Molecular epidemiology of Japanese encephalitis in East Asia.

Japanese encephalitis virus is genetically divided into five genotypes. Until the early 1990s, only genotype-3 JE virus was identified in Japan, then, the genotype shifted to type-1 countrywide. Based on our molecular epidemiological study of JEV in Vietnam, China, and Japan, 8 subgroups of genotype 1 JEVs are circulating in East Asia and, at least, 4 of these subgroups are chronologically related and frequently introduced to Japan through China. Possibly, this subgroup distribution is due to bird migration and/or infected mosquitoes which carry viruses for long distances. While in Japan, a small population of genotype 1 is circulating locally.