West Nile Virus

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West Nile virus (WNV) is a flavivirus commonly found in Africa, West Asia, and the Middle East. It is closely related to St. Louis encephalitis virus found in the United States. The virus can infect humans, birds, mosquitoes, horses and some other mammals. Until 1999, it had not previously been documented in the Western Hemisphere. The US virus is most closely related genetically to strains found in the Middle East.

West Nile fever is a mild disease characterized by flu-like symptoms that develop three to 15 days after being bitten by an infected mosquito. West Nile fever typically lasts only a few days and does not appear to cause any long-term health effects. Mild fever, headache, body aches, and occasional skin rash and swollen glands are the most common symptoms.

More severe disease can manifest as encephalitis (inflammation of the brain), meningitis (inflammation of the membrane around the brain and spinal cord), or meningoencephalitis (inflammation of the brain and the membrane surrounding it).

In the temperate zone of the world, West Nile encephalitis cases occur primarily in the late summer or early fall. In southern climates where temperatures are milder, WNV can be transmitted year round.

Cases of WNV in the United States

In 2001, there were 66 human cases of severe WNV in the US, including nine deaths. Among those with severe illness due to WNV, case-fatality rates range from 3 to 15 percent and are highest among the elderly. Less than 1 percent of persons infected with WNV will develop severe illness. View a map of WNV spread in the US, 1999-2002.

How do people get infected with WNV?

The principle route of human infection with WNV is through the bite of an infected mosquito. Mosquitoes become infected when they feed on infected birds, which may circulate the virus in their blood for a few days. The virus eventually finds its way into the mosquito's salivary glands. During subsequent blood meals, the virus may be injected into humans and animals, where it can multiply and potentially cause illness.

View a map of WNV cases in the US, 2006.

Basic Transmission Cycle of WNV

Mosquitoes become infected when they feed on infected birds. Infected mosquitoes can then transmit WNV to humans and animals while biting to take blood. The virus is located in the mosquito's salivary glands. During blood feeding, the virus may be injected into the animal or human, where it can multiply, possibly causing illness. View the WNV Transmission Cycle.

To learn more about West Nile virus, see Frequently Asked Questions about WNV.