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Last Updated: 01/21/15 08:51:44 AM

West Nile Virus Basics

What is West Nile Virus? 
West Nile Virus (WNV) is an arbovirus that can cause illness in humans, including encephalitis (inflammation of the brain). Mosquitos get the virus by feeding on infected birds and can then pass it on to other birds, and occasionally to other animals and people. The virus is not spread person-to-person.  

An arboviral encephalitis is a viral infectious disease affecting the brain that is transmitted by a mosquito or other insect vector. The virus attacks the brain, causing inflammation and swelling. Arboviruses belong to several groups of viruses that usually infect birds and are transmitted from bird-to-bird by mosquitos. The name "ar-bo-virus" comes from the fact that they are transmitted by arthropods (insects and other "bugs"). WNV is caused by a specific type of arbovirus known as a flavivirus, which was found previously only in Africa, eastern Europe and west Asia. WNV is closely related to other arboviruses such as St. Louis Encephalities (SLE), found in the U.S., Japanese Encephalities, found in southeast Asia, and Murray Valley Fever, found in Australia and New Guinea.  

In the U.S., WNV cases are most prevalent in late summer and early autumn; in Nevada, mosquito season is typically April through October.

West Nile Virus Background and History in Nevada
Prior to 1999, no WNV cases had been reported in the United States, although the virus had a wide distribution in Africa, Asia, the middle east and Europe and caused occasional epidemics. In 1999, the virus appeared in New York City, and was responsible for an encephalitis outbreak causing 62 human cases and seven deaths.

Between 1999 and 2003, WNV expanded from the northeastern U.S. to the south central states and westward, appearing on the west coast for the first time in 2002. Across the U.S., the number of human WNV cases in 2002 increased dramatically and greatly expanded in geographic range (4,161 reported cases with 277 deaths). The 2002 epidemic was the largest WNV epidemic ever reported and the largest of any reported arboviral encephalitis epidemic in the Western Hemisphere.

In 2003, WNV activity continued to increase, and was reported in 48 states and the District of Columbia. Today, WNV is firmly established throughout most of the US.

WNV was first detected in Nevada in 2004, and has been reported in all counties. 


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