Molecular epidemiology of Japanese encephalitis virus in Okinawa

In order to elucidate the molecular characteristics of Japanese encephalitis (JE) virus in Okinawa, 23 strains of JE virus isolated in a 25-year span were sequenced for the 240 nucleotides of the C-preM junction region and 111 nucleotides of the E gene region and compared with those of reference strains isolated in mainland Japan. The results of phylogenic analysis showed that although all the Okinawan isolates showed more than 96% homology in the nucleotide sequence in each region, they were chronologically divided into two groups: the old group (nine strains) and a new group (14 strains). On the other hand, in a comparison with reference strains in mainland Japan, the Okinawan isolates showed more than 94% nucleotide sequence homology in both regions, indicating that the Okinawan strains belong to the same genotype as that of JE strains in mainland Japan. The nucleotide homology of the old group was relatively higher than that of the new group. Among the 14 strains in the new group, 13 strains were isolated from mosquitoes collected from a pig farm from 1986 through 1992. These strains showed higher nucleotide divergence than the old group strains, isolated from mosquitoes and swine sera collected at several sites, in both regions. A nucleotide substitution at the position 1920 in the E gene was identified in three isolates. This substitution generated an aspargine-proline-threonine sequence capable of serving as an attachment site of carbohydrate.