Japanese encephalitis patients in Okayama Prefecture are very few yielding four patients in the southern district and only one patient in the northern district.
Examining the relationship between the cause with which the above fact (the low prevalence of the disease) came out, and the date on which swine turned out positive in their sera, the following results were obtained.
1) The date showing 50 percent positive HI reaction in swine came more lately in this year, 1970 than past years of 1965-1969, taking the date of the 24th in August in the southern district and the 31th in the same month in the northern district.
2) The shortness of the active term of mosquitoes of Culex Tritaeniorhynchus owing to the late coming of the date showing HI positive reaction in swine serum can be pointed out as the cause of the low prevalence of the disease.
3) It can be said in 1965 year to 1970 that there is a tendency of the lower morbidity rate in both southern and northern district according to the later coming of 50 per cent positive HI reaction in swine. And the correlations between the date of the 50 per cent positive HI reaction and the morbidity rate of the disease were shown as the value of -0.75 in the southern district and -0.68 in the northern district in Okayama Prefecture.
4) With the test made in the term of the 28th, July to the ninth, September, the positive HI reaction was not observed in domestic fowls.
