West Nile FAQ

Who is at risk for getting West Nile encephalitis?

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. It is unknown if immunocompromised persons are at increased risk for West Nile virus (WNV) disease.

How do people get infected with WNV?

The main route of human infection with WNV is 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 later blood meals (when mosquitoes bite), the virus may be injected into humans and animals, where it can multiply and possibly cause illness.

Additional routes of human infection became apparent during the 2002 West Nile epidemic. It is important to note that these other methods of transmission represent a very small proportion of cases. Investigations have identified WNV transmission through transplanted organs and through blood transfusions. See Blood Transfusions and Transmission: Questions and Answers for more information.

There is one reported case of transplacental (mother-to-child) WNV transmission. This case is detailed in a MMWR Dec 20, 2002 article. There is also one reported case of transmission of WNV through breast-milk. See questions and answers concerning WNV and breastfeeding for more information on this topic.

Although transmission of WNV and similar viruses to laboratory workers is not a new phenomenon, two recent cases of WNV infection of laboratory workers have been reported. These cases are detailed in a MMWR Dec 20, 2002 article.

What is the basic transmission cycle of the West Nile virus?

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.

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 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.

Can you get West Nile encephalitis from another person?

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.

Is a woman's pregnancy at risk if she gets infected with West Nile virus?

There is one documented case of transplacental (mother-to-child) transmission of WNV in a human. Although the newborn in this case was infected with WNV at birth and had severe medical problems, it is unknown whether the WNV infection itself caused these problems or whether they were coincidental. More research will be needed to improve our understanding of the relationship, if any, between WNV infection and adverse birth outcomes.

Nevertheless, pregnant women should take precautions to reduce their risk for WNV and other arboviral infections by avoiding mosquitoes, using protective clothing, and using repellents containing DEET (See using repellent safely). When WNV transmission is occurring in an area, pregnant women who become ill should see their health care provider, and those whose illness is consistent with acute WNV infection, should undergo appropriate diagnostic testing.

What are the symptoms of West Nile virus (WNV) infection?

Infection with WNV can be asymptomatic (no symptoms), lead to West Nile fever, or severe West Nile disease.

It is estimated that about 20% of people who become infected with WNV will develop West Nile fever. Symptoms include fever, headache, tiredness, and body aches, occasionally with a skin rash (on the trunk of the body) and swollen lymph glands. While the illness can be as short as a few days, even healthy people have reported being sick for several weeks.

The symptoms of severe disease (also called neuroinvasive disease, such as West Nile encephalitis or meningitis or West Nile poliomyelitis) include headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, and paralysis. It is estimated that approximately 1 in 150 persons infected with the West Nile virus will develop a more severe form of the disease. Serious illness can occur in people of any age, however people over age 50 and some immunocompromised persons (for example, transplant patients) are at the highest risk for getting severely ill when infected with WNV.

Most people (about 4 out of 5) who are infected with West Nile virus will not develop any type of illness (an asymptomatic infection), however you cannot know ahead of time if you'll get sick or not when infected.

What is the incubation period in humans (time from infection to onset of disease symptoms) for the West Nile disease?

The incubation period is usually two to 15 days.

How long do symptoms last?

Symptoms of West Nile fever will generally last a few days, although even some healthy people report having the illness last for several weeks. The symptoms of severe disease (encephalitis or meningitis) may last several weeks, although neurological effects may be permanent.

What is meant by West Nile encephalitis, West Nile meningitis, West Nile poliomyelitis, “neuroinvasive disease,” and West Nile fever?

The most severe type of disease due to a person being infected with West Nile virus is sometimes called “neuroinvasive disease,” because it affects a person's nervous system. Specific types of neuroinvasive disease include: West Nile encephalitis, West Nile meningitis, West Nile meningoencephalitis, and West Nile poliomyelitis. Encephalitis refers to an inflammation of the brain; meningitis is an inflammation of the membrane around the brain and the spinal cord; meningoencephalitis refers to inflammation of the brain and the membrane surrounding it; and poliomyelitis refers to an inflammation of the spinal cord.

West Nile fever is another type of illness that can occur in people who become infected with the virus. It is characterized by fever, headache, tiredness, aches and sometimes rash. Although the illness can be as short as a few days, even healthy people have been sick for several weeks.

If I have West Nile fever, can it turn into West Nile encephalitis?

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

West Nile fever is characterized by symptoms such as fever, body aches, headache, and sometimes swollen lymph glands and a rash. West Nile fever generally lasts only a few days, though in some cases symptoms have been reported to last longer, even up to several weeks. West Nile fever does not appear to cause any permanent health effects. There is no specific treatment for WNV infection. People with West Nile fever recover on their own, though symptoms can be relieved through various treatments (such as medication for headache and body aches, etc.).

Some people may develop a brief, West Nile fever-like illness (early symptoms) before they develop more severe disease, though the percentage of patients in whom this occurs is not known.

Occasionally, an infected person may develop more severe disease such as “West Nile encephalitis,” “West Nile meningitis” or “West Nile meningoencephalitis.” Encephalitis refers to an inflammation of the brain; meningitis is an inflammation of the membrane around the brain and the spinal cord; and meningoencephalitis refers to inflammation of the brain and the membrane surrounding it. Although there is no treatment for WNV infection itself, the person with severe disease often needs to be hospitalized. Care may involve nursing IV fluids, respiratory support, and prevention of secondary infections.

What can I do to reduce my risk of becoming infected with the West Nile virus?

Here are preventive measures that you and your family can take:

Protect yourself from mosquito bites:

  • Apply insect repellent to exposed skin. Generally, the more active ingredient a repellent contains the longer it can protect you from mosquito bites. A higher percentage of active ingredient in a repellent does not mean that your protection is better—just that it will last longer. Read more on insect repellent active ingredients. Choose a repellent that provides protection for the amount of time that you will be outdoors.
    • 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.
    • For detailed information about using repellents, see the insect repellent use and safety questions.
  • Spray clothing with repellents containing permethrin or another Environmental Protection Agency (EPA)-registered repellent since mosquitoes may bite through thin clothing. Do not apply repellents containing permethrin directly to exposed skin. Do not apply repellent to skin under your clothing.
  • When weather permits, wear long-sleeve shirts and long pants whenever you are outdoors.
  • Place mosquito netting over infant carriers when you are outdoors with infants.
  • Consider staying indoors at dawn, dusk, and in the early evening, which are peak mosquito biting times.
  • Install or repair window and door screens so that mosquitoes cannot get indoors.

Help reduce the number of mosquitoes in areas outdoors where you work or play, by draining sources of standing water. This way you reduce the number of places mosquitoes can lay their eggs and breed.

  • At least once or twice a week, empty water from flower pots, pet food and water dishes, birdbaths, swimming pool 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.

Note: Vitamin B and "ultrasonic" devices are Not effective in preventing mosquito bites.


What can be done to prevent outbreaks of the West Nile virus?

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. Details about effective prevention and control of West Nile virus can be found in CDC's Guidelines for Surveillance, Prevention, and Control (286 KB, 111 pages).

Is there a vaccine against West Nile encephalitis?

No, but several groups are working towards developing a vaccine.

Where can I get information about the use of pesticide sprays that are being used for mosquito control?

The federal agency responsible for pesticide evaluation is the EPA. Visit their website for detailed answers to the questions about pesticides used for mosquito control.

Can WNV cause illness in dogs or cats?

A relatively small number of WNV infected dogs and WNV infected cats have been reported to the 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.

How can my veterinarian treat my cat or dog if they are/may be infected with WNV?

There is no specific treatment for WNV infection. Full recovery from the infection is likely. Treatment would be supportive (managing symptoms, if present) and consistent with standard veterinary practices for animals infected with a viral agent.

Does my dog / cat becoming infected pose a risk to the health of my family or other animals?

There is no documented evidence of dog or cat-to-person transmission of WNV. The evidence suggests that dogs do not develop enough virus in their bloodstream to infect more mosquitoes. Cats develop slightly higher levels of virus in their bloodstream, but it is unclear if this would be enough to infect mosquitoes. It is very unlikely that cats would be important in furthering the spread of the virus. *

If your animal becomes infected with WNV, this suggests that there are infected mosquitoes in your area. You should take measures to prevent mosquitoes from biting you (use repellent and wear protective clothing).

Veterinarians should take normal infection control precautions when caring for any animal (including birds) suspected to have this or any viral infection.

How do cats and dogs become infected with the West Nile virus?

Dogs and cats become infected when bitten by an infected mosquito. There is also evidence that cats can become infected with the virus after eating experimentally infected mice. *

Can I become infected with WNV if a dog with the virus bites me?

Preliminary studies have not been able to detect virus in the saliva of infected dogs. This suggests that dog bites pose a low risk, if any, of transmission of WNV from dogs to other animals or people. *

Is there a vaccine for cats or dogs?

No, there is not a West Nile vaccine for cats or dogs.

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

No. There is no reason to destroy an animal just because it has been infected with the West Nile virus. Full recovery from the infection is likely. Treatment would be supportive and consistent with standard veterinary practices for animals infected with a 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.

What are "larvicides" and "adulticides"?

Larvicides are products used to kill immature mosquitoes. They can be either biological (such as toxin from specific bacteria that is lethal to mosquito larvae but not to other organisms) or chemical products, such as insect growth regulators, surface films, or organophosphates. Larvicides are applied directly to water sources that hold mosquito eggs or larvae. When used well, larvicides can help to reduce the overall mosquito by limiting the number of new mosquitoes that are produced.

Adulticides are products used to kill adult mosquitoes. Adulticides can be applied from hand-held sprayers, truck-mounted sprayers or using airplanes. Adulticides, when used well, can have an immediate impact to reduce the number of adult mosquitoes in an area, with the goal of reducing the number of mosquitoes that can bite people and possibly transmit West Nile virus.

Both larvicides and adulticides are regulated by the U.S. Environmental Protection Agency (EPA).

What is CDC's position regarding the use of chemical mosquito control?

Chemical control measures are one part of a comprehensive and integrated mosquito management program. An integrated program is the most effective way to prevent and control mosquito-borne disease. An integrated mosquito management program should include several components:

  • Surveillance (monitoring levels of mosquito activity, and where virus transmission is occurring)
  • Reduction of mosquito breeding sites
  • Community outreach and public education
  • The ability to use chemical and biological methods to control both mosquito larvae and adult mosquitoes

Control measures, including the decision to use chemical adulticides (pesticides to kill adult mosquitoes) should be based on surveillance data and the risk of human disease. The CDC's Revised Guidelines for Surveillance, Prevention, and Control of West Nile Virus in the U.S., 2003 [254 KB, 77 pp] provides detailed guidance about the use of control measures, including suggestions for a phased response and the actions that are possible at different levels of virus activity.

Are pesticides harmful to people?

Effect on human health is one of the primary factors considered in the regulation of pesticides. Pesticides that can be used for mosquito control have been judged by the EPA not to pose an unreasonable risk to human health. People who are concerned about exposure to a pesticide, such as those with chemical sensitivity or breathing conditions such as asthma can reduce their potential for exposure by staying indoors during the application period (typically nighttime).

A recently published study (MMWR, July 11, 2003) examined illnesses in nine states associated with exposure to pesticides used to control mosquito populations from 1999-2002. This study found that the "application of certain insecticides poses a low risk for acute, temporary health effects among person in areas that were sprayed and among workers handling and applying insecticides."

For more information on pesticides and health, consult the U.S. Environmental Protection Agency, which oversees the registration of these chemicals. The National Pesticide Information Center (NPIC) can also provide information through a toll-free number by calling 1 (800) 858-7378.

What should I do if I think that I am having health problems because of pesticides used in my area?

If you are experiencing health problems for any reason, it is important to see your health care provider promptly. If you are experiencing severe health problems, go immediately to an emergency room.

How does pesticide spraying affect the environment?

A great deal of research must be done before pesticides can be used in the environment. The best source for finding out about the pesticides used in your area and their effect on specific types of wildlife, is with he U.S. Environmental Protection Agency, which oversees the registration of these products. The National Pesticide Information Center (NPIC) can also provide information through a toll-free number by calling 1 (800) 858-7378.

What training is required for workers who apply pesticides?

Each state has mandated training and experience requirements that must be met before an individual can commercially apply pesticides. In New York state, for example, certified pesticide apprentices must be at least 16 years old, have completed an eight-hour core training course on safety issues and the use of pesticides, and have at least 40 hours of pesticide use experience in the field under the direct supervision of a certified pesticide applicator.

In addition, these applicators must follow the instructions and precautions that are printed on the pesticide label. All pesticide products are required to have a label, which provides information including instructions on how to apply the pesticide and precautions to be taken to prevent health and environmental effects. All labels are required to be approved by the EPA.

Where can I get information regarding the safety of specific pesticides?

Questions concerning specific pesticides can be directed to the EPA, as this agency has responsibility for registration of pesticides. Many issues are addressed on the EPA's Mosquito Control website.

The National Pesticide Information Center (NPIC) provides pesticide information and questions about the impact of pesticide use on human health. The NPIC is cooperatively sponsored by Oregon State University and the EPA. The NPIC can be reached online or toll-free by calling 1 (800) 858-7378.

How can I find out what type of pesticides are being used in my area?

Your local mosquito control program or health department can give information about the type of products being used in an area. Mosquito control activities are most often handled at the local level, such as through county or city government. Check with your health department or in the "blue" (government) pages of the phone book for the contacts in your area.


Center for Disease Control and Prevention (CDC): The CDC is a source of information on disease control, and their internet website includes a listing of state health departments. Call 972-221-6400 or visit the CDC's website for more information.