Evidence of frequent introductions of Japanese encephalitis virus from south-east Asia and continental east Asia to Japan.

The Japanese encephalitis virus (JEV) circulating in Japan consists of viruses with multiple phylogenetic origins. Phylogenetic analysis revealed that some JEV strains have recently migrated from south-east and continental east Asian countries. One phylogenetic subcluster of the JEV strains circulating in Japan was closely related to viruses isolated in Vietnam and China's inland region while other JEV subclusters were related to viruses isolated in Shanghai, China. One virus subcluster, however, was isolated solely in Japan and was not found in any other Asian country. Therefore, our data suggests that the JEVs that have remained or are circulating in Japan include a mixture of viruses that have previously migrated from south-east and continental east Asian countries.