Frequently Asked Questions

West Nile Virus

What is West Nile virus?
Where did West Nile virus come from?
What are the symptoms of West Nile virus infection?
How soon after exposure do symptoms appear?
If I have West Nile fever, can it turn into West Nile encephalitis?
What if I am pregnant or am nursing?
How do people get West Nile virus?
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?
Can I get West Nile virus directly from birds?
Can West Nile virus be spread from person-to-person?
How is West Nile virus diagnosed?
Is there a West Nile virus vaccine for humans?
What is the treatment?
How can I protect myself from West Nile virus?
Is DEET a carcinogen?
How do I report a mosquito problem?
How can I report a sighting of dead bird(s) in my area?
What should hunters do to protect themselves against West Nile virus?

Other Mosquito-borne Diseases

What is an arbovirus?
What is encephalitis?
What are the symptoms of arthropod-borne encephalitis?
How long is the incubation period?
What animals may carry arboviruses?
How many types of arboviruses may cause encephalitis?
How is arboviral encephalitis diagnosed?
What is the treatment for encephalitis?
How can outbreaks of arboviral encephalitis be prevented?

More Frequently Asked Questions

What is West Nile virus?
West Nile virus is closely related to St. Louis encephalitis virus, also found in the United States. The virus can infect humans, birds, mosquitoes, horses, and some other mammals. Most persons infected with this virus show no symptoms, although occasional infections can result in serious illness and even death.

Where did West Nile virus come from?
West Nile virus has been commonly found in humans, birds, and other animals in Africa, Eastern Europe, Western Asia, and the Middle East, but until 1999 had not previously been documented in the Western Hemisphere. The U.S. viral strain is most closely related genetically to strains found in Israel.

What are the symptoms of West Nile virus infection?
Most people infected with this virus do not have any symptoms. Some people experience a mild illness characterized by fever, headache, tiredness, aches, and sometimes a rash. Although the illness can be as short as a few days, even healthy people have been sick for several weeks. The more severe type of West Nile virus disease is sometimes called “neuro-invasive disease” because it affects a person’s nervous system. This severe illness can include encephalitis (inflammation of the brain), meningitis (inflammation of the tissues that cover the brain and spinal cord), or meningoencephalitis (inflammation of the brain and the membrane surrounding it). Severe manifestation of West Nile virus disease is marked by a rapid onset of a high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, paralysis, and in the most severe cases, death.

How soon after exposure do symptoms appear?
Symptoms usually appear 2 to 15 days after exposure.

If I have West Nile fever, can it turn into West Nile encephalitis?
When someone is infected with West Nile virus they will typically have one of three outcomes: No symptoms (most likely), West Nile fever (in about 20% of people), or severe West Nile disease, such as meningitis or encephalitis (less than 1% of those who become infected). If you develop a high fever with severe headache, consult your health care provider.

What if I am pregnant or am nursing?
Pregnancy and nursing do not increase risk of becoming infected with West Nile virus. The U.S. Centers for Disease Control and Prevention recommends that pregnant women use an insect repellant that contains DEET to prevent infection with mosquito-borne diseases. In 2003, cooperation between CDC, state health officials, and health care providers resulted in the nationwide registration of 74 women with WNV infections during pregnancy. The risk that West Nile virus may present to a fetus is still being evaluated. Also, the risk to an infant infected through breast milk is unknown. Talk with your health care provider if you have concerns.

How do people get West Nile virus?
The West Nile virus, like most mosquito-borne viruses, is found in wild and domestic birds. When a mosquito feeds on an infected bird, it can pick up the virus and transmit it to other, uninfected birds. Occasionally, infective mosquitoes will feed on mammals such as horses, dogs, cats, and humans, and transmit the virus to them. Mosquitoes do not pick up the virus by taking a blood meal from a person infected with West Nile virus.

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?
No, even in areas where mosquitoes do carry the virus, very few mosquitoes—much less than 1%—are infected. Even if an infected mosquito bites a person, less than one percent will get severely ill. The chances you will become severely ill from any one-mosquito bite are extremely small.

Can I get West Nile virus directly from birds?
Although transmission of West Nile virus and similar viruses to laboratory workers is not a new phenomenon, two cases of West Nile virus infection in laboratory workers have been reported. There is no evidence that a person can get the virus from simply handling live or dead infected birds. However, persons should avoid barehanded contact when handling any dead animal and use gloves or double plastic bags to place the carcass in a garbage can.

Can West Nile virus be spread from person-to-person?
Multiple investigations have confirmed that West Nile virus transmission can occur through transplanted organs and/or blood products. The implementation of donor screening for the virus in 2003 has reduced the risk of transfusion-associated infection substantially by removing hundreds of units of potentially infectious blood products donated by donors who did not know they were infected. Unfortunately, the current screening protocol has not completely eliminated all risk of West Nile virus infection through transplanted organs and/or blood products. The U.S. Centers for Disease Control and Prevention, the U.S. Food and Drug Administration, and the blood collection agencies and industry continue to work on finding the best approach to use in the future to ensure the safety of the blood supply.

Beyond this particular unique transmission pathway, West Nile virus infection 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.

How is West Nile virus diagnosed?
If you or your family members develop symptoms such as high fever, confusion, neck stiffness, disorientation, muscle weakness, and severe headache, you should see your health care provider immediately. Your health care provider will assess your risk for West Nile virus infection. If you are determined to be at high risk, your provider will draw a blood sample and send it to a laboratory for verification.

Is there a West Nile virus vaccine for humans?
No, but several companies are working towards developing a vaccine.

What is the treatment?
There is no specific treatment for viral infections, other than to treat the symptoms and provide supportive care. In more severe cases, intensive supportive therapy is indicated, often involving hospitalization, intravenous fluids and nutrition, respiratory support, prevention of secondary infections, and good nursing care. Elderly persons are at highest risk for developing severe illness due to West Nile viral infection, so these individuals should promptly seek medical care if infection is suspected.

How can I protect myself from West Nile virus?
It is not necessary to limit outdoor activities. However, you can and should try to reduce your risk of being bitten by mosquitoes. Mosquitoes are most active at dawn and dusk or when there is heavy cloud cover and high humidity, so use extra precaution at these times of the day:

• 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 that contains DEET sparingly to exposed skin.
• The more DEET a repellent contains the longer time it can protect you from mosquito bites. A higher percentage of DEET in a repellent does not mean that your protection is better—just that it will last longer. Choose a repellent that provides protection for the amount of time that you will be outdoors.
• DEET concentrations higher than 50% do not increase the length of protection.
• Repellents may irritate the eyes and mouth, so avoid applying repellent to the hands of children.
• Do not apply repellent to cuts, wounds, and sunburned or irritated skin.
• Wash treated skin with soap and water after returning indoors.
• 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.

Reducing the mosquito population around your home and property can be accomplished by eliminating standing water:

• At least once or twice a week, change the water in flowerpots, pet food and water dishes, and birdbaths.
• At least once or twice a week, empty water from swimming pool and boat covers, buckets, barrels, and cans.
• Check for clogged rain gutters and clean them out.
• Remove discarded tires, and other items that could collect water.
• Be sure to check for containers or trash in places that may be hard to see, such as under bushes or under your home.
• Use landscaping to eliminate standing water that collects on your property.

In addition to eliminating standing water in your yard, make sure all windows and doors have screens, and that all screens are in good repair.

Is DEET a carcinogen?
DEET is not a recognized or suspected carcinogen. In addition, in a recent review of the scientific literature about DEET, the U.S. Environmental Protection Agency (EPA) could find "no toxicologically significant effects in animal studies."

However, DEET can cause serious irritation to the eyes, nose, and mouth. Do not apply repellents to a child’s hands since children often put their hands in their mouths or rub their eyes. Acute DEET toxicity has been linked to overuse or repeated use on children. In spite of these cautions, the American Academy of Pediatrics Committee on Environmental Health recently stated: "Insect repellents containing DEET with a concentration of 10% appear to be as safe as products with a concentration of 30% when used according to the directions on the product labels."

For over 40 years, insect repellents containing DEET have safely prevented millions from being bitten by mosquitoes and ticks. However, because DEET is directly applied to the human skin and clothing, the EPA requires clear, common sense use directions, label warnings, and restrictions on all DEET product labels. EPA believes that such common sense measures will be especially protective of children and other individuals more sensitive to chemical substances.

Non-DEET repellents have not necessarily been as thoroughly studied as DEET, and may not be safe for use on children.

How do I report a mosquito problem?
The Missouri Department of Health and Senior Services (DHSS) does not have any legal authority to regulate or eliminate mosquito habitat. However, if you have not done so already, you may want to discuss this matter with your local public health department or city public works agency.

Often in Missouri communities, especially in rural areas, there are no ordinances that regulate mosquito-breeding habitat. Sometimes, however, if a public health issue is at stake, a local health department or public works agency can persuade a property owner to "do the right thing." If you feel strongly that your county or city should have an ordinance regulating mosquito-breeding habitat, you and your neighbors may want to discuss your concerns with your city alderman or councilman, or even go to a city council meeting.

How can I report a sighting of dead bird(s) in my area?
Because weather conditions and the amount of mosquito breeding habitat will vary from year to year, reporting and testing dead birds is a good way to find current West Nile virus activity in an area. If you find a dead crow, blue jay, sparrow, grackle, or bird of prey (hawk, owl, or eagle) in your yard or neighborhood, please call your local city or county health department to report it. They may ask you to bring in the dead bird to test for West Nile virus.

In addition, the Missouri Department of Health and Senior Services gathers information on bird mortality throughout the state with Dead Bird Reporting Form.

What should hunters do to protect themselves against West Nile virus?
The West Nile virus has been isolated in white tail deer, rabbits, squirrels, and dove. Hunters should follow the usual precautions when handling wild animals. They should wear gloves when handling and cleaning animals to prevent blood exposure to bare hands. Meat should be cooked thoroughly. If hunters anticipate being exposed to mosquitoes, they should apply insect repellents to clothing and skin according to label instructions.

What is an arbovirus?
Arbovirus is a made-up word. It is created from the three words: arthropod, borne, and virus. The arthropods are a large group of animals with segmented bodies, hollow, jointed legs, and an exoskeleton instead of a vertebral column. This group is also called the invertebrates and includes insects (bee, mosquito, moth), arachnids (spiders, ticks, mites), and crustaceans (shrimp, crab). Thus an arbovirus is a virus that is carried and spread by insects, usually blood-sucking insects. The arboviruses are a large group of viruses found worldwide. In the United States, mosquitoes most commonly spread arboviruses.

What is encephalitis?
Encephalitis usually refers to brain inflammation caused by a virus and less commonly by bacteria. Arboviral encephalitis refers to those viruses transmitted to people by arthropods, usually mosquitoes. Because people are not important in the life cycle of most arboviruses, arbovirus infections are generally rare in humans.

Epidemics are infrequent but unpredictable; the annual disease incidence in the United States varies from 150 to over 4,000 cases a year.

What are the symptoms of arthropod-borne encephalitis?
The majority of arboviral human infections are asymptomatic or result in a nonspecific flu-like syndrome. When symptoms occur, they usually start suddenly, with fever, chills, headache, muscle aches, and tiredness. In rare cases, infection may lead to encephalitis, with the potential for a fatal outcome or permanent impairment to the nervous system. Fortunately, only a small proportion of infected persons progress to encephalitis. Symptoms of encephalitis are drowsiness, stiff neck, confusion, convulsions, tremors, and coma.

How long is the incubation period?
The incubation period is usually 2 to 6 days, but ranges from 2 to 14 days, and may be up to 21 days in immunocompromised people.

Can you get encephalitis from another person?
Direct person-to-person transmission of arboviruses does not occur. The principle route of infection for arboviral encephalitis is through the bite of an infected mosquito. However, additional routes of arboviral infection have become apparent since 1999 when West Nile virus was first documented in the United States. It is important to note that these other methods of transmission represent a very small proportion of cases:

• Investigations in 2002 confirmed West Nile virus transmission through transplanted organs and/or blood products.
• There are documented cases of transplacental (mother-to-child) transmission of West Nile virus in humans. The risk that West Nile virus may present to a fetus is still being evaluated. Also, the risk to an infant infected through breast milk is unknown. More research is needed to improve our understanding of the relationship - if any - between West Nile virus infection and adverse birth outcomes.

Finally, although transmission of West Nile virus and similar viruses to laboratory workers is not a new phenomenon, two cases of West Nile virus infection of laboratory workers have been reported.

What animals may carry arboviruses?
In general, birds are often the source of arboviral infection for mosquitoes. The notable exception is the LaCrosse encephalitis virus, which is carried by small mammals such as chipmunks or squirrels. Wild birds carry eastern equine encephalitis, St. Louis encephalitis, and western equine encephalitis. West Nile virus also infects wild birds, which appear to serve as its primary reservoir. In addition, West Nile virus has been isolated in at least 36 different species of mammals and reptiles, including horses, cats, dogs, bats, chipmunks, skunks, squirrels, and domestic rabbits.

West Nile virus does not appear to cause extensive illness in dogs or cats and there is no documented evidence of animal-to-person transmission of the virus. In addition, dogs and cats do not produce significant levels of West Nile virus in their blood to infect a mosquito with the virus.

How many types of arboviruses may cause encephalitis?
There are over 100 different viruses found in wild animals that may cause this disease in humans. In the United States, eastern equine encephalitis, western equine encephalitis, California/LaCrosse encephalitis, St. Louis encephalitis, and West Nile viruses account for the overwhelming majority of cases.

How is arboviral encephalitis diagnosed?
Since the disease cannot be distinguished from other causes of encephalitis based on symptoms, the diagnosis requires laboratory tests. Several different laboratory tests may need to be performed to rule out bacterial meningitis as the cause of the illness.

What is the treatment for encephalitis?
Because the arboviral encephalitides are viral diseases, antibiotics are not effective for treatment. No effective antiviral drugs have yet been discovered. Treatment is supportive, attempting to deal with problems such as swelling of the brain, loss of the automatic breathing activity of the brain, and other treatable complications like bacterial pneumonia.

How can outbreaks of arboviral encephalitis be prevented?
The most important measure to control the disease is to eliminate mosquitoes. Since it is not practical to try to kill all mosquitoes, barrier methods such as window screens can be used. Additionally, homeowners can control the environment around their homes by eliminating artificial or natural breeding pools for mosquitoes such as old tires, slowly draining ditches, swimming pool and boat covers, buckets, barrels, and cans.

It is not necessary to limit outdoor activities. However, you can and should try to reduce your risk of being bitten by mosquitoes. Mosquitoes are most active at dawn and dusk or when there is heavy cloud cover and high humidity, so use extra precaution at these times of the day. If you do go outdoors when mosquitoes are biting, wear long pants and long sleeved shirts and apply a mosquito repellent that contains DEET to exposed skin areas.

In Missouri, most human cases of arboviral encephalitis occur from June through September, when biting insects are most active. Annual community clean-up campaigns in late winter and early spring can help reduce mosquito-breeding sites and raise awareness that human beings are responsible for creating a large proportion of mosquito problems.