Nationwide surveillance of West Nile virus targeting mosquitoes and dead birds from April 2004 through March 2007 in Japan

We conducted nationwide West Nile virus (WNV) surveillance targeting mosquitoes and dead birds to reveal whether the virus and its potential vectors are present in Japan. A total of 12 766 mosquitoes and 230 dead birds were collected in April 2004-March 2005 (the 2004-2005 period), 10 755 mosquitoes and 267 dead birds in April 2005-March 2006 (the 2005-2006 period), and 8624 mosquitoes and 245 dead birds in April 2006-March 2007 (the 2006-2007 period). The species of most of the mosquitoes collected over the 3 years were Culex tritaeniorhynchus (47.82%) and Anopheles sinensis (28.49%), and other species included Aedes albopictus (6.75%), the Culex pipiens group (Cx. pipiens pallens and Cx. pipiens molestus: 5.37%), Aedes vexans nipponii (2.54%), Armigeres subalbatus (1.08%), and Aedes japonicus (0.95%). As for the dead birds, most were Passeriformes (456 specimens), which included several crow species, and the other orders included Anseriformes, Columbiformes and Ciconiiformes (78, 66 and 36 specimens, respectively). All the specimens tested negative for WNV RNA by reverse-transcriptase polymerase chain reaction (RT-PCR) in the 2004-2005 period and by real-time RT-PCR in the 2005-2006 and the 2006-2007 periods, respectively. Our surveillance provided no evidence for WNV in Japan as of the end of the surveillance period, but on the other hand, it revealed that several species of potential WNV vectors are distributed widely in Japan, which suggests that WNV in principle could be transmitted by the potential vectors if introduced. Thus, it is essential to take continued precautions against WNV introduction.