Abstract
Seasonal rise in the HI antibody in young swine sera against Japanese encephalitis (JE) virus was investigated in Kumamoto, Japan, from 1965 to 1983. Time of appearance of positive antibody in 50% of the sera were studied annually. From 1965 to 1967, 50% of sera became positive by the end of June. But thereafter, the 50% positive time was delayed, in general, to the mid July in the years from 1968 to 1973, and up to the early August after 1974.
Besides this trend, variations in the 50% positive time were also observed from year to year. During the 11 years from 1970 to 1980, the date of the 50% positive time was positively correlated with the date of the 6th day when the daily mean temperature became over 27°C (r=0.65, n=11, p<0.05) . The number of human JE patients in Kumamoto also tended to increase when the 50% positive time appeared earlier. The secular trend in the variation of the 50% antibody positive time in swine sera, however, was thought to be caused by some other factors than meteorological ones. Some probable explanations of the secular and drastic changes were discussed.
