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West Nile Virus

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Also known as: WNV

What is West Nile virus?

West Nile virus (WNV) is an infectious disease that was first discovered in Uganda, Africa in 1937. It then spread slowly through the Middle East, Eastern Europe, and West Asia. The first U.S. cases of WNV were reported in New York in 1999. Since then, the viral infection has moved steadily west and south and is now found in Canada and every state in the U.S. except Hawaii and Alaska.  According to the Centers for Disease Control and Prevention (CDC), WNV is the leading cause of encephalitis due to arboviruses in the country. Outbreaks of cases begin to appear in the summer and increase through the fall, and the medical community tracks its emergence and promotes awareness and means for its prevention.

WNV is a mosquito-borne virus and belongs to the flavivirus group that causes illness in many regions of the world. Other flaviviruses are responsible for conditions such as dengue fever and yellow fever. These infections can affect both humans and a variety of animals. WNV tends to be seasonal in the United States, but in tropical areas of the world, flavivirus infections may occur year round.

Birds are the primary WNV hosts and carriers, and mosquito bites are the most common route of transmission. When a mosquito bites an infected bird, the mosquito becomes infected and can then transmit the virus to other animals or humans that it bites. It is estimated that about 1 in 200 mosquitoes harbors the virus.

Although WNV is usually not transmitted person-to-person, there have been cases of WNV being passed on to others through blood donations, organ transplants, and rarely from a mother to her baby during pregnancy, delivery, or through breast milk while nursing. Blood centers in the United States now routinely screen donated units of blood for the presence of WNV.

For the most current numbers of confirmed human cases of WNV in the U.S. and the number of deaths attributed to it as the cause, visit the CDC's West Nile Virus web site.

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