The Topic of This Month Vol. 32, No. 12 (No. 382)

Norovirus food poisoning in Japan as of 2011
(IASR 32: 352-353, December 2011)

Norovirus (NoV) is a major cause of sporadic infectious gastroenteritis, outbreak of gastroenteritis and food poisoning in winter seasons (IASR 31: 312-314, 2010).   Patientsf stools and vomits are main sources of infections.   The virus spreads from person to person through contacts with contaminated fingers or in communities through contaminated environments.   NoV-contaminated foods cause food poisoning.   The present article summarizes the epidemiological characteristics of NoV-related food poisoning in recent years based on Statistics of Food Poisoning, Ministry of Health, Labour and Welfare (MHLW) (

1. Statistics of Food Poisoning
Contribution of NoV to food poisonings: NoV, together with Campylobacter , has been in the top two in the total number of incidents among the food poisonings every year since 2004 (IASR 31: 1-3, 2010).   It has been continuously at the top in the total number of cases since 2001; it has occupied nearly half of the total food poisoning cases.   Since August 2003 when Nov became an independent identity separate from “small round structured virus”, which includes NoV, sapovirus (SaV), etc, in the etiological agent classification (IASR 24: 309-310, 2003), NoV occupied 99% of all the viral food poisonings.

Seasonal incidence variation: The data from 2002/03 season to 2010/11 season (each season starts in September and ends in August next year) are shown in Table 1.   In 2006/07 season, NoV genotype GII/4 (see p. 365 of this issue) caused an unprecedented outbreak (IASR 28: 277-278, 2007) with 513 incidents involving total 30,852 cases (60 cases/incident).   In 2010/11 season, NoV food poisoning decreased to 242 incidents consisting of 6,490 cases (27 cases/incident) (as of November 1, 2011).

Food preparation facilities: Among facilities responsible for food poisoning (Table 1), restaurants occupy 64% of the incidents, followed by hotels (13%), caterers (8%), and workplaces (5%).   In terms of the total number of cases, however, restaurants occupy 44% followed by caterers (20%) and hotels (19%).   In terms of the number of cases per incident, the top is food manufacturers producing bread, rice cake, confectionery etc (120) followed by caterers (100), schools (76) and hotels (62).

Implicated foods: There are two major transmission routes of NoV to foods; accumulation of NoV in bivalves grown up in a sea area where sewage contaminated by NoV from patientsf stools flows in and contamination during food preparation by a virus-excreting food handler through direct contact with contaminated fingers or indirectly via cooking utensils.   Occasionally, food poisoning caused by consumption of water from a NoV-contaminated well occurs.   The incidents related to the oyster consumption were around twenty per season during 2005/06-2008/09 seasons, but they slightly increased in 2009/10 and 2010/11 seasons (Table 2).   In 2010/11 season, there were 7 incidents from May to August with a peak in June, to which Crassostrea nippona , “summer oyster” was responsible (see pp. 354, 359 & 360 of this issue), and 5 incidents caused by the fried oyster.   Since 2002/03 season, there have been 23 incidents traced back to bivalves other than oyster, such as, freshwater clam, short necked clam, common orient clam (Meretrix lusoria ) and scallop, among which 12 incidents were caused by soy marinated or alcohol preserved freshwater clam.   NoV food poisoning suddenly increased to 118 in 2006/07 season, which was caused by consumption of catered meals or boxed lunches.   Banquet dishes, course dishes, catered meals, boxed lunches, sushi , school lunches that did not contain oyster were probably contaminated by NoV-carrier food handlers.   There have been four incidents attributable to well water or ground water since 2002/03 season.   In general, NoV food poisoning caused by oyster has a peak in January, while that due to other foods than oyster in December.

Large-scale NoV food poisoning: There were 12 large-scale NoV food poisoning outbreaks from 2002/03 to 2010/11 seasons, which involved 500 or more cases (Table 3).   Of 12 outbreaks, 5 occurred in 2006/07 season.   As food preparation facilities, caterers were responsible for 7 outbreaks.

2. Virus detection from foods
Detection of NoV from implicated foods in food poisoning outbreaks: According to the “Outbreak Report with Pathogen Detection” submitted by the prefectural and municipal public health institutes to National Institute of Infectious Diseases, NoV detection from implicated foods was successful in 67 incidents (5%) among 1,341 outbreaks of foodborne (identified or suspected) NoV infection in 2002/03-2010/11 seasons.   In half of the 67 NoV-positive incidents, NoV was detected from oyster and other bivalves (total 36 incidents).   Other NoV-positive foods were “foods other than bivalves” (22 incidents), water (2 incidents, IASR 26: 150-151 & 330-331, 2005) and “unknown/no description” (7 incidents).

Detection method of viruses from foods: Detection of viruses from foods is crucial for identifying responsible foods, for assessing risk of food contamination, and for establishing preventive measures.   Various methods are being developed (see pp. 355 & 357 of this issue) and some have been used successfully for virus detection from foods directly (see p. 364 of this issue).   For detection of viruses present on the surface of foods (frequently suspected in a case attributable to contamination by a virus carrier), the method used for detection of hepatitis A virus from semi-dried tomato ( is useful (IASR 31: 320-321, 2010).

Though less frequently, viruses other than NoV are detected from bivalves (see pp. 360, 361 & 363 of this issue).   Searching for SaV and other gastroenteritis viruses in parallel with NoV is desirable.

For tracing the contamination route(s), testing environmental swab samples for viruses is recommended (see p. 358 of this issue).   For investigating a food poisoning involving a wide area, which is often brought about by contamination of raw materials, sharing of epidemiological and virus genome sequence data among communities and local governments is indispensable (see pp. 354 & 363 of this issue).

3. Preventive measures
(1) Bivalves and other food materials with risk of NoV contamination should be heated so that temperature of the inner center of the food is maintained above 85°C for 1 minute or longer, and should be handled so as to avoid cross-contamination.

(2) Infection control including hand wash and other standard hygienic measures should be implemented in food handling.   Foods that are served without further cooking should not be handled with bare hands.   Disposable gloves should be used instead.

(3) Vomit should be promptly and appropriately disposed so as to prevent contamination of foods and cooking environment.

(4) Presence of asymptomatic NoV infections should be always taken into account.   Healthy condition of food handlers should be maintained, such as, through regular health checks.

(5) Attention should be paid to trends of infectious gastroenteritis and NoV detection.   For more information, refer to “Flash report of norovirus in Japan, 2011/12 season” (

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