Shift in Japanese encephalitis virus (JEV) genotype circulating in northern Vietnam: implications for frequent introductions of JEV from Southeast Asia to East Asia

1 Department of Virology, National Institute of Hygiene and Epidemiology, 1 Yersin Street, Hanoi, Vietnam 2 Department of Virology, Institute of Tropical Medicine, Nagasaki University, Sakamoto 1-12-4, Nagasaki City 852-8523, Japan 3 Division of Epidemiology, Department of Infectious Diseases, Oita Medical University, Hasama-machi, Oita, Japan 4 Department of Vector Ecology and Environment, Institute of Tropical Medicine, Nagasaki University, Sakamoto 1-12-4, Nagasaki, Japan 5 Department of Epidemiology, Entomology Laboratory, National Institute of Hygiene and Epidemiology, 1 Yersin Street, Hanoi, Vietnam 6 CREST, Japan Science and Technology Corporation, Saitama 332-0012, Japan Correspondence Kouichi Morita moritak{at}net.nagasaki-u.ac.jp This study analyses the evolutionary relatedness of 16 Japanese encephalitis virus (JEV) isolates (nine from Vietnam and seven from Japan) to previously published JEV strains using E gene sequence data. Vietnamese and Japanese strains isolated between 1986 and 1990 were found to cluster in genotype 3. However, more recent Vietnamese and Japanese strains isolated between 1995 and 2002 grouped within genotype 1, now a dominant though previously unreported genotype in Vietnam. In addition, in this study, strains isolated between 1995 and 2002 were more closely related to those isolated in the 1990s than to the older genotype 1 strains. Recently, the introduction of JEV genotype 1 into Japan and Korea has also been reported. Hence this genotype shift phenomenon may be occurring throughout all East Asia. Further studies on JEV ecology are needed to clarify the mechanism of JEV genotype 1 spread to new territories. These authors contributed equally to this work.