The Topic of This Month Vol.18 No.6(No.208)
Complying with the National Epidemiological Surveillance of Infectious Diseases (NESID) program, about 500 sentinel hospitals report aseptic meningitis cases every month and collect specimens; prefectural and municipal public health institutes (PHIs) conduct virus isolation and identification for surveying etiological agents of meningitis and report the results to the Infectious Disease Surveillance Center.
Aseptic meningitis cases increase in number every summer, and such enteroviruses as echovirus (E) and group B coxsackievirus (CB) are isolated as the etiological agents. The main serotypes of the isolates are changing every year (Fig. 1). The monitoring of the trend of enteroviruses up to the present time has found that E30 caused nationwide epidemics during 1989 through 1991 (see IASR, Vol. 13, No. 8), and E30 (1983), E6 (1985), and E7 (1986) increased suddenly and provoked unexpected epidemics.
The aseptic meningitis cases reported in 1996 numbered 1,519 (2.98 cases per sentinel hospital), being fewer than 1,619 cases reported in 1995 (3.14 cases per sentinel hospital) and the fewest ever reported. The aseptic meningitis cases at ages of 0-4 years accounted for 44%, 5-9 years 38%, 10-14 years 11%, and >=15 years 7.6%. The ratio of 0-year cases (18%) was as high as that in the preceding year (Fig. 2).
In 1996, virus isolation was reported with 296 meningitis cases (as of May 20, 1997). Being different from usual years, as there was no large epidemics caused by enterovirus of certain serotypes, reports on isolation of any serotype did not exceed 100 (Fig. 1). Reports on isolation of E7 counted 67 (23%) and those of CB4 42 (14%) (Table 1). Those of CB5 and CB3, which were isolated frequently in 1995, decreased in number from 269 to 26 and from 80 to 5, respectively (see IASR, Vol. 17, No. 3).
Attention has been paid to the trend of E7 since reports on its isolation was increasing from the summer of 1995 to October (Fig. 3) (see IASR, Vol. 17, No. 3). Reports on its isolation totaled 226 from 23 laboratories with a largest number in July also in 1996. A larger number of reports came from the Chugoku, Shikoku, and Kyushu districts in 1995, whereas such reports came from the Kinki (see IASR, Vol. 17, No. 11) and Tokai districts in 1996 (Table 2). To the contrary, reports on isolation of CB4 were the largest in number among those of enteroviruses in 1996, totaling 319 from 37 laboratories in the whole country (Table 2).
Being different from meningitis cases due to enteroviruses, the diagnosis of mumps meningitis is usually made not virologically but clinically; reports on isolation of mumps virus, therefore, are very few. Such reports numbered 40 (14%) in 1996, being on a similar level as usual (see Table 1 and IASR, Vol. 15, No. 9).
In Fig. 4 shown are the ages in years of children and in months of infants from which E7, CB4 or mumps virus was isolated. Of the cases from which CB4 was isolated in 1996, meningitis was frequently diagnosed among 0-year infants, but more rarely in 1- to 2-year cases. The ages of 0-year infants are detailed in months; the ratio of the cases aged 0 to 1 month was high and that of those aged over 2 months was low. A similar tendency was seen with CB5, which was frequently isolated in 1995 (see IASR, Vol. 17, No. 3), showing characteristics of CB (see IASR, Vol. 16, No. 8). In contrast to this, cases from which mumps virus was isolated included few 0-year infants.
Besides those described above, a few reports on isolation of some noteworthy meningitis-related viruses conducted by PHIs in compliance with the infectious agent surveillance program in 1996 are presented in the following. In Osaka Prefecture, a local meningitis epidemic due to E4 occurred in March, not in the enterovirus-prevailing season, and some elementary schools or their classes were closed (see IASR, Vol. 17, No. 6). Besides, E20, which was not isolated for 8 years after 1988, was isolated from six cases in Wakayama Prefecture (three were 0-, 7-, and 8-year meningitis cases) (see IASR, Vol. 18, No. 1), and E21, which was not isolated for 5 years after 1991, was isolated also from six cases in Shimane and other prefectures (two were 7- and 9-year meningitis cases) (see IASR, Vol. 18, No. 2).
In surveying for enteroviruses, antisera for serotyping are indispensable. To make up for the insufficiencies of the antisera now on market, the National Institute of Infectious Diseases with cooperation of some PHIs have prepared serum pools for echovirus type differentiation and are trying to apply them in the surveillance (see IASR, Vol. 18, No. 3).