Serological epidemiology of Japanese encephalitis virus infections in Ishigaki and Iriomote Islands, Okinawa

Abstract

Hemagglutination-inhibition (HI) tests against Japanese encephalitis virus were carried out on healthy residents in Ishigaki Island between late July and early August from 1967 to 1972 except for 1969. In 1967 and 1968, HI positive rates in the group of over 20 years of age were 81.6% and 79.1% respectively. The HI positive rates and high antibody titers showed a tendency to increase with age. However, extending from 1967 to 1972, it had been revealed that the almost all residents of the group ofyounger age under 19 years old were HI negative. A small number of HI positive persons of the youngage had experiences in living outside of Ishigaki Island such as Okinawa Island, Miyako Island orTokyo. In 1970, the HI tests were also carried out on residents in Iriomote Island. The results showedsame pattern as those of Ishigaki Island. On the other hand, in 1968, the HI tests were carried out onpigs, cattles and goats in Ishigaki Island. Although positive rates of these animals were relatively low, asmall number of the animals born and bred in Ishigaki Island showed HI positive.The results suggested that Japanese encephalitis virus had been widespread in Ishigaki and IriomoteIslands until about 20 years before the beginning of survey but thereafter the residents had beendelivered from the virus infection. It was thought that this event was closely related with exterminatorymeasures against malaria in these areas since 1945. However, it may be possible that Japanese encephalitisvirus has been still alive in these areas from the survey of the domestic animals.