EPIDEMIOLOGIC AND ECOLOGICAL-STUDIES OF JAPANESE ENCEPHALITIS IN OKINAWA, SUBTROPICAL AREA IN JAPAN .2. PREVALENCE OF JAPANESE ENCEPHALITISANTIBODY IN RESIDENTS IN OKINAWA, MIYAKO AND ISHIGAKI ISLANDS

During 1989 to 1990, human sera were collected by age groups in Okinaw a (the northern, central and southern areas), Miyako and Ishigaki isla nds and examined for the neutralization (N) antibodies to two strains, Nakayama (vaccine strain) and C307 (Okinawan strain), of Japanese enc ephalitis (JE) virus. In Okinawa island, the N antibody positive rate to C307 was higher than that to Nakayama, while in Miyako and Ishigaki islands, the positive rate to Nakayama was higher than that to C307, suggesting that JE virus transmission rate was higher in Okinawa than in Miyako and Ishigaki islands. In Okinawa Prefecture, JE vaccine had not been administered to most of residents over 31 years of age at the time of serum collection. In residents over 31 years old, the positiv e rate to C307 was highest in the north of Okinawa (83.3%) and was low est in Miyako (26.3%), with the second lowest in Ishigaki (33.3%). The distribution of N antibody titers to C307 gave hyperbolic patterns in the 0-5 age groups in Miyako and Ishigaki, and also in the 31-40, 41- 50 age groups in Miyako and the 41-50 age group in Ishigaki, suggestin g low rates of natural infection in these 4-5 decades in both islands. In residents of ages subjected to JE vaccine, a characteristic patter n was obtained, in which the curves to Nakayama shifted to higher tite rs than those to C307, suggesting that the first antigenic stimulation was caused by vaccine, not by natural infection of JE virus.