Abstract
The mosquito-borne Japanese encephalitis virus (JEV) causes encephalitis in man but not in pigs. Complete genomes of a human, mosquito and pig isolate from outbreaks in 1982 and 1985 in Thailand were sequenced with the aim of identifying determinants of virulence that may explain the differences in outcomes of JEV infection between pigs and man. Phylogenetic analysis revealed that five of these isolates belonged to genotype I, but the 1982 mosquito isolate belonged to genotype III. There was no evidence of recombination among the Thai isolates, but there were phylogenetic signals suggestive of recombination in a 1994 Korean isolate (K94P05). Two sites of the genome under positive selection were identified: codons 996 and 2296 (amino acids 175 of the non-structural protein NS1 and 24 of NS4B, respectively). A structurally significant substitution was seen at NS4B position 24 of the human isolate compared with the mosquito and pig isolates from the 1985 outbreak in Thailand. The potential importance of the two sites in the evolution and ecology of JEV merits further investigation.