Variation in the incidence and causal factors of Japanese Encephalitis in Okayama Prefecture from 1950 to 1989

Variation in the incidence and causal factors of Japanese Encephalitis in Okayama Prefecture from 1950 to 1989 were surveyed, with results revealing, first, that incidence rates of Japanese Encephalitis peaked first in 1950 and then again in 1956 and 1964, with cases of Japanese Encephalitis rarely found since 1971. Second, positivity rates of hemoagglutination inhibition antibody tests decreased after July 1967. Third, the numbers of swine per pigsty has increased annually, indicating that large-scale pigsties have been constructed. Fourth, the prevalence of culex tritaeniorhynchus decreased beginning in 1965 though it has gradually recovered since 1974. Fifth, the vaccination rate of the inhibitants of the area increased beginning in 1957, peaked in 1967, and has since gradually declined. Finally, the relationship between changes in causal factors of Japanese Encephalitis and the declining incidence of the latter in Okayama Prefecture since 1966 are discussed.