Abstract
West Nile virus maintains natural infection cycle between birds and mosquitoes. It has been known that about 200 species of birds are infected with West Nile virus and the virus is isolated from more than 40 species of mosquitoes. This suggests that West Nile virus has an ability to be transmitted by many species of mosquitoes and infect many kinds of animals. Approximately 20% of infected humans develop symptoms. West Nile fever, an acute febrile illness, is the main disease, and meningitis and encephalitis (meningoencephalitis) occasionally occur. Cases with flaccid paralysis or polyneuritis have been recently reported. Thus, West Nile virus causes multiple types of symptoms in humans. The endemic area has expanded in North America and Siberia. West Nile virus may enter Japan in the near future; therefore, we should keep paying attention to the endemic and epidemic situations in the world.
