Ecological Studies on Japanese Encephalitis Virus: Results of Investigations in Nagasaki Area, Japan, in 1969, 1970 and 1971

Serial surveys on the ecology of Japanese encephalitis virus in Nagasaki area were made during from 1969 to 1971. In total, 11,229 hibernated female mosquitoes of Culex tritaeniorhynchus were caught in early spring every year for virus isolation, however it was unsuccessful. The significant increase of hemagglutination inhibition antibody possessing rate in the sera of slaughtered pigs in early spring was not found in these three years. Virus isolations from the vector mosquitoes in epidemic season were made from 1st to 26th of August in 1969, from 19th of July to 16th of August in 1970 and from 13th to 27th of July in 1971, respectively. Although the isolation efficiencies were not remarkably different at the highest level in a certain limited period in epidemic season during the years 1964 to 1971, the periods for JE virus isolation from the vector mosquitoes became shorter in the years from 1968 to 1971 than from 1964 to 1967. It was considered as one of the reasons that the number of the vector mosquitoes was smaller during the epizootic from 1968 to 1971 than from 1964 to 1967. Subsequently, it was noted that the encephalitis case sbecame to decrease in number in recent years.