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What is the West Nile Virus?

Mosquito

West Nile is a flavivirus similar to those associated with past occurrences of Western Equine Encephalitis and St. Louis Encephalitis. The West Nile Virus has been present in Africa and the Middle East for many years. It was first identified by scientists in Equatorial Africa near the Nile River in the 19th Century. It's name is derived from its location of discovery.

Certain species of mosquitoes are known as "vectors" in the spread of the West Nile Virus. These mosquitoes are infected with the virus when they feed on the blood of infected birds. While birds in the Corvid family, including ravens, crows and blue jays, are most often mentioned as carriers of the West Nile Virus, these birds frequently die from the infection. Among the ways through which West Nile is first identified in a new geographic area is the detection of dead crows and ravens that were infected with the virus.

After an incubation period of 5 to 15 days, a mosquito infected with the virus can transmit it to birds, animals and humans when it bites again. Humans and animals are known as "end hosts" for the virus, since it cannot be transmitted from one infected person to another nor from one infected animal to another. Most people infected with the West Nile Virus by an infected mosquito will develop no symptoms. Some people may experience mild influenza-like symptoms. A very small number (less than 1%) of people who are infected will become severely ill. Elderly people, newborn infants, some very young children and people with a depressed immune system are most at risk for severe illness from the West Nile Virus. Healthy people that are bitten by an infected mosquito typically develop antibodies to the virus.

Protecting Yourself

  • Inspect all window and door screens to make sure they are in good condition and free of holes.
  • When you are outdoors in areas that have mosquitoes, wear long sleeve shirts and long pants.
  • Use mosquito repellent. Repellents greatly reduce your exposure to mosquitoes when applied to exposed clothes and skin.
  • Avoid outdoor activities at times when mosquitoes are actively feeding, mostly in the early morning and at dusk.
  • Reduce mosquito breeding areas on your property by cleaning gutters to eliminate standing water, emptying unnecessary water in buckets, pots, animal dishes, etc. outside your house and by safely eliminating mosquito larvae in outdoor ponds and water features.

West Nile Virus—Frequently Asked Questions