A survey was conducted on the prevalence of Japanese B encephalitis occurring in Okayama Prefecture from the standpoint of seasonal variation, locality, age and sex during the year 1960, and the results were compared with the national statistics. The results were as follows:
1. The total number of patients with Japanese B encephalitis was 89, 29 deaths occured and the morbidity rate was 5.1. This 5.1 morbidity rate is of moderate prevalence for Okayama Prefecture ranking 5th place in the nation; the rate 5.1 is about 3 times the national morbidity rate.
2. The peak period of this disease in the southern district of Okayama Prefecture comes about one week earlier than that in the northern district.
3. The incidence of the disease during 1960 differed completely from the usual regional trend, showing a higher morbidity rate in such regions as Wake, Jodo, Oda, Katsuta Counties and Niimi City. This is in marked contrast to the usual morbidity rate which was higher in such districts as Kurashiki City, Asakuchi County and Kojima City. This seems to indicate that the prevalency of this disease is moving from the south-western to the south-eastern and north-western districts.
4. The mortality rates of 1960 in the northern and southern districts did not change much from the years 1958 and 1959. The mortality rate in the northern district in 1960 was 60 per cent which is not an appreciable difference from 1950; however, in the southern district the mortality rate was about 42 per cent in the period of 1950-1955 and it decreased to 21.9 per cent in 1960. This decrease in the mortality rate is about one half of former years and is believed to be due to the improvement of medical facilities in the southern districts.
5. The number of vaccinated individuals was 370, 000 and the rate of vaccination was 24.7 per cent compared to the total population. The morbidity rate in the vaccinated group was 1.9 per cent and 6.2 per cent in the unvaccinated group showing a significant decrease in the morbidity in the vaccinated group. In the
x2 test of the difference between the morbidity rates of the vaccinated group and the unvaccinated group, the risk of error proved to be less than one per cent.
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