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.
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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.
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A. The principle route of human infection with West Nile virus 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 possibly cause illness. Additional routes of infection have become apparent during the 2002 West Nile epidemic. It is important to note that these other methods of transmission represent a very small proportion of cases. A recent investigation has confirmed WNV transmission through transplanted organs. Investigations of other patients who developed WNV infection within several weeks of receiving blood products or organs are ongoing to determine whether WNV was transmitted by transfusion or transplantation in any of these cases. There is one reported case of transplacental (mother-to-child) WNV transmission. Although transmission of WNV and similar viruses to laboratory workers is not a new phenomenon, two recent cases of WNV infection of laboratory workers have been reported.
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A. No. Even in areas where mosquitoes do carry the virus, very few mosquitoes - much less than 1% - are infected. If the mosquito is infected, less than 1% of people who get bitten and become infected will get severely ill. The chances you will become severely ill from any one mosquito bite are extremely small.
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A. No. West Nile encephalitis is NOT transmitted from person-to-person. For example, you cannot get West Nile virus from touching or kissing a person who has the disease, or from a health care worker who has treated someone with the disease.
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A. Infected mosquitoes are the primary source for West Nile virus. Although ticks infected with West Nile virus have been found in Asia and Africa, their role in the transmission and maintenance of the virus is uncertain. However, there is no information to suggest that ticks played any role in the cases identified in the United States.
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A. There is no evidence that a person can get the virus from handling live or dead infected birds. However, persons should avoid bare-handed contact when handling any dead animals and use gloves or double plastic bags to place the carcass in a garbage can.
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A. West Nile virus is transmitted by infectious mosquitoes. There is no documented evidence of person-to-person or animal-to-person transmission of West Nile virus. Normal veterinary infection control precautions should be followed when caring for a horse suspected to have this or any viral infection.
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A. With West Nile virus infection, the viremia usually is transient, and people clear the virus very quickly. Blood centers will take precautions (see preceding question and answer) to be sure that donors who have been diagnosed with West Nile virus have fully recovered before donating.
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A. No, but several companies are working towards developing a vaccine.
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A. Visit the American College of Physicians website: "Mosquitoes and mosquito repellents: A clinician's guide" (Mark S. Fradin, MD. Annals of Internal Medicine. June 1, 1998;128:931-940). You can also find information on insect repellents containing DEET at the Environmental Protection Agency (EPA) website, and at www.deet.com.
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A. Most infections are mild, and symptoms include fever, headache, and body aches, occasionally with skin rash and swollen lymph glands. More severe infection may be marked by headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, paralysis, and, rarely, death.
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A. Usually 3 to 15 days.
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A. Contact your health care provider if you have concerns about your health. If you or your family members develop symptoms such as high fever, confusion, muscle weakness, and severe headaches, you should see your doctor immediately.
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A. A relatively small number of WNV infected dogs (<40) and only 1 WNV infected cat have been reported to CDC during 2003. Experimentally infected dogs showed no symptoms after infection with WNV. Some infected cats exhibited mild, nonspecific symptoms during the first week after infection--for the most part only showing a slight fever and slight lethargy. It is unlikely that most pet owners would notice any unusual symptoms or behavior in cats or dogs that become infected with WNV.
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A. No. There is no documented evidence that West Nile virus is transmitted between horses. However, horses with suspected West Nile virus should be isolated from mosquito bites, if at all possible.
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A. Visit the USDA website Animal and Plant Health Inspection Service (APHIS), or U.C. Davis Center for Equine Health.
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A. Because of their outdoor exposure, game hunters may be at risk if they become bitten by mosquitoes in areas with West Nile virus activity. The extent to which West Nile virus may be present in wild game is unknown. Surveillance studies are currently underway in collaboration with the U.S. Geological Survey (USGS) National Wildlife Health Center (in Madison, Wisconsin) and with state and local wildlife biologists and naturalists to answer this question.
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A. Hunters should follow the usual precautions when handling wild animals. If they anticipate being exposed to mosquitoes, they should apply insect repellents to clothing and skin, according to label instructions, to prevent mosquito bites. Hunters should wear gloves when handling and cleaning animals to prevent blood exposure to bare hands and meat should be cooked thoroughly.

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