The Topic of This Month Vol. 30, No. 6 (No. 352)

Japanese encephalitis, Japan, 2003-2008
(IASR 30: 147-148, June 2009)

Japanese encephalitis (JE), a serious and acute form of encephalitis, is caused by JE virus (JEV) transmitted by the bite of infective mosquitoes, Culex tritaeniorhynchus.  JE is a category IV notifiable infectious disease in the National Epidemiological Surveillance of Infectious Diseases under the Infectious Diseases Control Law enacted in April 1999. Prefectural public health institutes (PHIs) participating in the National Epidemiological Surveillance of Vaccine-Preventable Diseases have monitored herd immunity among humans and JEV infection in pigs.  This article describes the trend of JE in 2003-2008 (for the data of preceding years, see IASR 24: 149-150, 2003).

Incidence of JE: A special intensive immunization program targeting at all age groups, particularly the elderly and children in 1967-1976 successfully reduced the number of JE cases to several dozen in the 1980s and even to nine or less from 1992 on (Fig. 1).

In six years between 2003 and 2008, total 33 JE cases were reported. They were mostly in September.  The date of the onset of symptoms of the first patient of the year was on May 27 (2008, in Ibaraki Prefecture) and that of the latest was on October 30 (2003, in Hiroshima Prefecture) (Fig. 2).  All the 33 JE cases occurred in 16 prefectures in the western part of Japan (Fig. 3); among them six cases occurred in Fukuoka Prefecture.

In 2005-2007 when incidence was relatively higher, among 24 cases reported, 17 cases occurred in the Kyushu and Shikoku districts.  In other districts, 2 cases (1 deceased) occurred in Aichi Prefecture in 2006-2007, 2 cases in Ishikawa Prefecture in@2007 and 2 cases in Ibaraki Prefecture in 2008.  The numbers of male and female cases were 19 and 14, respectively.  Twenty-eight cases (85%) were aged 40 years or more, and 6 cases were 65-69 years old.  There were 2 cases aged 25-34 years, and 3 cases below 20 years (Fig. 4).  A 3-year-old case in 2006 had received no vaccination at all (see p. 153 of this issue).

There were total 4 fatal cases, which were reported at the time of notification or later as additional information. One case in 2004 was in its 20s, one case in 2006 was in its 60s, and two cases in 2007 were in its 40s and in its 80s.

Antibody prevalence among general population (see p. 149 of this issue): Approximately 3,200 people in 11 prefectures were surveyed in 2008 to determine antibody prevalence (Fig. 5).  Neutralizing antibody positive (antibody titer U≥10) rate in different ages revealed existence of two age groups whose antibody prevalence was low, a groups of 6 months to 5 years (<15%), and a group of 30-64 years (<50%).  Comparison with the previous surveys revealed that the former group is expanding to the right, i.e., to advanced ages since 2000 and the antibody prevalence in the latter group has been decreasing since 2004.  In contrast, the antibody prevalence among 9-24 years and that of ≥65 years of age remained high, 80% and ≥50%, respectively.

Until the beginning of 2005, the JE vaccine was in the regular vaccination and was given in a series of three stages, the first stage consisting of 2 primary doses at 3 years of age and a booster dose at 4 years of age, the second stage a booster dose at 9-12 years of age, and the third stage a booster dose at 14-15 years of age.  On May 30, 2005, however, a notice on gWithholding the use of JE vaccine in the regular vaccination (recommendation)h from the Director, Tuberculosis and Infectious Diseases Control Division, Ministry of Health, Labour and Welfare (MHLW) (Announcement No. 0530001) was issued and the third stage immunization was abolished on July 29, 2005.  Since then, the JE vaccination rate dropped sharply, which well explains the low JE immunization status group of young children and its rightward shift with time (Fig. 5).

JEV infection in pigs (see p. 151 of this issue): PHIs have been testing pigs, an amplifier of JEV, that are brought to slaughterhouses during summer (5-8 months old).  Emergence of JEV HI antibody positive pigs, i.e., primary infection rate among pigs of the corresponding year, has been used as an indicator of the JEV activity (Fig. 6).  It usually starts in the South and progresses to the North.  In recent years, the earliest detection of antibody-positive pigs has been around May in Okinawa and around July in other parts of Japan, all west of Toyama Prefecture.

In 2008, by the end of October, of the 35 prefectures that surveyed pig sera, there were 34 prefectures that detected JEV antibody-positive pigs, and 24 among them detected it in more than 50% of the pigs.  In 2003-2008, the JE patients were found in the prefectures with higher incidence of antibody positive pigs (

Virus isolation/detevtion: JEV was detected from one case in Hiroshima Prefecture in 2002 (genotype III), one case in Shizuoka Prefecture (genotype I), and one deceased case in Aichi Prefecture (genotype I).  The majority of recent JEV isolates in Japan has been genotype I (see p. 153 of this issue).  A JE virus detected from pigs in Ishigaki Island, Okinawa Prefecture in 2005 was genotype III and was close to isolates from Taiwan in 1985-1996 (see p. 155 of this issue).  Isolation of JEVs from wild boars in Hyogo Prefecture in December 2008 and May 2009 (genotype I) suggested implication of wild boars as an amplifier of the agent (see p. 156 of this issue).  It is important to continue the isolation/detection of JEV or its genome from patients, pigs, wild boars and mosquitoes for the purpose of surveillance of JEV.

Conclusion: The decrease of JE cases in recent years can be attributed to three factors, (1) the regular vaccination that gave sufficient protective immunity to children, (2) decreased population of mosquitoes due to decreased paddy fields and switch of cultivation method of rice to the one unfavorable for mosquito larvae (Kamimura, Med Entomol Zool 49 (3): 181-185, 1998); and (3) keeping pig farms away from residential areas.

In recent years, however, the JE epidemiology appears changing.  While the elderly used to be the majority of the patients, recent JE occurred among children and middle-aged people, too.  Some JE cases occurred in prefectures where no JE case had been reported. JE antibody positive pigs were detected in prefectures with no reported human JE cases. It is possible that infective mosquitoes are now present all over Japan from Okinawa to Hokkaido.  JE should be always included in differential diagnoses of encephalitis or encephalopathy during summer.

On February 23, 2009, a new freeze-dry tissue culture JE vaccine was approved for production and sale.  On March 19, the vaccination advisory board, MHLW, concluded that grecognizing persisting risk of JEV infection in Japan, the role played by vaccine is crucially importanth, and proposed preferential vaccination to children who have no JE immunity.  Subsequently, on June 2, 2009, MHLW revised the Rules of Immunization Practice to the effect that the new vaccine is included in the first stage of the regular vaccination (see p. 157 of this issue). 

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