The Topic of This Month Vol.20 No.8(No.234)

Japanese encephalitis, 1991-1998

Japanese encephalitis (JE) is serious acute encephalitis transmitted by mosquitoes, Culex tritaeniorhynchus, harboring JE virus. The surveillance for JE conducted by National Epidemiological Surveillance of Vaccine-Preventable Diseases accomplishes confirmation of notified patients (based on the Individual Case Card), understanding the immune status of the people, and monitoring JE virus infection in pigs. The following deals with the incidence of JE during 1990s (1991-1998) (see IASR, Vol. 9, No. 1 and Vol. 13, No. 2 concerning the surveillance results before 1990).

Incidence of JE: Although annual JE cases counted at thousands mainly among children during 1950s, they decreased to fewer than 1,000 in 1965. They exceeded 2,000 in 1966, including the largest number of cases among those aged over 55 years [Ogata, Clinical Virology (in Japanese), Vol. 13, No. 2, p. 150-155, 1985]. From 1967 to 1976, vaccination was given actively to not only children but also to adults including the aged as execution of a special countermeasure, resulting in rapid decrease of cases to tens per year in 1980s (Fig. 1).

During the eight years from 1991 through 1998, a total of 35 cases were reported (Table 1). Although 13 cases were reported in 1991, four or fewer cases have been reported in a year since 1992. There were approximately equal numbers of male and female cases, being 18 and 17, respectively. Of 30 cases with obvious prognosis, five (17%) passed away, 18 (60%) recovered leaving some sequela, and the rest seven (23%) recovered leaving no sequela. Records of vaccination are retrievable for 12 cases; 10 were not vaccinated at all and the remaining two were not vaccinated in the last 3 years.

The onset of the disease occurred mostly in August in 21 cases (60%), particularly in late August (August 16-31) in as many as 14 cases (Fig. 2). The earliest onset took place on July 27 in Ehime Prefecture and the latest one on October 17 in Nagano Prefecture. The number of cases per district was the largest, 16, in Kyushu, followed by six each in Shikoku and Chubu districts (Fig. 3). It is noteworthy that the cases in Kyushu and Shikoku districts counted at 22, accounting for over 60% of all cases. No case was reported in Tohoku or Hokkaido district. There were six cases in Nagasaki, five in Kumamoto and four in Ehime Prefectures. Of the 35 cases, one was at the age of seven and all the other 34 cases were over 40 years. Ten cases were at the age of 60-69 years, 11 at 70-79 years, and five over 80 years; 26 cases in all (83%) were over 60 years (Fig. 4). After 1995, all the cases but the 7-year one were over 60 years. Japanese encephalitis, therefore, can be regarded as a kind of encephalitis affecting mainly the aged at present in Japan.

JE antibody prevalence among general population: About 2,000 people in 10 prefectures were surveyed for the antibody prevalence in 1996 (Fig. 5). The positives (neutralizing antibody titer >=1:12) accounted for about 60% in the 0 to 4-year and about 90% in the 5 to 29-year groups. Although the positives accounted for about 70% in the 30 to 59-year group, those exceeded 80% again in the over 60-year group. At present, JE vaccine is given in three series in a standard schedule for regular vaccination. They are two primary immunizations at 3 years of age and a booster at 4 years for the first series, a booster at 9-12 years for the second series, and a booster at 14-15 years for the third series. The antibody prevalence was correlated to the vaccination history in children under 14 years of age; the antibody-positive rate was higher among vaccinees than among nonvaccinees (Fig. 5a). The geometric mean titer of positives was over 1:32 in any age group (Fig. 5b). That in those younger than 10 years was higher also in vaccinees than in nonvaccinees. Most cases reported during the past eight years were older than 60 years, but neither the antibody-positive rate nor the geometric mean titer of positives of those aged over 60 years was necessarily lower than those of other age groups.

JE virus infection in pigs: Pigs are known as an amplifier of JE virus. During 1965-1994, public health institutes in the 47 prefectures in the whole county surveyed pigs (5-8 months old) brought to slaughter houses for the positive rate of JE HI antibody (= rate of infection in that year) to use as an indicator of the prevalence of JE virus (Fig. 6). Antibody-positive pigs appear every year in around May in Okinawa and in around July in the other prefectures of Western Japan. The areas in which antibody-positive pigs appear go up north as months pass by until they appear in all prefectures but Hokkaido before October (see p. 187-188 of this issue). It seems that the appearance of antibody-positive pigs is being delayed as compared with that in 1960s. In Tohoku district, no case has been found since 1991, whereas antibody-positive pigs have been found, indicating the presence of JE virus-infected mosquitoes.

Conclusion: JE cases that had been counting at more than 100 until the beginning of 1970s have decreased to 2-4 per year in the past several years. Although several explanations can be given to this decrease, the following three seem to be most relevant. (1) By vaccination against JE, most children acquired protective immunity to JE virus in their preschool period. (2) The population of C. tritaeniorhynchus has been reduced due to the decrease in paddy fields where C. tritaeniorhynchus multiplies and to the change of the rice-producing method (Kamimura, Med. Entomol. Zool., Vol. 49, No. 3, p. 181-185, 1998). (3) The pig breeding conditions have changed: pigs are being raised in places remote from the human residential area. So, even if C. tritaeniorhynchus is infected with JE virus by biting infected pigs, it will be less likely that they fly to the residential area and bite humans. Nevertheless, JE virus-infected mosquitoes can be found in all areas but Hokkaido in the summer season in Japan. JE is still a disease with a high fatality rate, frequently leaving sequela, if symptoms develop. Besides, as many as 30,000 to 40,000 cases are being reported every year in Asia (see p. 189 of this issue). Thus, JE is a disease requiring caution even today.

Addendum: In the Law Concerning the Prevention of Infectious Diseases and Medical Care for Patients of Infections enforced in April 1999, JE is classified as one of the category IV notifiable infectious diseases.

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