Genetic characterization of early isolates of Japanese encephalitis virus: genotype II has been circulating since at least 1951.

Japanese encephalitis virus (JEV) consists of five genotypes (GI-V). Phylogenetic characterization of 16 JEV strains isolated from the 'USSR', Japan and Korea during the 1930-1970s revealed that 15 strains fell into GIII, confirming that GIII was the predominant genotype of JEV in Japan and Korea between 1935 (isolation of the prototype strain; a GIII virus) and the 1990s (when GI supplanted GIII). One of the Korean isolates fell into GII, demonstrating that GII has been circulating for at least 19 years longer than previously thought. Formerly, GII was associated with endemic disease and this genotype had never been isolated north of Southern Thailand. Additionally, the northern border of GIII prevalence was extended from Japan to the 'USSR'.