Molecular evolution of Japanese encephalitis virus isolates from swine in Oita, Japan during 1980–2009

In order to identify the patterns of genetic change of Japanese encephalitis virus (JEV) strains circulating in Oita, the complete envelope (E) gene has been sequenced for 35 isolates from swine in a 30-year span. Based on nucleotide and deduced amino acid sequences, the genetic variation was examined, phylogeny was estimated and selection pressures were also analyzed. This study demonstrated that the major genotype (G) of JEV isolates had shifted from GIII to GI in the mid-1990s in Oita. The intensities of selection acting on the Oita GIII and GI strains were found to be almost same. It suggests that the intensity of selection might not be the reason for such a genotype shift observed in Oita. Pairwise comparisons revealed the high conservation of the E gene at the protein level. Compared with the Oita GIII strains, all the Oita GI strains shared four amino acid changes at E129 (T–M), E222 (A–S), E327 (S–T) and E366 (A–S). Among all 70 JEV isolates involved in this paper, the GI strains shared only one amino acid change at E222 (A–S) in comparison with the GIII strains. No strong evidence for positive selection was found, the JEV evolution has generally been subject to strong purifying selection, but one ongoing evolutionary pathway was found to be under relaxed purifying selection in Oita. This study is a localized example of JEV molecular evolution in nature.