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

Laboratory and Epidemiology Communications

Characteristics of Norovirus Outbreaks during a Non-Epidemic Season
Tatsuya Miyoshi, Kiyoko Uchino, Mitsuko Matsuo, Yoshiharu Ikeda, Hisayoshi Yoshida,
Hisako Sibata1, Fumitoshi Fujii1 and Tomoyuki Tanaka*
Sakai City Institute of Public Health, Sakai 590-0953, and 1Sakai City Public Health Center, Sakai 590-0078, Japan
Communicated by Tatsuo Miyamura(Accepted February 13, 2006)
Norovirus (NoV) is a major cause of acute gastroenteritis
worldwide and it is generally accepted that epidemics of NoV
infections and outbreaks occur in winter. We report here twooutbreaks of NoV infection during a non-epidemic season,
between July and August 2005, in Sakai City, Osaka, Japan.
Case 1: On July 15th, 2005, a resident in a nursing home
for the elderly (total number of residents: 120; staff: 100)began vomiting. Between the 15th and 23rd of July, the number of people suffering vomiting and diarrhea increased to 37(infected residents: 21; infected staff: 16) (Fig. 1). The transmission of NoV in this case was considered to be personto-person because no suspicious foods, such as contaminatedfood or shellfish, were identified. Using reverse transcriptasepolymerase chain reaction (RT-PCR), the NoV genome wasdetected in stool specimens from 14 of the infected residents
and 2 of the infected staff members. The partial capsid genesequence revealed identical sequences, and phylogenetic
analysis classified the virus as genogroup II, genotype 1 (GII/*Corresponding author: Mailing address: Sakai City Institute of
Public Health, Kaicho Higashi 3-2-8, Sakai 590-0953, Japan.
E-mail: tanaka-tom@city.sakai.osaka.jp140Fig. 1. Daily incidence of patients in Case 1, a nursing home for theelderly. The total number of daily cases does not show a sharp pattern,
but rather a dull one with a tailing pattern.

person-to-person contact. The NoV genome was detected instool specimens from 4 children and 1 staff member by RTPCR; the virus was classified as belonging to genogroup II,
genotype 6 (GII/6) (1).
The genotypes detected in Cases 1 and 2 were GII/1 andGII/6, respectively, both of which were different from genotype GII/4 which had been prevalent in the preceding winterseason. We are also examining an outbreak of both NoV GI/
4 and GII/6 which occurred in a nursery in Sakai City in May
2005 (submitted for publication). These data suggest thatseveral different genotypes were co-circulating in the summer
season in this area.
We did not observe any peak of infection in these two cases.
We did, however, observe that most of the patients showed
only mild symptoms which included vomiting and diarrhea
once or twice per day, but recovered quickly. Based on theseclinical signs, it was difficult to arrive at a precise diagnosisof NoV infection because in summer, these symptoms arecommonly regarded as “summer cramp” or “summer stress”.
However, we suspect that these symptoms are likely to be
manifestations of NoV infections. Therefore, it is importantto screen for NoV during the non-epidemic season using RTPCR or antigen enzyme-linked immunosorbent assay (ELISA)(2) in order to prevent the spread of NoV.

Fig. 2. Daily incidence of patients in Case 2, a nursery school, showing
multiple peaks, which is a the characteristic NoV infection pattern in
non-epidemic season.

This article was published in the Infectious Agents Surveillance Report (IASR), vol. 26, p.19-20, 2005 in Japanese.
REFERENCES1) (1).
Case 2: On August 2nd, 2005, 3 children under 5 years ofage and 1 staff member at a nursery school (total number ofchildren: 146; staff: 35) experienced vomiting and diarrhea.
Between the 6th and 12th of August, 17 more children and 2
more staff members suffered the same symptoms (Fig. 2).
The transmission of NoV was considered to have occured by1. Katayama, K. (2002): Phylogenetic analysis of the complete genome of 18 Norwalk-like viruses. Virology, 299,
225-239.
2. Tanaka, T. (2003): Evaluation of Norovirus antigen
detection ELISA kit. Jpn. J. Med. Pharm. Sci., 50, 709714 (in Japanese).

141