WEST NILE VIRUS INFORMATION

Human illness from West Nile virus is rare, even in areas where the virus has been reported. The chance that any one person is going to become ill from a mosquito bite is low.

You can further reduce your chances of becoming ill by protecting yourself from mosquito bites. To avoid mosquito bites:

• Apply insect repellent containing DEET (N,N-diethyl-meta-toluamide) when you're outdoors. For details on when and how to apply repellent, see Insect Repellent Use and Safety in our Questions and Answers pages. See also Using Insect Repellent Safely from the EPA.

• When possible, wear long-sleeved clothes and long pants treated with repellents containing permethrin or DEET since mosquitoes may bite through thin clothing. Do not apply repellents containing permethrin directly to exposed skin. If you spray your clothing, there is no need to spray repellent containing DEET on the skin under your clothing.

• Consider staying indoors at dawn, dusk, and in the early evening, which are peak mosquito biting times.

• Limit the number of places available for mosquitos to lay their eggs by eliminating standing water sources from around your home. Learn more on the Prevention of West Nile Virus question and answer page.

• Check to see if there is an organized mosquito control program in your area. If no program exists, work with your local government officials to establish a program. The American Mosquito Control Association can provide advice, and their book Organization for Mosquito Control is a useful reference. Another source of information about pesticides and repellents is the National Pesticide Information Center, which also operates a toll-free information line: 1-800-858-7378 (check their Web site for hours).

Overview Updated

Q: What are West Nile virus, West Nile fever, and West Nile encephalitis?
A. West Nile virus 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.

West Nile fever is a case of mild disease in people, characterized by flulike symptoms. West Nile fever typically lasts only a few days and does not appear to cause any long-term health effects.

More severe disease due to a person being infected with this virus can be West Nile encephalitis, West Nile meningitis, or West Nile meningoencephalitis. Encephalitis refers to an inflammation of the brain, meningitis is an inflammation of the membrane around the brain and the spinal cord, and meningoencephalitis refers to inflammation of the brain and the membrane surrounding it.

Q. Where did West Nile virus come from?
A. West Nile virus has been commonly found in humans and birds and other vertebrates in Africa, Eastern Europe, West Asia, and the Middle East, but until 1999 had not previously been documented in the Western Hemisphere. It is not known from where the U.S. virus originated, but it is most closely related genetically to strains found in the Middle East.

Q. Historically, where has West Nile encephalitis occurred worldwide?
A. See the map describing distribution of flaviviruses, including West Nile virus:

Q. How long has West Nile virus been in the U.S.?
A. It is not known how long it has been in the U.S., but CDC scientists believe the virus has probably been in the eastern U.S. since the early summer of 1999, possibly longer.

Q. I understand West Nile virus was found in "overwintering" mosquitoes in the New York City area in early 2000. What does this mean?
A. One of the species of mosquitoes found to carry West Nile virus is the Culex species, which survives through the winter, or "overwinters," in the adult stage. That the virus survived along with the mosquitoes was documented by the widespread transmission during the summer of 2000.

Q. Is West Nile virus now established in the Western Hemisphere?
A. The continued expansion of West Nile virus in the United States indicates that it is permanently established in the Western Hemisphere.

Q. Is the disease seasonal in its occurrence?
A. In the temperate zone of the world (i.e., between latitudes 23.5° and 66.5° north and south), West Nile encephalitis cases occur primarily in the late summer or early fall. In the southern climates where temperatures are milder, West Nile virus can be transmitted year round.

About the Virus, the Disease, and Its Spread

West Nile virus is spread by the bite of an infected mosquito, and can infect people, horses, many types of birds, and some other animals.

Most people who become infected with West Nile virus will have either no symptoms or only mild ones. However, on rare occasions, West Nile virus infection can result in severe and sometimes fatal illnesses.

There is no evidence to suggest that West Nile virus can be spread from person to person or from animal to person.

See the Overview of West Nile Virus section of our Questions and Answers pages and the Background page for details.

PLEASE NOTE: CDC is not a hospital or clinical facility; we do not see patients and are unable to diagnose your illness, provide treatment, prescribe medication, or refer you to specialists.

If you have a medical emergency, contacting CDC is not the proper way to get immediate help. If you are a patient, please see your health care provider or the nearest emergency room. If you are a health care provider, please contact your state epidemiologist or local health department.

Reporting Dead Birds

Dead birds in an area may mean that West Nile virus is circulating between the birds and the mosquitoes in that area. Over 110 species of birds are known to have been infected with West Nile virus. Although birds, particularly crows and jays, infected with WN virus can die or become ill, most infected birds do survive.

The public can play an important role in monitoring West Nile virus through reporting dead birds to state and local health departments. However, in some areas, birds are no longer being collected. Inaddition, state and local agencies have different policies for collecting and testing birds. See the Links to State and Local Government Sites page to find information about reporting dead birds in your area.