Abstract
A survey was done on the prevalency of Japanese B encephalitis in Okayama Prefecture during the year 1961 from the standpoint of seasonal variation, age and sex as well as locality and the following results were obtained:
1. The number of encephalitic patients was 79, the number of deaths was 29, and the mortality rate was 36.7 per cent. Its prevalency in Okayama Prefecture was intermediate, ranking eleventh in Japan with 2.1 times the national morbidity rate.
2. As for the incidence of patients according to the season, the peak period in the southern district of Okayama Prefecture is about ten days earlier than that in the northern district.
3. This disease increased after 1960 in Mitsu, Kibi Counties, and Takahashi City instead of such districts of Okayama City, Tamano City, Asakuchi County and Kurashiki City where it used to occur frequently. The trend during 1960 and 1961 has been to move from the former districts of the south toward the adjacent northern districts.
4. In 1961 the mortality rate in the northern distrct was 65.4 per cent and 22.6 per cent in the southern district, showing a distinct decrease. There is an increasing morbidity in old people in the southern district. One of the reasons for the decrease in the mortality rate is due to the improvement in medical facilities since 1958.
5. The number of persons receiving preventive vaccination in 1961 was 415, 000; the vaccination rate was 24.8 per cent. The morbidity rate was 1.4 per cent in the vaccinated group and 5.8 per cent in the unvaccinated group. The difference of the morbidity rate between the vaccinated group and the unvaccinated group is significant. The error in the statistics obtained from using the x2 test is under 0.1 per cent.
