2009/10 season GII/4 caused only 41% of the outbreaks and GII/2 caused 35%, which increased from 3% in the previous 2008/09 season (Table 2 and p. 320 of this issue).

Route of infection: In the 2006/07 season, 861 of 1,388 outbreaks were attributable to person-to-person transmission.  However, such incidents decreased to 474 in the 2007/08 season.  The number of outbreaks attributable to food decreased by half from the 2006/07 to 2008/09 season (from 262 to 138) (Table 2).

Places responsible for infection: In the 2006/07 season, the most frequent place of infection was at elderly nursing homes, hospitals, and welfare facilities.  The outbreaks at these places decreased yearly, however, in the 2009/10 season, many incidents involving nursery schools were reported; the spread of infection was mostly attributable to person-to-person infection (Table 2).

4. Statistics of Food Poisoning in Japan: According to the food poisoning statistics of the Ministry of Health, Labour and Welfare (MHLW), the number of NoV food poisoning incidents was the highest in the 2006/07 season (513 incidents and 30,852 patients).  In the succeeding years, however, it decreased (365 incidents/15,835 patients in 2007/08, 274 incidents/10,885 patients in 2008/09, and 301 incidents/9,187 patients in 2009/10).  With an exception of an incident that involved 1,734 patients in the 2006/07 season, the most common number of patients involved in a single food-poisoning incident is between 17-32 persons (385 incidents) followed by 33-64 persons (331 incidents) and then 9-16 persons (295 incidents) (Fig. 4).  Among the facilities that were responsible for the food poisoning incident, restaurants were the most frequent (915 incidents), followed by hotels (194 incidents), and then caterers (147 incidents).  The most contaminated food was a combined prepared food (163 incidents) followed by fish, which included shellfish (103 incidents).

5. Measures to be taken against NoV and future challenge: To prevent NoV infections, the surveillance data of infectious gastroenteritis and NoV detection should be carefully followed.  Persons working in nursing homes, restaurants, catering and other such businesses should observe regular health checks and proper hand washing.  Because NoV outbreaks may occur in the “off-season” (see p. 321 of this issue), hygienic controls targeting NoV should be conducted throughout the year.

MHLW published “Guidance on prevention of food poisoning caused by norovirus” on October 12, 2007 (  To prevent NoV food contamination by asymptomatic virus carriers, food-handlers should follow the basic principles of hygienic practices.  In addition, persons involved in food preparation should be repeatedly checked until they test negative (see p. 319 of this issue).

To identify the source of food poisoning and to prevent further spreading, detection and characterization of the virus is crucial.  NoV detection methods from food samples need to be established and standardized (see p. 315 of this issue).  Virus sequence information is indispensable for the detection and tracing of contaminated food that may involve large geographical areas.  For this purpose, V-Nus Net Japan is now being established to share the information on the virus sequence (see p. 315 of this issue).  Though less frequently detected than NoV, sapovirus (SaV) has been implicated in some large-scale food poisoning incidents (see p. 322-324 of this issue).  Therefore, SaV screening should also be included in the testing.

In the past, inadequate handling of vomit exposed many persons to NoV, which lead to large-scale outbreaks (IASR 28: 84, 2007 & 29: 196, 2008).  Therefore, not only feces but also vomit should be carefully and appropriately disposed.  As the NoV was detected for a long period from a vacuum cleaner that was used to clean up a vomit spill, the dust in the cleaner should be carefully disposed (see p. 317 of this issue).

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