West Nile Virus

West Nile virus (WNv) is an emerging infectious disease, spread primarily by mosquitoes and birds, that was first detected in Uganda in 1937.  It was considered to be of minimal public health importance until childhood febrile epidemics were recognized in Egypt 20 years later.   

Since then a mild strain of the virus became prevalent in parts of Europe, Russia and Israel.  The lack of background immunity in resident birds and humans, combined with the evolution of more virulent strains, most likely set the scene for the extensive WNv epidemics documented in northern latitudes in more recent years. 

The virus has now spread beyond its traditional boundaries to North America, where it was first detected in New York City in 1999.  A rapidly organized and widespread intensive emergency response by New York City, as well as expanded ongoing mosquito control efforts in surrounding areas, proved insufficient to contain the virus and prevent it from over- wintering before dispersing into the remainder of the New World.  Within six years, WNv had moved from east to west coasts, spreading across all of the contiguous 48 states of the USA, and invading parts of southern Canada, Mexico and Central and South America.

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