passive latex agglutination or a multiplex polyVerocytotoxin- reversed
merase chain reaction assay (3,5,6).
If the production of verocytotoxin is confirmed by theproducing laboratory,
the case is considered symptomatic VTEC.
Persons associated with the initial case (e.g., family memEscherichia coli, bers) may be further examined for VTEC at the doctor’s
When a doctor diagnoses either symptomatic orJapan, 1999–2004 discretion.
asymptomatic VTEC infection, he or she has to report thisMio Sakuma,* Mitsuyoshi Urashima,*†and Nobuhiko Okabe*†In 1999, an infectious disease prevention law wasenacted in Japan that affected the nationwide infectious
surveillance system. A total of 19,304 laboratory-confirmed
verocytotoxin-producing Escherichia coli cases werereported through 2004. The annual incidence was
2.74/100,000 population; its fluctuation over time andspace was associated with climate, socioeconomic, andpopulation factors.

riggered by 2 major outbreaks of verocytotoxin-producing Escherichia coli (VTEC) in Japan (1,2), thenationwide surveillance system of the National Institute ofInfectious Diseases (NIID) was reengineered in April 1999
by enacting a new infectious disease prevention law to better ascertain the state of laboratory-confirmed VTEC cases
across the nation. In this study, we used these nationwide,
population-based surveillance data to determine the infectious status of VTEC and to explore factors that affect theincidence of VTEC.

TThe Study
Since the new surveillance system under the new law
began, all laboratory-confirmed VTEC cases are reported
and counted in Japan. Under this system, stool samples orrectal swabs are obtained from patients when the clinician
suspects hemorrhagic enterocolitis due to pathogenic E.
coli based on clinical symptoms such as hemorrhagic colitis. These specimens are sent to laboratories at the hospital,private companies, national institutions in each prefecture,or the NIID. To maintain high levels of sensitivity andspecificity to detect VTEC, the protocol and training inthese laboratories fall under the guidance of the NIID. At
these laboratories, the specimens are cultured on specificmedia such as CHROMagar O157 (Kanto Co. Ltd., Tokyo,Japan) or cefixime-tellurite sorbitol MacConkey agar
(Oxoid, Unipath Ltd., Hampshire, UK); specific antibodies
against each serotype of E. coli are used (3,4). If the existence of pathogenic E. coli is confirmed, the ability to produce verocytotoxin from isolates is investigated by using*Jikei University School of Medicine, Tokyo, Japan; and †National
Institute of Infectious Diseases, Tokyo, Japan

event to the local health center immediately and manage
the cases to prevent further spread of the disease. The number of VTEC cases in Japan is totaled for each prefecture
weekly. In this study, we used this surveillance data reported from April 1999 to October 2004 (287 weeks), whichwere retrieved from the Infectious Agents Surveillance
Report published by NIID. Climate variables, which weresummarized weekly, were retrieved from meteorologic
agencies in the capitals of the 47 prefectures. Considering
the incubation period between infection and reporting adiagnosis of VTEC, we used the climatic conditions fromthe 2-week period before each case was reported. We also
used annual socioeconomic data for each of the 47 prefectures (7), including the following information: population
density, percentage of children (<15 years of age), percentage of elderly (>65 years of age), average number of persons in the household, number of livestock (beef cattle,dairy cattle, hogs, and chickens) per person in the prefecture, and average income. All statistical analyses were performed by using Stata 8.0 software (Stata Corp. LP,College Station, TX, USA).
Conclusions
Nationwide, 19,304 cases of VTEC were reported during the study. The annual incidence was 2.74 per 100,000.
The highest number that occurred in a prefecture was 63
VTEC cases per 1,000,000 during 1 week in a single prefecture. More than 16 VTEC cases were observed in 10%of 13,489 weeks (287 weeks × 47 prefectures), and nocases were reported in 57.6% of 13,489 weeks. Age distribution of patients indicates that the number of VTEC cases
was highest in children <5 years of age and fewer cases
were reported in older age groups. A total of 65 outbreaks,
defined as >11 laboratory-confirmed VTEC cases in a certain time frame and area, were reported during the study.
The biggest outbreak occurred during September 2003 in
Kanagawa prefecture and included 252 symptomatic and197 asymptomatic cases of VTEC.
The change in VTEC cases over time is shown inFigure 1, on which the average air temperature (>25°C)
during each week of the summer season is overlaid.
Though the annual incidence showed no clear tendency toincrease or decrease during this study, a marked seasonal
oscillation pattern with peaks centered in July and August
was shown.

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DISPATCHES

Figure 1. The annual oscillation of verocytotoxin-producing
Escherichia coli (VTEC) cases during the study period. In addition
to the VTEC cases, the average air temperature (>25°C) during
each week of the summer season is overlaid in the graph.

The geographic distribution of VTEC cases per
100,000 per year in each of the 47 prefectures indicatedthat a relatively higher incidence of VTEC was clustered inwestern sections of several Japanese prefectures and northeastern sections of 2 Japanese prefectures (Figure 2). The
4 prefectures with the highest annual incidences were rural
areas: Saga (9.2/100,000), Ishikawa (7.9/100,000), Akita(5.8/100,000), and Iwate (5.8/100,000). Conversely, theprefectures with the lowest incidences were near urbanareas: Yamanashi (1.3/100,000), Ibaraki (1.1/100,000),
Niigata (0.9/100,000), and Shizuoka (1.4/100,000).
The association of climate and socioeconomic factors
with the fluctuation of VTEC cases was estimated by using
multiple regression analyses (Table). Within the climate
variables, average air temperature of the day, wind speed,
and the number of sunny days were significantly associated with the incidence of VTEC cases per 100,000 per week

per prefecture. By adjusting for these 3 climate variables
as well as calendar months, associations between 7 socioeconomic variables and VTEC incidence/100,000 population per week per prefecture were analyzed. Results
indicated that the following population-related factors
were strong risk factors for VTEC incidence: a higher percentage of elderly people in the prefecture, higher population density, higher number of persons in a household of
the prefecture, and higher percentage of children. The following socioeconomic factors in the prefecture showed apositive association with VTEC incidence: lower average
income in the prefecture and greater number of beef cattle
per person. On the other hand, the number of chickens perperson was negatively associated with VTEC incidence.
Moreover, this multiple regression model showed thatthese population, socioeconomic, and climate factors
could statistically explain 31% of the variability of VTEC
incidence.
We cannot determine a causal relationship because of
the nature of the ecologic study that we used in thisresearch. However, the results imply that higher beef cattle density, higher population density, and more persons
per household might increase the risk of developing VTEC
infection.
Because our surveillance data were collected from different regions of Japan, we compared them on the assumption that 1) people seek care with the same frequency in all
regions when they are ill, 2) doctors request stool specimens with the same frequency in all regions, and 3) laboratories test for VTEC with the same standards in all

Figure 2. Average number of VTEC cases per 100,000 population
per year in each of 47 prefectures from 1999 to 2004, Japan.
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Escherichia coli, Japan, 1999–2004

regions. Thus, some degree of observation bias may exist
even under control of the law, which is a limitation of thisstudy. In addition, the number of cases includes not only
symptomatic but also asymptomatic VTEC, which may
also raise the incidence rate in the Japanese surveillancesystem.
In conclusion, we showed a high annual incidence of
VTEC of 2.74 per 100,000 that was associated with climate, socioeconomic, and population factors. However,
because this was an ecologic study, further longitudinalstudies are necessary to address these complicated associations.
Acknowledgment
The authors thank Yoshikatsu Eto for critical review of themanuscript.
Dr Sakuma is a PhD candidate at the Jikei University Schoolof Medicine, Tokyo, and a pediatrician at the Jikei University
hospital. Her research focuses on the study of trace elements incord blood and association of a child’s development, behavior,and health condition with the distribution of trace elements.
The opinions expressed by authors contributing to this journal do
not necessarily reflect the opinions of the Centers for Disease
Control and Prevention or the institutions with which the authors
are affiliated.

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Address for correspondence: Mitsuyoshi Urashima, Division of ClinicalResearch & Development, The Jikei University School of Medicine, 325-8, Nishi-shimbashi, Minato-ku, Tokyo 105-8461, Japan; fax: 81-35400-1250; email: urashima@jikei.ac.jp

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