Frequently Asked Questions about West Nile Virus


What is West Nile virus?

West Nile virus is a mosquito-borne virus that can infect humans, birds, horses and other mammals. Outbreaks of West Nile virus have occurred in Africa, Egypt, Israel, Asia, Romania, Russia and France. Before 1999, West Nile virus had never before been found in the Western Hemisphere. West Nile virus first appeared in North America in New York City in 1999. The virus was most likely introduced by an infected bird or mosquito that was imported from a country where the virus is common. Since 1999, the virus has spread across the continental United States. Visit for more information on West Nile virus nationally.

Signs and Symptoms

What are the symptoms of West Nile virus?

Most people who are infected with West Nile virus either have no symptoms (80%) or experience a mild or moderate illness (20%) with symptoms such as fever, headache, fatigue, or body aches before fully recovering. Some persons may also develop a rash or swollen lymph glands. Symptoms can last for as short as a few days, though even healthy people have become sick for several weeks.

While rare (< 1%), some individuals, particularly persons 60 years of age and older, can develop serious disease from West Nile virus infection. Also known as West Nile neuroinvasive disease, illness may include encephalitis (inflammation of the brain), meningitis (inflammation of the membrane around the brain and spinal cord), or acute flaccid paralysis (a polio-like syndrome in which muscles become very weak or paralyzed). Symptoms of this more severe form of disease may include headache, high fever, stiff neck, nausea and vomiting, confusion, coma, tremors, convulsions, muscle weakness and/or paralysis. At its most serious, West Nile virus can cause permanent neurological damage and death.

Who is most at risk for getting severe West Nile virus disease after being bitten by an infected mosquito?

Anyone who is infected can potentially develop severe West Nile virus disease (e.g. encephalitis, meningitis, or acute flaccid paralysis). However, persons older than 60 have the highest risk of becoming severely ill, and there is increasing evidence that immunocompromised persons (e.g., people who are taking immunosuppressive medications such as prednisone, or people who have received organ transplants) are at higher risk for severe West Nile virus disease too.

What proportion of people die when infected with West Nile virus?

Fewer than 1% of people infected with West Nile virus develop encephalitis, and among those hospitalized with West Nile encephalitis, the case fatality rate ranges from 10% to 15%. Therefore, fewer than 1 in 1,000 people infected with West Nile virus die. Individuals over the age of 60 are at greatest risk for the severe complications of West Nile virus.


How is West Nile virus spread?

West Nile virus is predominantly spread to humans by the bite of an infected mosquito. In 2002 other methods of human transmission were discovered but appear to be rare. It has now been shown that West Nile virus can be transmitted to humans who receive transfusions of infected blood or blood products, or who receive infected organs through transplantation. Currently, routine testing of donated blood products is conducted to reduce this risk of transmission. Also, in 2002 there was one case of transmission from a pregnant woman to her fetus, and one probable case of mother-to-baby transmission through breast milk. Additionally, two laboratory workers were accidentally infected following injuries while handling infected birds. You can not get West Nile virus from another person (with the exception of blood products and organ donations from an infected person). West Nile virus is NOT spread by casual contact such as touching, kissing, or caring for someone who is infected.

How long does it take to get sick after a bite from an infected mosquito?

If illness does occur, symptoms generally appear between 3 to 15 days of being bitten by an infected mosquito.

Can I get West Nile virus directly from birds?

In 2002, CDC reported two human cases of West Nile virus in laboratory workers who became infected after injuries while handling infected dead birds. It is extremely unlikely that members of the general public would be exposed to West Nile virus in this manner. However, as always, when handling a dead bird or animal for disposal, use gloves to carefully place the bird in double-plastic bags and then place in the outdoor trash.

Besides mosquitoes, can I get West Nile virus directly from other insects or ticks?

Infected mosquitoes are the primary source of West Nile virus and caused the recent outbreaks in the United States. Although several types of ticks in Africa and Europe have been found to be infected with West Nile virus, there is no evidence that ticks or other insects in this country are able to transmit the virus.

Children and Pregnant Women

Are children and infants at greater risk for severe West Nile virus disease?

Children and infants are not at greater risk than other individuals for becoming infected with West Nile virus but they can potentially develop severe West Nile virus disease. In the years from 1999-2008, 589 children under age 20 years in the US were reported with West Nile encephalitis, meningitis or acute flaccid paralysis. This is 5% of all reported cases of severe West Nile virus disease for these years.

If I am pregnant and get infected with West Nile virus, can it affect my fetus?

Maybe. It is unknown how often transmission occurs during pregnancy and it is difficult to predict the effects of infection on every baby. In 2002 there was one case of transmission of West Nile virus from mother to fetus during pregnancy. The newborn was infected with West Nile virus at birth and had severe neurological problems, including abnormalities of the brain and retinas. It is possible that West Nile virus caused these abnormalities in the baby; however, it is not proven. The Centers for Disease Control and Prevention (CDC) studied outcomes of about 70 pregnant women who were infected with West Nile virus in 2003 or 2004. Most of the infants born to these mothers seemed normal. Three infants had West Nile virus infection that could have been acquired from the mother during pregnancy. Three different infants in this study were born with malformations that could have been caused by the mother’s West Nile virus infection, but there is no evidence that West Nile virus was the cause of the deformities.

Can West Nile virus be transmitted through breast milk?

Possibly. It appears that West Nile virus may be transmitted to infants through breast milk. In 2002, a woman developed encephalitis due to West Nile virus acquired through a blood transfusion she received shortly after giving birth. Laboratory analysis showed evidence of West Nile virus in breast milk collected from the mother soon after she became ill. She had been breastfeeding her infant and approximately 3-4 weeks after birth the infant tested positive for the West Nile virus. Because of the infant's minimal outdoor exposure, it is unlikely that the infection was transmitted by a mosquito – it is more likely that the infant became infected through the mother’s breast milk. However the infant had no symptoms of West Nile virus and remained healthy.

The health benefits of breastfeeding have been well established. The risk for West Nile virus transmission through breastfeeding is not fully understood, and the new findings from 2002 do not change current breastfeeding recommendations.

If you have confirmed West Nile virus infection, decisions regarding continuing breastfeeding should be made in consultation with both your doctor and your baby's doctor.

If I am pregnant or breastfeeding, should I be tested for West Nile virus?

There is no need to be tested for West Nile virus if you are pregnant or breastfeeding unless you are ill during West Nile season (in New York City this means June through October). Pregnant women who develop an unexplained fever or become ill with other symptoms compatible with possible West Nile virus infection such as fever, headaches, stiff neck, nausea and vomiting, confusion, muscle weakness, or your eyes become sensitive to light, you should see a doctor immediately.

Should parents spray insect repellent on their children before they go to school?

Children who attend school, camp or go on trips outdoors during the daytime are at low risk for mosquito bites. However, mosquitoes can still be active during the day in areas where there are weeds, tall grass, bushes or areas with known high mosquito activity. Precautions should be taken to protect your child from mosquito bites if a child will be in or near areas where mosquitoes are active, or if playing outside at dusk, night time or at dawn. For more information about repellents and children, go to the Repellents (( )and Personal Protective Measures (( section.

Testing and Treatment

How is West Nile virus treated?

There is no specific treatment for West Nile virus. Most people who become infected will get better on their own. In more severe cases, intensive supportive therapy is indicated, i.e., hospitalization, intravenous (IV) fluids and nutrition, airway management, ventilatory support (ventilator) if needed, prevention of secondary infections and proper nursing care.

Although there is currently no specific therapy proven to be effective for treating West Nile neuroinvasive disease, there are a couple of clinical trials underway. Not every patient with West Nile infection is eligible for this treatment. Patients or their families who want more information about these experimental treatments should ask their medical providers about them. Medical providers may contact the Bureau of Communicable Disease at the New York City Department of Health and Mental Hygiene for more information.

What should I do if I think I have West Nile virus?

People with mild or moderate symptoms should recover completely, although some symptoms may last for weeks, and do not usually require any specific medication. However, if you develop symptoms such as fever, headaches, stiff neck, confusion, muscle weakness, or your eyes become sensitive to light, you should consult your doctor and be tested for West Nile virus.

If I get bitten by a mosquito, should I be tested for West Nile virus?

Most mosquitoes are not infected with West Nile virus and illnesses related to mosquito bites are uncommon. There is no need to be tested for West Nile virus unless the person bitten is ill and has symptoms suggesting possible West Nile virus infection. If after someone in your family gets bitten they develop symptoms such as fever, headaches, stiff neck, confusion, muscle weakness, or your eyes become sensitive to light, you should consult your doctor about getting them tested.

Is there a vaccine for West Nile virus?

There is currently no human vaccine for West Nile virus, but several companies are working towards developing one. There are West Nile virus vaccines that have been approved for use in horses.

Mosquito Control in New York City

What is the City doing to address the problem of West Nile virus?

New York City is working to reduce the risk of West Nile virus infection. The main goal is to decrease the number of adult mosquitoes by eliminating breeding sites wherever possible and applying larvicides (to kill the immature larval form of the mosquito) to areas of standing water that cannot be drained completely. The City also regularly tests mosquitoes for West Nile virus throughout the spring and summer.

Is the City planning to spray pesticides?

New York City may spray pesticides to target adult mosquitoes if there is a risk to human health from West Nile virus. Spraying will be concentrated in areas most at risk for disease occurrence and will be conducted by experienced and licensed applicators who are required to follow New York State Department of Environmental Conservation (NYSDEC) and EPA requirements. The City is hopeful that with an early and aggressive campaign against mosquito breeding areas, the need for the spraying of pesticides will be reduced.

At what time of day will the city be spraying?

New York City will only spray for mosquito control in the evenings or preschool hours of the morning.

Will the public be notified in advance about spraying activities?

Residents can learn about adulticiding schedules in advance through public service announcements, the media, the City's website (, spray schedule at, or by calling 311, the City's Information Line. DOHMH will provide notification at least 24 hours prior to a spray event.

Do pesticides sprayed to eliminate adult mosquitoes pose health risks to people and pets?

In the amounts of pesticides used, following guidelines set by the EPA and the NYSDEC, health risks to people and pets are relatively low. However, some people may be more sensitive to pesticides and may experience short-term eye or throat irritation or a rash. To reduce your chance of exposure, follow the suggestions below. Anyone experiencing adverse reactions to pesticides should seek medical care or call 311 or the NYC Poison Control Center at (212) POISONS (764-7667) or 1-800-222-1222.

If the City sprays pesticides in an area where I am, what should I do during the spraying?

If pesticide spraying occurs, DOHMH recommends that all individuals take the following precautions to avoid direct exposure to pesticides and reduce the risk of reactions:

  • Whenever possible, stay indoors during spraying.
  • Some individuals are sensitive to pesticides. Persons with asthma or other respiratory conditions are encouraged to stay inside during spraying since there is a possibility that spraying could worsen these conditions.
  • Air conditioners may remain on. But if you wish to reduce the possibility of indoor exposure to pesticides, set the air conditioner vent to the closed position, or choose the recirculate function.
  • Remove children's toys, outdoor equipment and clothes from outdoor areas during spraying. If outdoor equipment and toys are exposed to pesticides, wash with soap and water before using again.
  • Wash skin and clothing exposed to pesticides with soap and water.
  • Always wash your produce thoroughly with water before cooking or eating.

Anyone who has a bad reaction to pesticides should seek medical care or call 311 or the NYC Poison Control Center at (212) POISONS (212-764-7667) or 1-800-222-1222.


Dogs and Cats

Can West Nile virus cause illness in dogs or cats?

West Nile virus has been confirmed in some domestic animals, including pet birds, dogs and cats. A study done in Queens in 1999 showed that 5% to 11% of dogs tested had been exposed to West Nile virus, though none were clinically ill. Infected cats may exhibit mild, nonspecific symptoms during the first week after infection--for the most part only showing a slight fever and slight lethargy. It is important to remember that pets with neurologic disease should be first evaluated for othermore likely causes of illness, including rabies.

Can infected dogs or cats be carriers and transmitters of West Nile virus to humans?

West Nile virus is predominantly transmitted by infected mosquitoes. There is no evidence that dogs and cats can transmit West Nile virus to humans. Veterinarians should take normal infection control precautions when caring for animals suspected of having West Nile virus or any other viral infection.

How do dogs or cats become infected with West Nile virus?

Dogs and cats become infected the same way humans become infected: by the bite of an infected mosquito.

Can a dog or cat infected with West Nile virus infect other dogs or cats?

There is no documented evidence that West Nile virus is transmitted between dogs and cats.

What should I do if I think my dog, cat, or pet might have West Nile virus?

While dogs and cats do not appear to suffer clinical illness from West Nile virus, if you think your pet is ill, you should see your veterinarian promptly. Pets can develop neurologic diseases from many other causes, and these should be ruled out before testing for West Nile virus is considered.

Should a dog or cat infected with West Nile virus be euthanized?

As animals infected with West Nile virus do not transmit the virus to people or other animals, there is no reason to euthanize an infected animal. Treatment would be supportive and consistent with standard veterinary practices for animals infected with any viral agent.

Can I use insect repellent on my pets?

DEET-based repellents, which are recommended for humans, are not approved for veterinary use (largely because animals tend to ingest them by licking.) Talk with your veterinarian for advice about the appropriate product for use on your pet.

Blood and Organ Donation

If a person was diagnosed with West Nile virus, can they still donate blood?

A person can donate blood once the person has fully recovered from West Nile virus and there is no evidence to suggest that the virus is still in their blood. However, people who have recently been diagnosed with West Nile virus or who have had a recent illness with fever should not donate blood, other body fluids or organs for at least one month following their illness.

Can I get West Nile virus from donating blood?

It is not possible for a person to get West Nile virus or any infectious disease from giving blood.

I have more questions about West Nile Virus and the blood supply!

Please contact the New York Blood Center at 1-800-688-0900.

More Information

If I travel outside New York City, do I need to worry about West Nile virus?

Yes. West Nile virus has spread throughout the United States and is present in other parts of the world as well. In some areas of the country, particularly the south and the west, the West Nile virus transmission season lasts longer. People can become infected with West Nile virus early in the spring or as late in the year as November or December. So when you travel, especially during the spring, summer and fall, you should check with the local health department wherever you go to see if West Nile virus, or any other arboviruses ( ), are present. If yes, then you should take proper precautions to avoid mosquito bites.

Should I report dead birds to the Department of Health and Mental Hygiene?

No, unless you are reporting a group of 10 or more dead birds of any species, or a group of 3 or more dead waterbirds. The Health Department will no longer take reports of individual dead birds to monitor for West Nile virus. These tests had limited value as an early warning system for the virus. The Department will continue to take reports of groups of dead birds (10 or more of any species or 3 or more waterbirds). To report these clusters, call 311. If you need to dispose of a dead bird, use disposable rubber gloves to carefully place it in a double plastic bag, and then put the bag in the outdoor trash. Wash your hands with warm soapy water afterwards.

Last Updated: August 31, 2012