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Overview
Q. What is West Nile Virus?
A. The West Nile Virus should actually be called West Nile Encephalitis. "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. It was not until 1999 that the virus was documented in the Western Hemisphere. It is not known from where the U.S. virus originated.
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 Center for Disease Control scientists believe the virus has probably been in the eastern U.S. since the early summer of 1999 when the first case was reported, possibly longer.
Q. Where has the West Nile virus been found so far?
A. As of December 2002, West Nile virus has been found in 44 states. In Ohio in 2002, it was identified in all 88 counties.
Q. How many cases of West Nile encephalitis in humans have occurred in the U.S.?
A. Since the virus was first identified in this country, there have been 3,852 human cases in 39 states and 232 fatalities. Most of these cases were from 2002.
Q. What precautions are needed to prevent a recurrent outbreak?
A. Active sampling for West Nile virus (i.e., surveillance) in mosquito and bird populations will greatly enhance state and local governments' early detection systems.
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Transmission
Q. How do people get West Nile encephalitis?
A. Mainly by the bite of mosquitoes infected with West Nile virus. Even in areas where mosquitoes do carry the virus, usually less than 1 in 500 mosquitoes are infected.
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 mosquitoes do carry the virus, very few mosquitoes are infected. About 1 in 500 mosquitoes are infected and about 1 in 300 of the people who get bitten and become infected will become 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, birds, or animals?
A. The principle route of human infection with West Nile virus is through the bite of an infected mosquito. Additional routes of infection have become apparent during the 2002 West Nile epidemic. Other methods of transmission include blood transfusion, organ transplantation, mother-to-child (ingestion of breast milk and transplacental) and occupational.
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 are 12% for hospitalized patients and are highest among the elderly.
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Prevention & Symptoms
Q. Is there a vaccine against West Nile encephalitis?
A. Not at this time, but several companies are working towards developing a vaccine.
Q. Who is at risk for getting West Nile encephalitis?
A. All residents of areas where virus activity has been identified are at risk. Persons older than 50 years 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. Typically 3 to 14 days.
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Testing & Treating
Q. If you think you have symptoms of West Nile virus, what should you 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 family 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. 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. At this time, there is no specific therapy. In severe cases, intensive supportive therapy is indicated, often involving hospitalization.
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Animals & Birds
Q. Do birds infected with West Nile virus die or become ill?
A. In the 1999 New York area epidemic, there was a large die-off of American crows. Through September 2002, West Nile virus has been identified in at least 160 species of birds found dead in the United States. Most of these birds were identified through reporting of dead birds by the public.
Q. How can I report a sighting of dead bird in my area?
A. Please contact your state or local health department.
Q. Can West Nile virus cause illness in dogs or cats?
A. West Nile virus has been isolated from several dead cats. A serosurvey of dogs and cats in the epidemic area showed a low infection rate.
Q. Has West Nile virus caused severe illness or death in horses?
A. Yes, while data suggest that most horses infected with West Nile virus recover, West Nile virus has caused deaths in horses yearly since the initial outbreak in 1999.
Q. Does West Nile virus affect other animals?
A. The virus may also affect sheep, cattle, and other animals. Mosquitoes can only become infected by feeding on birds.
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Mosquito Control
Q. What types of mosquitoes carry the virus?
A. To date, 36 species of mosquito have been documented as carrying the virus. Several of these species are common in Lake County.
Q. Is spraying the answer?
A. No, in several cities where mosquito-borne diseases erupt repeatedly, widespread aerial spraying of pesticides that target adult mosquitoes (known as adulticides) rarely, if ever occurs. Through effective monitoring, these cities can identify infected areas a month before any human comes down with the virus. These infected areas can then be individually treated, thereby saving lives and money. Widespread spraying may also kill many beneficial insects as well as other wildlife species.
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To Reduce Your Chances Of Being Affected
- 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.
Note: Vitamin B and "ultrasonic" devices are NOT effective in preventing mosquito bites.
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Popular Wildlife Misconceptions
- “Swamps are mosquito breeding grounds” is a popular misconception. There are many species of wildlife in a natural swamp or wetland that feed on mosquito larvae. Small puddles and open containers containing water actually produce more mosquitoes.
- Purple martins and bats are natural mosquito control. While beneficial, purple martins and bats actually eat few mosquitoes. Martins feed during the day when few mosquitoes are out and bats feed mainly on insects such as beetles. Mosquitoes are a very small percentage of either diet.
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Mosquito Control Measures
- Dispose of any water-holding containers including discarded tires.
- Drill holes in the bottom of containers that are left outdoors.
- Clean clogged roof gutters annually.
- Turn over large containers or wheelbarrows when not in use and do not allow water to stagnate in birdbaths.
- Thoroughly clean livestock-watering troughs monthly.
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For More Information
Lake County General Health District
Pennsylvania Department of Health
West Nile Virus Fact Sheet, Daily Surveillance Update
United States Geological Survey
West Nile Virus
Center for Disease Control
Ohio Department of Health
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