West Nile Virus Frequently Asked Questions

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West Nile Virus
According to the Centers for Disease Control and Prevention, West Nile virus (WNV) is the leading cause of viral encephalitis in the United States. Originally discovered in Africa in 1937, WNV was first detected in the United States in 1999 in New York City. Since then it has caused seasonal epidemics of West Nile virus; fever and severe disease. West Nile virus is transmitted to humans and animals through the bite of an infected mosquito. Mosquitoes become infected with WNV when they feed on infected birds.

Q. Where did West Nile virus come from?

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

Q. How long has West Nile virus been in the U.S.?

A. It is not known exactly how long the virus has been in the U.S., but Centers for Disease Control scientists believe it has probably been in the eastern U.S. since the early summer of 1999 or possibly longer.

Q. How do people get infected with West Nile virus?

A. Humans get West Nile virus (WNV) 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 gets 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.

Other methods of transmission which have been reported, though they are a very small proportion of cases include WNV transmission through: transplanted organs, blood transfusions, or transplacental, from mother-to-child.

Q. If I live in an area where birds or mosquitoes with West Nile virus have been reported and a mosquito bites me, am I likely to get sick?

A. No. Even in areas where mosquitoes do carry the virus, very few mosquitoes are infected. If the mosquito is infected, less than 1% of people who get bitten and become infected will get severely ill. Approximately 1 in 150 people get severely sick.

Q. Can you get West Nile encephalitis from another person?

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.

Q. Besides mosquitoes, can you get West Nile virus directly from other insects or ticks?

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. There is no information to suggest that ticks played any role in the cases identified in the United States.

Q. Can you get West Nile virus directly from birds?

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. If you find a dead bird with no signs of trauma, please report it here.

Q. Can I get infected with West Nile virus by caring for an infected horse?

A. West Nile virus is transmitted by the bite of an infected mosquito. There is no documented evidence of person-to-person or animal-to-person transmission of West Nile virus.

Q. If a person has had West Nile virus, can they still donate blood?

A. With West Nile virus infection, the virus is usually short lived and people clear it very quickly. Blood centers take appropriate precautions to be sure that donors who have been diagnosed with West Nile virus have fully recovered before donating.

Q. Is there a vaccine against West Nile encephalitis?

A. No, but several companies are working towards developing a vaccine.

Q. Where can I get more information on mosquito repellents?

A. For a variety of information regarding repellent, visit the Center for Disease Control website at: http://www.cdc.gov/ncidod/dvbid/westnile/qa/insect_repellent.htm. You can also find information on insect repellents containing DEET at www.deet.com.

Q. What are the symptoms of West Nile encephalitis?

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.

Q. What is the incubation period in humans (i.e., time from infection to onset of disease symptoms) for West Nile encephalitis?

A. Usually 3 to 15 days.

Q. I think I have symptoms of West Nile virus. What should I do?

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.

Q. Can West Nile virus cause illness in dogs or cats?

A. A relatively small number of WNV infected dogs (<40) and only 1 WNV infected cat have been reported to CDC. 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.

Q. Can a horse infected with West Nile virus infect horses in neighboring stalls?

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.

Q. Where can I get more information on horses and West Nile virus?

A. Visit the California Department of Food and Agriculture (CDFA) website at http://www.cdfa.ca.gov/AHFSS/Animal_Health/wnv_info.html or the U.C. Davis Center for Equine Health at http://www.vetmed.ucdavis.edu/CEH/.

Q. Are duck and other wild game hunters at risk for West Nile virus infection?

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.

Q. What should wild game hunters do to protect against West Nile virus infection?

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.