Recent change of the antigenicity and genotype of Japanese encephalitis viruses distributed on Okinawa Island, Japan.

In this study, five isolates of Japanese encephalitis virus (JEV) were obtained from swine serum samples collected on Okinawa Island, Japan, between 2002 and 2003. All five JEV isolates belonged to genotype 1, and JEV isolates obtained from the island before 1992 were genotype 3. Genotype 1 was known to be distributed from northern Thailand to Cambodia and recently expanded to Australia, Vietnam, the Republic of Korea, and Japan. However, phylogenetic analysis showed that the source of the newly emerging genotype 1 in Asia is different from that in Australia. Sero-epidemiologic investigations showed that serum samples collected from 1985 to 1988 from JEV-immune swine neutralized both the Naha Meat 54 strain (1985 JEV Okinawan isolate from swine, genotype 3) and the Oki 431S strain (2002 JEV Okinawan isolate from swine, genotype 1), and many samples collected in 2002 neutralized the Oki 431S strain but not the Naha Meat 54 strain. These results strongly suggest that the genotype and antigenicity of JEV on Okinawa Island have changed significantly over the past decade.