Jpn. J. Infect. Dis., 59, 2006

Laboratory and Epidemiology Communications

A Food-Borne Norovirus Outbreak at a Primary School in Wakayama Prefecture
Seiji Morioka*, Tooru Sakata, Atsuko Tamaki, Takahide Shioji, Akira Funaki,
Yasuo Yamamoto, Hiroomi Naka1, Fumio Terasoma1, Kenji Imai1 and Koji Matsuo2
Tanabe Public Health Centre, Wakayama 646-8580; 1Wakayama Prefectural Research Centre of Environment andPublic Health, Wakayama 640-8272; and 2Shirahama Hamayu Hospital, Wakayama 649-2211, Japan
Communicated by Takaji Wakita(Accepted April 18, 2006)
ing chop board and knives, water taps in the canteen, canteen
tables, table towels, were also examined for noroviruses. Stool
specimens were collected from 4 symptomatic persons (ateacher and 3 children), and from the 2 cooks. In this municipality, breads were made and delivered by a bakery for all
the primary schools. Although there was no indication ofa widespread outbreak throughout the municipality at thebeginning of the survey, we also obtained stool samples frombakery workers. All the specimens were tested by the realtime PCR method (1).
For interviews, we tried to apply a master-table method.
However, each lunch except for the frozen sample had been
completely eaten, and the method was not applicable. Thehead teacher normally ate the same lunch as the pupils, but
he had not done so on 22 November due to his attendanceat an out-of-town conference. He was asymptomatic, but
the vice head teacher who had eaten the same lunch as thepupils was affected. A girl was absent on 22 November due
to a severe toothache, and she was also asymptomatic. Thesecircumstances suggested that the lunch on 22 November was
the cause of the outbreak.

We experienced a norovirus (Genogroup II) outbreak at a
primary school in the southern part of Wakayama Prefecturethat was highly comparable with the prior school lunch outbreak in November 2005.
On Thursday, 24 November 2005, the head teacher of a
primary school reported by telephone to Tanabe Public HealthCentre that 69 pupils out of 93 (74%) were absent due tovomiting and diarrhea, and that the school doctor (KM) had
decided to close the school until the following Monday, 28November. The school at that time consisted of 93 students,
11 teachers, and 2 cooks.
An investigation was preformed on all 106 persons bytelephone interviews and direct interviews at their own homes
to determine whether they had nausea, vomiting, or diarrhea,
and when their symptoms manifested. For hygienic investigations, a frozen sample of the lunch served on 22 November
was available, and swab specimens from cooking tools, includ*Corresponding author: Mailing address: Tanabe Public HealthCentre, 23-1 Asahigaoka, Tanabe City, Wakayama 646-8580,
Japan. E-mail: smorioka@m6.dion.ne.jp
205explanation by authorities. Therefore, we conducted a conference for the P.T.A. at the school on Saturday, 26 November,
after getting results of norovirus detections. Although we had
conducted a campaign for washing hands with a chlorinatedstream and careful treatments for ingested foods and stools
on 25 November, some familial infections had already occurredby that time. At the conference, we again emphasized theimportance of washing hands.
Since the establishment of a norovirus detection method,
many outbreaks have been reported throughout Japan,
especially from kindergartens (2) and nursing homes (3).
Previously the virus was called SRSV (small round structuredvirus) and it was often isolated from oysters (4). Althoughoysters were not used in the lunch of 22 November 2005,
seafoods such as squid and shrimp were served. Even thoughnorovirus was not detected from the lunch, the synchronous
appearance of symptomatic individuals well supports that theincident was food-borne. It is widely accepted that oil and fat
in the food often prevent the isolation of noroviruses in thefood. The same mechanism may have occurred in this outbreak.
Needless to say, school lunch must be safe (5). As forthis school, the kitchen twice recorded a score of 97 points(out of a possible 100), in May 2004 and May 2005, fromhygienic inspectors from Tanabe Public Health Centre, andthis was almost the highest mark attainable. We should notethat more than 1,000 food poisoning cases are reported every
year from schools in Japan (5). Retrospective evaluations
following this outbreak found no false maintenance at school
before or after the incident. We should consider that there isalways a possibility of food poisoning, even when everyone
pays full attention to proper hygiene.
With regard to secondary disseminations, we experienced
14 additional cases from symptomatic families, and one ofthem was confirmed as norovirus positive. Although 85 cases
could have caused widespread familial infections, only 14
cases from 10 families were found. We think that this may
have resulted from our hygienic campaign and early explanation of the incident for the P.T.A.

Figure 1 summarizes the distribution of symptomatic individuals by 6-h period. There was no diarrheal incident before
22 November. Eighty-five individuals out of 105 lunch eaters(81%) showed symptoms. Of these 85 persons, 74 (87%) wereaffected synchronously between noon on 23 November to 6a.m. on 24 November (24 h to 42 h after the lunch of 22November). We detected norovirus (Genogroup II) from 4
affected stools, while other specimens turned out to benorovirus negative.
Table 1 shows the ingredients and macro-nutrients of thelunch. The champon was served after cooking at 100°C, andall the vegetables were washed 3 times in a chlorinated stream.
The lunch specimens were completely negative for norovirus.
The canteen had a large room for grade 3 to grade 6 students
and a smaller room for grade 1 and 2 students. The incidence
rate in the large room was 91%, and that in the smaller room
was 66%, but we found no logical explanations for thisdiscrepancy. There was a possibility that asymptomatic eaters
were also affected but not apparently so. Although the 2 cooks
ate the lunch dish before serving the teachers and children,
both of them were norovirus negative. Additional investigations clarified that the bread served on 22 November was notcausative, since symptomatic claims from other schools wereabsent.
For the control of this outbreak and the prevention ofnorovirus infections to other schools, we communicated theinformation about this outbreak to the health section and theEducational Board of the municipality. The information was
also delivered to hospitals and local medical association members so that symptomatic patients would be treated carefully.
The P.T.A. of the school suggested the need for an early

We are grateful to all the students and their families fortheir contribution to the study, and to all the public healthnurses who conducted telephone interviews and investigations of the infected population.
This article appeared in part in the Infectious Agents
Surveillance Report (IASR), vol. 27, p.13-14, 2006 in Japanese.

Fig. 1. Distribution of symptomatic cases in November 2005.
Table 1. Menu of the lunch on 22 November 2005
bread

REFERENCES

a dish of noodles with seafood and vegetables (champon)
Chinese noodle
squidshrimp
bean sprouts (moyashi)carrotChinese cabbage (hakusai)
Jew’s ear (kikurage)
Welsh onion (naga-negi)

1. Kageyama, T., Kojima, S., Shinohara, M., Uchida, K.,Fukushi, S., Hoshino, F. B., Takeda, N. and Katayama,
K. (2003): Broadly reactive and highly sensitive assay
for Norwalk-like viruses based on real-time quantitative
reverse transcription-PCR. J. Clin. Microbiol., 41, 15481557.
2. Akizuki, M., Yoshizawa, Y., Hiki, M. and Ogata,T. (2004): Six incidents of norovirus gastroenteritis
outbreaks occurring in various groups comprising fromchildren to elderly people, December 2003-January 2004,Ibaraki. Infect. Agents Surveillance Rep., 25, 156-157(in Japanese).
3. Tanaka, S. (2005): Norovirus outbreak at a nursing home
for the aged. J. Public Health Practice, 69, 690-691 (inapple salad
apple
cucumbercabbageyogurt206

Japanese).
4. Kurimura, T. and Osato, T. (2000): Caliciviridae. p. 327328. In Osato, T. (ed.), Medical Virology. 2nd ed. Nankodo, Tokyo (in Japanese).

5. Sakagami, Y. (2005): Recent view points on hygienic
maintenance of school lunch -protection of food poisoning-. Jpn. J. School Health, 47, 304-311 (in Japanese).

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