What is West Nile Virus?

Photo courtesy USGS West Nile flavivirus

West Nile virus is a potentially fatal disease that is spread by the bite of infected mosquitoes. In most of North America, it occurs primarily during the late summer and early fall (between July and mid-September). In the winter, the weather is too cold for most species of mosquitoes to survive. But in warmer climates, West Nile can be transmitted year-round.

West Nile virus belongs to a group of disease-causing viruses called flaviviruses, which are spread by ticks and mosquitoes. Other flaviviruses include yellow fever, Japanese encephalitis and dengue. (See Stanford.edu: Flavivirus to learn more about this class of viruses.) In very rare cases, West Nile has also been transmitted through blood transfusions, organ transplants and breastfeeding and has been passed from mother to baby during pregnancy.

Photo courtesy MorgueFile Crows are one of the bird species especially susceptible to West Nile.

Mosquitoes catch West Nile virus from biting infected birds -- more than 130 different bird species are known to have been infected with West Nile, and more than 40 species of mosquito can carry the West Nile virus. The virus circulates through the mosquito's bloodstream and into its salivary glands. Then, when the mosquito bites a human (or animal), it transmits the virus into their bloodstream.

Once in the human body, the West Nile virus multiplies and may cross the blood-brain barrier. Normally, this barrier prevents bacteria and viruses from getting into the brain. But a few viruses, including Herpes simplex virus, Eastern equine encephalitis and West Nile virus, are able to cross this barrier. Scientists do not yet fully understand the exact mechanism by which this occurs, but it may be due to a substance released during the immune response to these viruses. An immune-cell protein called TLR-3 recognizes the West Nile virus and activates an immune-cell response, leading to the secretion of a substance called TNF-alpha. This substance temporarily breaks down the blood-brain barrier, allowing the West Nile virus to slip through and gain access to the brain and central nervous system. Its entry causes the immune system to launch an inflammatory response, leading to the most serious West Nile symptoms of brain and/or spinal cord inflammation.