Epidemiological Analysis of the Japanese Encephalitis Virus Spread from Mosquitoes to Pigs

The five years survey results of antibody rise in summer against Japanese Encephalitis Virus in pigs in Natori district of Miyagi Prefecture were analyzed with regard to (1) the time of antibody rise, (2) the steepness of the antibody positive slope to reach to the height of 100%, (3) the level of antibody titers and (4) the detection rate of 2-ME sensitive antibodies. These serological results obtained in pigs were finally correlated to the isolation rate of the Japanese Encephalitis Virus from mosquitoes collected in the same area and to the incidence of Japanese Encephalitis patients among 1, 800, 000 residents in Miyagi Prefecture. On the basis of this limited 5 years survey, one can conclude as follows: 1. A hundred percent infection among pigs and a consistent isolation of the virus from mosquitoes have been the case for these 5 years. Thus the cyclic outbreak between mosquitoes and pigs existed every year, even in the absence of the spread to human. 2. The time of initial and consistent isolation of virus from mosquitoes coincided completely with the time of antibody rise among pigs, as was pointed out by us 5 years ago. 3. The earlier the appearance of initial antibody rise among pigs, the higher the incidence of infection among human. 4. A minor pre-outbreak and a minor post-outbreak of infection among pigs were noticed before and after the main outbreak among pigs. The fact was clearly illustrated by pursuing the 2-ME sensitive antibodies. 5. The height of antibody titers in pig sera was assumed to be a reliable indicator to predict the amount of virus harbored by mosquitoes. The titer in pig sera was lowest in a year of the least incidence of human infections, irrespective of the fact that 100% of pigs were infected every year.