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Health Tips
West Nile Virus
Overview of West Nile Virus
Centers for Disease Control and Prevention
Q. What is West Nile encephalitis?
A. "Encephalitis" means an inflammation of the brain and can be caused by viruses and bacteria, including viruses transmitted by mosquitoes. West Nile encephalitis is an infection of the brain caused by West Nile virus, 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.
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. 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.
Symptoms of West Nile Virus (CDC)
Centers for Disease Control and Prevention
Q. Who is at risk for getting West Nile encephalitis?
A. All residents of areas where virus activity has been identified are at risk of getting West Nile encephalitis; persons over 50 years of age have the highest risk of severe disease.
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.
Transmission of West Nile Virus (CDC)
Centers for Disease Control and Prevention
Q. How do people get West Nile encephalitis?
A. People become infected by the bite of a mosquitoes infected with West Nile virus.
Q. What is the basic transmission cycle?
A. Mosquitoes become infected when they feed on infected birds, which may circulate the virus in their blood for a few days. Infected mosquitoes can then transmit West Nile virus 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 may multiply, possibly causing illness.
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 the virus is circulating, very few mosquitoes are infected with the virus. Even 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.
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. However, some evidence exists that indicates West Nile encephalitis can be transmitted from person to person via organ transplant.
Q. Is a woman's pregnancy at risk if she gets West Nile encephalitis?
A. There is no documented evidence that a pregnancy is at risk due to infection with West Nile virus.
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. However, there is no information to suggest that ticks played any role in the cases identified in the United States.
Q. How many types of animals have been found to be infected with West Nile virus?
A. Although the vast majority of infections have been identified in birds, WN virus has been shown to infect horses, cats, bats, chipmunks, skunks, squirrels, and domestic rabbits.
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.
Q. Can I get infected with West Nile virus by caring for an infected horse?
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.
Q. How does West Nile virus actually cause severe illness and death in humans?
A. Following transmission by an infected mosquito, West Nile virus multiplies in the person's blood system and crosses the blood-brain barrier to reach the brain. The virus interferes with normal central nervous system functioning and causes inflammation of brain tissue.
Q. What proportion of people with severe illness due to West Nile virus die?
A. Among those with severe illness due to West Nile virus, case-fatality rates range from 3% to 15% and are highest among the elderly. Less than 1% of persons infected with West Nile virus will develop severe illness.
Q. If a person contracts West Nile virus, does that person develop a natural immunity to future infection by the virus?
A. It is assumed that immunity will be lifelong; however, it may wane in later years.
Prevention of West Nile Virus (CDC)
Centers for Disease Control and Prevention
Q. What can be done to prevent outbreaks of West Nile virus?
A. Prevention and control of West Nile virus and other arboviral diseases is most effectively accomplished through integrated vector management programs. These programs should include surveillance for West Nile virus activity in mosquito vectors, birds, horses, other animals, and humans, and implementation of appropriate mosquito control measures to reduce mosquito populations when necessary. Additionally, when virus activity is detected in an area, residents should be alerted and advised to increase measures to reduce contact with mosquitoes.
Q. Is there a vaccine against West Nile encephalitis?
A. No, but several companies are working towards developing a vaccine.
Q. What can I do to reduce my risk of becoming infected with West Nile virus?
A. - Stay indoors at dawn, dusk, and in the early evening.
- Wear long-sleeved shirts and long pants whenever you are outdoors.
- Spray clothing with repellents containing permethrin or DEET since mosquitoes may bite through thin clothing.
- Apply insect repellent sparingly to exposed skin. An effective repellent will contain 35% DEET (N,N-diethyl-meta-toluamide). DEET in high concentrations (greater than 35%) provides no additional protection.
- Repellents may irritate the eyes and mouth, so avoid applying repellent to the hands of children.
- Whenever you use an insecticide or insect repellent, be sure to read and follow the manufacturer's DIRECTIONS FOR USE, as printed on the product. · Install or repair window and door screens so that mosquitoes cannot get indoors.
- Note: Vitamin B and "ultrasonic" devices are NOT effective in preventing mosquito bites.
Q. Where can I get more information on mosquito repellents?
A. Visit the American College of Physicians website: "Mosquitoes and Moswuito Repellents: A Clinician's Guide" (Mark S. Fradin, MD. Annals of Internal Medicine, June 1, 1998: 128:931-940 - http://www.acponline.org/journals/annals/01jun98/mosquito.htm). You can also find information on insect repellents containing DEET (http://www.epa.gov/opp00001/citizens/deet.htm) at the Environmental Protection Agency (EPA) website. Another good source of information about pesticides and repellents is the National Pesticide Information Center (http://npic.orst.edu/), which also operates a toll-free information line: 1-800-858-7378 (check their website for hours).
Q. Where can I get information about the use of pesticide sprays that are being used for mosquito control?
A. The federal agency responsible for pesticide evaluation is the Environmental Protection Agency (EPA - www.epa.gov).
Testing and Treating West Nile Encephalitis in Humans (CDC)
Centers for Disease Control and Prevention
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. How do health care providers test for West Nile virus?
A. Your physician will first take a medical history to assess your risk for West Nile virus. People who live in or traveled to areas where West Nile virus activity has been identified are at risk of getting West Nile encephalitis; persons older than 50 years of age have the highest risk of severe disease. If you are determined to be at high risk and have symptoms of West Nile encephalitis, your provider will draw a blood sample and send it to a commercial or public health laboratory for confirmation.
Q. How is West Nile encephalitis treated?
A. There is no specific therapy. In more severe cases, intensive supportive therapy is indicated, often involving hospitalization, intravenous fluids, airway management, respiratory support (ventilator), prevention of secondary infections (pneumonia, urinary tract, etc.), and good nursing care.
How You Can Protect Your Animals
United States Department of Agriculture - Animal and Plant Health Inspection Service
You can decrease the chances of your animals being exposed to the virus by decreasing their exposure to adult mosquitoes. The best way to do this is by removing any potential sources of water in which mosquitoes can breed.
- Dispose of any water-holding containers, including discarded tires.
- Drill holes in the bottom of containers that are left outside.
- Clean clogged roof gutters on an annual basis.
- Turn over plastic wading pools or wheelbarrows when not in use, and do not allow water to stagnate in birdbaths.
- Aerate ornamental pools or stock them with fish.
- Clean and chlorinate swimming pools that are not in use and be aware that mosquitoes can breed in the water that collects on swimming pool covers.
- Use landscaping to eliminate standing water that collects on your property; mosquitoes can breed in any puddle that lasts more than 4 days.
- Thoroughly clean livestock-watering troughs on a monthly basis.
Local mosquito-control authorities can help in assessing the mosquito breeding risks associated with your property.
Birds can also be protected by limiting their exposure to mosquitoes. In areas reporting large numbers of bird deaths, investigations are conducted and samples collected to determine the cause of the deaths. People finding dead birds should notify local health officials.
USDA - Veterinary Services - January 2002
Q. Must horses affected by the virus be euthanized?
A. No. Because horses are incidental hosts, it is highly unlikely that mosquitoes feeding on an infected horse could ingest enough of the virus to transmit it to other animals. Horses are humanely euthanized only when the viral infection is so severe they will not be able to recover.
Q. Will horses affected by the virus be quarantined?
A. No. Since infected horses do not appear to be carriers for the disease, it is unlikely a quarantine would be necessary.
Q. Are dogs and cats affected by the virus?
A. It is unlikely that dogs or cats will show signs of clinical illness, although any mammal or bird could potentially be exposed to the virus through mosquito bites. A survey of blood samples from dogs and cats in the New York City epidemic area showed a low infection rate.
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