Ecological studies on Japanese encephalitis virus. Survey of virus dissemination in Nagasaki area, Japan, in 1966, and 1967.

Serial survey on the ecology of Japanese encephalitis (JE) virus was made in 1966 and 1967. In spring in the both years, wild caught females of Culex tritaeniorhynchus and many other species including a great number of hibernated females were examined for JE virus. In 1967, besides the above, wild caught females in spring were forced to engorge blood from susceptible pigs, and some days after, the engorged ones were examined for the virus and the pigs for HI antibody. However, no virus was found in all the mosquitoes examined, and no antibody was detected in the pigs. Nevertheless, in mid-spring in 1967 2-ME sensitive antibody was found in a few of slaughtered pigs. This facts is very important and it seems necessary to set forward the program of investigations. Japanese common snakes of six species were examined for the reservoir of JE virus in nature, but it was concluded that snakes play minor role as a natural source of the virus transmission even in the epidemic season. Despite of the fact that the starting time of epizootic in pigs and of the occurrence of infected mosquitoes was nearly the same in 1966 and 1967, the duration and size of the infection in mosquitoes and of the epidemic in men were shorter and smaller in 1967 than in the previous year. The reason seems that the larger number of infected mosquitoes at the starting time in late June,1967 caused earlier and more rapid rise in the HI antibody possessing rate of pigs at the early days of epizootic in them reducing the remaining rate of susceptible pigs as faster as about a half-month than in 1966, and this, in turn, caused subsequent infections in mosquitoes and then in men to be shorter in duration and smaller in size.