The Topic of This Month Vol.24 No.12(No.286)

Outbreaks of norovirus infection, January 2000-October 2003

(IASR 2003; 24 : 309-310)

Norovirus is the main etiological agent of viral gastroenteritis. Since no cultured cells to propagate this virus are available, detection has depended upon morphological observation by electron microscopy. This virus has been called "small round structured virus (SRSV)". Recently, however, gene analysis of SRSV has markedly advanced and found that SRSV comprises viruses of two different genera, both belong to calicivirus family, Caliciviridae . It has been approved during the 2002 International Congress of Virology that "Norwalk-like virus", the tentative name of the genus, has been renamed Norovirus and "Sapporo-like virus", the other genus, Sapovirus (see p. 311 of this issue). Norovirus is classified at present into two genogroups (GI and GII), each comprising a number of genotypes (see p. 312 of this issue).

1. The Statistics of Food Poisoning in Japan: The regulation of the Food Sanitation Law was partly amended on May 30, 1997, adding "SRSV" and "other viruses" as the etiological agents of food poisoning (see IASR, Vol. 19, No. 1). After 1998, when these agents were counted as the etiological agents in the Statistics of Food Poisoning compiled by the Ministry of Health, Labour and Welfare, it is indicated that cases of food poisoning due to SRSV begin to appear in October and most cases occur in winter (Fig. 1).

Since SRSV notified as the etiological agent of food poisoning has mostly been identified as norovirus by PCR, "SRSV" a category of the etiological agent of food poisoning has been changed to "Norovirus " in conjunction with the partial amendment of the Food Sanitation Law on August 29, 2003. In this connection, SRSV other than noroviruses have been categorized in "other viruses" (see p. 314 of this issue).

2. Reports of norovirus detection from outbreaks: Apart from the afore-mentioned Statistics of Food Poisoning of Japan, prefectural and municipal public health institutes (PHIs) send "Outbreak Reports from Infectious Agent Surveillance" to Infectious Disease Surveillance Center, National Institute of Infectious Diseases. These reports include not only food poisoning incidents but also outbreaks due to person-to-person or unknown route of transmission of the agent. During the period from January 2000 to October 2003, outbreaks in which virus was detected from cases (cases of gastroenteritis or food poisoning, and kitchen staffs) numbered at 970 (Table 1), of which 911 yielded norovirus (in 823 outbreaks detected by PCR, 21 by EIA, and 67 by EIA and PCR). Of these, GII only was detected in 571 outbreaks, GI only in 86, and both GI and GII in 100. During 2001-2002, the ratio of GII increased (Table 1). By month, detection of GI + GII was concentrated on December-March (Fig. 2).

Route of transmission: The majority of outbreaks were ascribed to foodborne transmission, 10% to person-to-person transmission, and the other 40% to unknown route. The greater part of GI+GII-detected outbreaks was suspected of foodborne transmission, while GII only was detected in most outbreaks suspected of person-to-person transmission.

Scale of outbreaks: To find the distribution of scales of outbreaks, in 863 outbreaks in which the number of cases was reported, cases were totaled at every exponent of two. The highest distribution was seen with outbreaks involving 9-16 cases (181 outbreaks). Among outbreaks in which person-to-person transmission was suspected, those involving 17-32 and 33-64 cases predominated (Fig. 3), and the suspected places of infection involved schools (in 23 outbreaks), nursery schools and kindergartens (17 outbreaks), and welfare facilities (14 outbreaks) (see p. 319-320 of this issue).

In outbreaks in which foodborne transmission was suspected (including those of unknown route of transmission), the numbers of cases by place of consumption (probable place of infection) were compared (Fig. 4); eight or fewer cases were found in the majority of outbreaks at home. At restaurants and hotels, the scale was diverse, from small to large scales. Many outbreaks at welfare facilities, schools, and nursery schools and kindergartens involved more than 33 cases. Besides, 40-50% of outbreaks at business places (21 incidents) and hospitals (14 incidents) involved more than 33 cases.

Table 2 shows four outbreaks each involving more than 257 cases. The outbreak with the largest number of cases occurred from bread sprinkled with parched soybean flour and sugar served for school lunch at primary and junior high schools, and norovirus GII with the identical genotype was detected from cases, kitchen staffs, and parched soybean flour and sugar sprinkled on bread (see p. 315 of this issue).

Incriminated foodstuffs: Of the outbreaks suspected of foodborne transmission, incriminated foodstuffs were recorded in 287 outbreaks; oysters in 154 outbreaks, and other shellfish in 45 outbreaks, thus in many outbreaks; contamination of food materials was suspected. In other outbreaks, such composite ready-to-eat foods as party foods and boxed lunch were involved, and in many incidents it was not clarified whether the cause was contamination of food materials or cross contamination. On the other hand, there were four instances of cross contamination occurring during preparation of bread and confectionery (see IASR, Vol. 23, No. 10). In 55 outbreaks, norovirus was detected from food by PCR, of which genogroup GII was found in 35 outbreaks, GI in 12 outbreaks and GI+GII in one outbreak. In addition to bread sprinkled with parched soybean flour and sugar as described above, oyster was incriminated in 46 incidents, school lunch in two incidents, and different kinds of bivalves (see IASR, Vol. 22, No. 9, Vol. 23, No. 5 and p. 316 of this issue). In most incidents, identification of the food involved by virus detection was difficult, and in such incidents that norovirus was detected from bivalves, the genogroup did not coincide with that detected from cases (nine incidents).

3. Conclusion: To prevent outbreaks of food poisoning due to oysters and other shellfish, several countermeasures such as fixing the ingredient standard of oysters for consuming raw and discriminating from those for heat processing, and expressing the sea area of harvest (see IASR, Vol. 20, No. 11) have been taken. Nevertheless, incidents of food poisoning due to norovirus have not still been reduced. Recently, detection of norovirus from foodstuffs has become possible, and investigations for viral contamination of domestic and imported shellfish are being conducted (see p. 317 of this issue). Investigation on risk management for viral contamination of food has been started (see p. 319 of this issue).

Norovirus causes not only food poisoning but also epidemics of gastroenteritis in winter. Every year-end, detection of norovirus from child cases of infectious gastroenteritis is on the increase (see p. 321 of this issue). Gastroenteritis outbreaks in children are also on the increase in this season (see IASR, Vol. 22, Nos. 2 and 12). Since the middle of October 2003, outbreaks of gastroenteritis in a nursery school in Aomori, in elementary schools in Iwate and Fukuoka (see p. 322-324 of this issue), and in a kindergarten in Shiga prefecture have been reported. In 2001, a foodborne outbreak by a norovirus-infected child who served school lunch was reported (see IASR, Vol. 22, No. 9), and health observation of school children and personal hygiene particularly hand washing are important during norovirus prevalence. Outbreaks of influenza-like illness at the beginning of an influenza season may sometimes be caused by norovirus (see IASR, Vol. 21, No. 2) and special attention must be paid to the information of infectious agent surveillance in the community from late autumn to winter.

Return to the IASR HomePage
Return to the IASR HomePage(Japanese)

Back to Home