West Nile virus (WNV) was first isolated from a febrile patient in the West Nile district of Uganda in 1937. From 1937 to the early 1990s, human outbreaks, manifesting as mild febrile illnesses, were rarely reported in Israel and Africa. Since 1996, there have been outbreaks involving thousands of people in Romania, Russia, Israel, and the United States and Canada. More than 4,000 people were affected in the Ohio and Mississippi River basins in 2002. Eighty-five percent of human infections occur in August and September, consistent with the bird-mosquito-bird cycle. Increased age is a risk factor for mortality.
West Nile virus is a single-stranded RNA flavivirus belonging to the Japanese encephalitis virus antigenic complex. This complex con tains several viruses that cause encephalitis in humans: St. Louis encephalitis virus in the Americas, Japanese encephalitis virus in East Asia, and Murray Valley encephalitis virus and Kunjin virus in Australia. Two lineages of WNV exist. Only lineage 1 viruses cause human disease. The virus has minimally evolved genetically since being isolated in 1999 .
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