West Nile Virus
West Nile Virus (WNV) causes an infection in horses, birds and humans.  People and animals get the virus when they are bitten by an infected mosquito.  There is no evidence that West Nile virus can spread from one person to another or from an animal to a person.

WNV is transmitted by infected mosquitoes, primarily Culex, Aedes, and Ochlerotatus species.  Mosquitoes become infected after biting infected birds that serve as the primary host of the virus. The virus undergoes a reproductive cycle inside the mosquito, in which it must pass through the midgut wall, multiply in many tissues, and accumulate in the salivary glands of the mosquito. Mosquitoes salivate every time they bite, and they are capable of transmitting the virus 10 to 14 days after feeding on an infected bird, so bites after that time are infectious.

Effects on humans

In humans, the virus usually does not produce symptoms in people who are infected.  Those who do get ill from the virus usually experience symptoms within 5 to 10 days after being bitten by an infected mosquito.  Their symptoms can include fever, headaches, body aches, swollen glands and a rash.  People above age 50 face the greatest risk of severe illness due to West Nile Virus.

Less than one percent of infected people develop encephalitis from West Nile Virus.  This severe illness can cause convulsions, paralysis and even death.

As of November 2005, there have been 19,224 confirmed human cases of WNV in the United States. Last year, 105 fatalities were attributed to the virus. 

There is no specific treatment for West Nile infections. After having West Nile Virus, patients may be immune to the virus in the future.

The Centers for Disease Control offers more detailed information on West Nile Virus.


Home page   Protect yourself     Control mosquitoes     What is West Nile Virus?