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The topic of This Month Vol.35 No.7(No.413)

Epidemiology of Norovirus in Japan, 2010/11-2013/14 seasons

(IASR 35: 161-163, July 2014)

Norovirus (NoV) is an RNA virus with 5 genogroups GI-GV, among which GI and GII cause human infection.  GI genogroup contains at least 9 genotypes and GII genogroup is composed of 22 genotypes (see p. 173 of this issue).  Persons infected with norovirus shed large quantities of virus in stool and vomit; virus is present in patients’ stool for 10-21 days (occasionally for >1 month) after disappearance of clinical signs and symptoms (IASR 31: 319-320, 2010).  NoV causes food poisonings. Person-to-person transmission occurs through contact (often via contaminated fingers) or inhalation of droplets of fresh vomit or dried-up vomitus (IASR 28: 84, 2007 & 29:196, 2008).

1. Notification of infectious gastroenteritis reported under the National Epidemiological Surveillance of Infectious Diseases (NESID) and detection/isolation of NoV:  Infectious gastroenteritis that includes NoV infection is reported from approximately 3,000 pediatric sentinels in Japan (http://www.niid.go.jp/niid/images/iasr/35/409/de4091.pdf).  Each year, the number of infectious gastroenteritis cases increase towards the end of the calendar year forming a sharp peak at week 49 to 51, with approximately 18 cases reported per sentinel per week; after a temporary decline, NoV activity increases again, forming a broad arch shaped curve, from week 5 to 25 of the next year (Fig. 1; http://www.niid.go.jp/niid/en/10/2096-weeklygraph/1647-04gastro.html).

Prefectural and municipal public health institutes (PHIs) report isolation/detection of causative agents of infectious gastro-enteritis, including NoV, to NESID using a case-based reporting form (IASR 31: 75-76, 2010).  Among the reported pathogens, NoV is the most commonly reported, followed by rotavirus and sapovirus.  NoV is dominant from November/December and continues to be reported through May, although rotavirus becomes more common from February (Fig. 1) (IASR 35: 63-64, 2014).

Among pathogens reported to NESID as causative agents of sporadic cases of infectious gastroenteritis, NoV occupies about one third of the pathogens detected from children 3 years old or younger (Fig. 2).  With increase in age, the proportion of NoV increases.

NoV detected during the 2010/11-2013/14 seasons was mostly of GII type (Fig. 1 & https://www0.niid.go.jp/niid/idsc/iasr/Byogentai/Pdf/data11e.pdf).  Among NoVs detected from infectious gastroenteritis patients 0-15 years of age, GII/4 was dominant in the 2006/07-2009/10 seasons but GII/3 became the most frequent (50%) in the 2010/11 season (Table 1 in p. 163 of this issue & IASR 31: 312-314, 2010).  Since the 2011/12 season, GII/4 once again became more dominant, and made up nearly 80% of the NoV detected during the 2012/13 season; many of the outbreaks during this season were due to the GII/4 variant (Sydney 2012) (see p. 165, 167, 168 & 169 of this issue and IASR 33: 333-334 & 334-335, 2012), which was epidemic abroad (IASR 34: 45-49, 2013).

2. NoV detected from outbreak cases:  Information of pathogen detection in food poisonings, complaints attributed to foods, as well as outbreaks of gastroenteritis due to person-to-person transmission are reported from PHIs as outbreak events.

During the 2010/11-2013/14 seasons, 517-815 outbreaks were reported annually (Table 2 in p.163 of this issue). November and December were high seasons (Fig. 3).  Among outbreaks with known genotype information, GII/3 was the most common during the 2010/11 season, while GII/4 became dominant since the 2011/12 season.

For the 2010/11-2013/14 seasons, foods were responsible for 700 outbreaks, person-to-person for 1,256 and 593 outbreaks were of unknown cause.  The place most frequently suspected as the source of infection was nursery schools, followed by restaurants, nursing homes for the elderly, and primary schools.  Person-to-person transmission was frequent in nursery and primary schools and nursing homes for the elderly.  Foodborne infection was most frequently attributed to restaurants (Table 2 in p. 163 of this issue).

During the 2010/11 season, foods were suspected in 141 of the 648 outbreaks. Person-to-person transmission was most common and suspected for 355 outbreaks, accounting for more than half of the outbreaks; frequently suspected locations were nursery schools, restaurants and primary schools.  GII/3 was frequently found in nursery and primary schools while nursing homes for the elderly was more associated with GII/4.

For the 2011/12-2013/14 seasons, person-to-person transmission was suspected for 212, 394, and 295 outbreaks in each respective season.  During the 2012/13 season, nursing homes for the elderly were suspected for 141 outbreaks.  GII/4 was dominant and responsible for 500 outbreaks; among them, 176 and 86 outbreaks occurred in nursing homes for the elderly and nursery schools, respectively, where person-to-person transmission is suspected as the main route of infection.  There were 59 outbreaks associated with restaurants.

3. Statistics of Food Poisoning: Statistics for food poisoning managed by the Ministry of Health, Labour and Welfare (MHLW) contains data on NoV-related events (IASR 32: 352-353, 2011, https://idsc.niid.go.jp/iasr/32/382/tpc382.html).  The number of NoV-related events was 293 (8,086 patients) in 2010/11, 317 (10,969 patients) in 2011/12, 437 (19,709 patients) in 2012/13, and 228 (8,903 patients) in 2013/14 (as of June 2, 2014).  In the 2012/13 season, the number of patients per outbreak exceeded 500 in three events (2,035, 1,442 and 526, respectively).  In the 2013/14 season, there was a large-scale outbreak involving 8,027 persons, causing food poisoning in 1,271 persons (see p. 164 of this issue).

During the 2010/11-2013/14 seasons, the most frequent number of patients per event was 17-32 (327 events) followed by 9-16 (310 events) and 33-64 (226 events)(Fig. 4).  The place most frequently suspected as the source infection was restaurants (906 events), followed by hotels (111 events) and caterers (85 events).

4. Preventive measures and challenges: In order to improve and promote prevention of norovirus infection, MHLW issues a public notice every season (in November 20, 2013 for the 2013/14 season) (http://www.mhlw.go.jp/topics/syokuchu/dl/131120_1.pdf).  For NoV prevention, attention should be paid to trends in infectious gastroenteritis and NoV detection informa-tion.

Infection control including hand washing, proper clothing and use of gloves in food handling areas and other standard hygienic measures should be implemented (IASR 33: 137-138 & 334-335, 2012).  Health condition of food handlers should be monitored and maintained through implementation of regular health checks (IASR 34: 265-266, 2013).  These measures should be maintained throughout the year as NoV activity is continuous.  For rapidly identifying the cause of food poisoning and for preventing further spread, standardized methods for detecting virus from foods need to be established.

Development of NoV vaccine is identified as a high priority for the MHLW’s master plan for immunization (MHLW, notification number 121, 2014; see http://www.mhlw.go.jp/stf/seisakunitsuite/bunya/kenkou_iryou/kenkou/kekkaku-kansenshou/kihonteki_keikaku/index.html).  Effective vaccine development requires comprehensive information, such as pathogen information obtained through NESID, knowledge on antigen variability and mutation/evolution of the major antigen determinant gene(s) (see p. 170 of this issue), and computational prediction of virus evolution (see p. 171 of this issue).

Copyright 1998 National Institute of Infectious Diseases, Japan

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