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WNV Description & Distribution

West Nile virus (WNV) is a mosquito-borne virus that can cause encephalitis (inflammation of the brain) and/or meningitis (inflammation of the lining of the brain and spinal cord).  Outbreaks of the disease caused by WNV have occurred in Egypt, Asia, Israel, South Africa and some parts of Europe and Australia.  The virus was first seen in the US in the fall of 1999 in New York City, and has since spread across the US to the Pacific Ocean, into several Canadian Provinces and possibly into Mexico.  WNV was first detected in Michigan in 2001 and has been detected in Michigan each year since 2001.

WNV primarily infects and multiplies in birds, which serve as reservoirs for the virus.  The virus is spread between birds mostly through the bite of an infected mosquito.  It has been detected in more that 162 species of birds, however corvids (crows and blue jays) are more severely affected than other species of birds and often die as a result of their infection.  When the level of virus circulating amongst birds and mosquitoes becomes high, horses and humans can be accidentally infected.  The virus also has been shown to infect several other types of mammals (including bats, cats, chipmunks, squirrels, skunks, and domestic rabbits) usually without causing clinical signs.