Ecologic Studies of Japanese Encephalitis Virus in Japan

Abstract

Summary Linear study of mosquitoes in several habitats near Tokyo between 1952 and 1957 established that the only species consistently infected with JE virus naturally was C. tritaeniorhynchus. Mosquito infection was a regular summer phenomenon limited to late June-early October. Density of infected mosquitoes at a study site is considered the important ecologic concept rather than frequency of infected mosquitoes in samples tested. Although infection frequencies varied yearly in one area (Shinhama) and were similar from place to place in single years, densities of infected mosquitoes may have been just the opposite because differences in total populations overcompensated for changes in infection frequencies. Natural frequencies of infected mosquitoes increased progressively during the summer to reach maxima of one in 2 to 20. Density of infected mosquitoes increased concomitantly with frequency until peak frequencies were reached and total populations declined enough to reduce the density of infected mosquitoes at a study site. Time of C. tritaeniorhynchus infection was independent of total population and the incidence of mosquito infection increased while the total population declined. It is therefore particularly important to emphasize that there is no direct, chronologic relationship between peak vector populations, and epidemic or epizootic infection. However, infection of vertebrates occurs coincident with, and as a consequence of, mosquito infection and is related directly to the time when maximal density of infected mosquitoes occurs in a specific area.