West Nile Virus Frequently Asked Questions
West Nile virus is a flavivirus commonly found in Africa, West Asia, and the Middle East. It is closely related to the St. Louis encephalitis virus which is also found in the United States. The virus can infect humans, birds, mosquitoes, horses and some other mammals.
Insect repellents can help reduce exposure to mosquitoes that may carry viruses such as West Nile virus, which can cause serious illness and even death. Using insect repellent allows you to continue to play and work outdoors with a reduced risk of mosquito bites.
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 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.
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 last as short as a few days, even healthy people have been sick for several weeks.
Apply repellent when you are going to be outdoors. Even if you don’t notice mosquitoes there is a good chance that they are around. Many of the mosquitoes that carry West Nile virus bite between dusk and dawn.
If you are outdoors around these times of the day, it is especially important to apply repellent. In many parts of the country, there are mosquitoes that also bite during the day, and some of these mosquitoes have also been found to carry West Nile virus.
It is not known how long it has been in the U.S., but CDC scientists believe the virus has probably been in the eastern U.S. since the early summer of 1999, possibly longer.
In general you should re-apply repellent if you are being bitten by mosquitoes. Always follow the directions on the product you are using. Sweating, perspiration or getting wet may mean that you need to re-apply repellent more frequently.
Repellents containing a higher concentration (higher percentage) of active ingredient typically provide longer-lasting protection.
No. Currently there is no West Nile virus vaccine available for humans. Many scientists are working on this issue, and there is hope that a vaccine will become available in the next few years.
Female mosquitoes bite people and animals because they need the protein found in blood to help develop their eggs. Mosquitoes are attracted to people by skin odors and carbon dioxide from breath. The active ingredients in repellents make the person unattractive for feeding.
Repellents do not kill mosquitoes. Repellents are effective only at short distances from the treated surface, so you may still see mosquitoes flying nearby.
No. This vaccine has not been studied in humans and could be harmful. The effectiveness of this vaccine in preventing West Nile virus infections in horses has yet to be fully evaluated, and its effectiveness in humans is completely unknown.
Veterinary vaccines are not manufactured with the same rigorous quality and purity standards required of human vaccines, nor are they required to undergo the extensive field testing required of human vaccines before they are licensed. For these reasons, veterinary vaccines and other veterinary drugs should never be used in humans.
The Centers for Disease Control and Prevention (CDC) recommends using products that have been shown to work in scientific trials and that contain active ingredients which have been registered with the U.S. Environmental Protection Agency (EPA) for use as insect repellents on skin or clothing.
When the EPA registers a repellent, they evaluate the product for efficacy and potential effects on human beings and the environment. EPA registration means that EPA does not expect a product, when used according to the instructions on the label, to cause unreasonable adverse effects to human health or the environment.
Of the active ingredients registered with the EPA, the CDC believes that two have demonstrated a higher degree of efficacy in the peer-reviewed, scientific literature. Products containing these active ingredients typically provide longer-lasting protection than others:
- DEET (N,N-diethyl-m-toluamide)
- Picaridin (KBR 3023)
Oil of lemon eucalyptus [active ingredient: p-menthane 3,8-diol (PMD)], a plant- based repellent, is also registered with EPA. In two recent scientific publications, when oil of lemon eucalyptus was tested against mosquitoes found in the United States it provided protection similar to repellents with low concentrations of DEET.
The main route of human infection with West Nile virus 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.
For more information access the Centers for Disease Control and Prevention (CDC) information on blood transfusions and West Nile virus at http://www.cdc.gov/ncidod/dvbid/westnile/qa/transmission.htm.
Typically, the more active ingredient a product contains the longer it provides protection from mosquito bites. The concentration of different active ingredients cannot be directly compared (that is, 10 percent concentration of one product doesn’t mean it works exactly the same as 10 percent concentration of another product.)
DEET is an effective active ingredient found in many repellent products and in a variety of formulations. A study conducted in 2002 found:
- A product containing 23.8 percent DEET provided an average of 5 hours of protection from mosquito bites
- A product containing 20 percent DEET provided almost 4 hours of protection
- A product with 6.65 percent DEET provided almost 2 hours of protection
- Products with 4.75 percent DEET were both able to provide roughly 1 and a half hour of protection
These examples represent results from only one study and are only included to provide a general idea of how such products may work. Actual protection will vary widely based on conditions such as temperature, perspiration, and water exposure.
Choose a repellent that provides protection for the amount of time that you will be outdoors. A product with a higher percent of active ingredient is a good choice if you will be outdoors for several hours while a product with a lower concentration can be used if time outdoors will be limited. Simply re-apply repellent (following label instructions) if you are outdoors for a longer time than expected and start to be bitten by mosquitoes.
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 one percent 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.
Check the product label if you have questions-–repellents must specify their active ingredients. In some cases you will note the chemical name in addition to/instead of the “common” name:
- DEET is N,N-diethyl-m-toluamide
- Picaridin is KBR 3023, sometimes known as Picaridin is KBR 3023, sometimes known as “Bayrepel”
- The active ingredient in oil of lemon eucalyptus is p-menthane 3,8-diol (PMD)
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.
Certain products which contain permethrin are recommended for use on clothing, shoes, bed nets, and camping gear, and are registered with EPA for this use. Permethrin is highly effective as an insecticide and as a repellent.
Permethrin-treated clothing repels and kills ticks, mosquitoes, and other arthropods and retains this effect after repeated laundering. The permethrin insecticide should be reapplied following the label instructions. Some commercial products are available pretreated with permethrin.
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 Insect Repellent). 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 West Nile virus infection, should undergo appropriate diagnostic testing.
Most of these repellents are sold at multiple retail, discount and drug stores. A wider selection may be available at “outdoor” stores or in hunting and camping sections.
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.
Always follow the recommendations appearing on the product label.
- Use enough repellent to cover exposed skin or clothing. Don’t apply repellent to skin that is under clothing. Heavy application is not necessary to achieve protection.
- Do not apply repellent to cuts, wounds, or irritated skin.
- After returning indoors, wash treated skin with soap and water. (This may vary depending on the product. Check the label.)
- Do not spray aerosol or pump products in enclosed areas.
- Do not spray aerosol or pump products directly to your face. Spray your hands and then rub them carefully over the face, avoiding eyes and mouth.
Although the vast majority of infections have been identified in birds, West Nile virus has been shown to infect horses, cats, bats, chipmunks, skunks, squirrels, and domestic rabbits.
Use of repellent products may cause skin reactions in rare cases. Most products also note that eye irritation can occur if product gets in the eye. If you suspect a reaction to a product, discontinue use, wash the treated skin, and call a poison control center. If product gets in the eyes flush with water and consult health care provider or poison control center. If you go to a doctor, take the product with you.
There is a national number to reach a Poison Control Center near you: (800) 222-1222.
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.
Repellent products must state any age restriction. If there is none, EPA has not required a restriction on the use of the product.
According to the label, oil of lemon eucalyptus products should not be used on children under 3 years.
Since it is the most widely available repellent, many people ask about the use of products containing DEET on children. No definitive studies exist in the scientific literature about what concentration of DEET is safe for children. No serious illness has been linked to the use of DEET in children when used according to manufacturer’s recommendations.
The American Academy of Pediatrics (AAP) Committee on Environmental Health updated their recommendation for use of DEET products on children in 2003, citing: “Insect repellents containing DEET (N,N-diethyl-m-toluamide, also known as N,N-diethyl-3-methylbenzamide) with a concentration of 10 percent appear to be as safe as products with a concentration of 30 percent when used according to the directions on the product labels.” AAP recommends that repellents with DEET should not be used on infants less than 2 months old.
Parents should choose the type and concentration of repellent to be used by taking into account the amount of time that a child will be outdoors, exposure to mosquitoes, and the risk of mosquito-transmitted disease in the area.
If you are concerned about using repellent products on children you may wish to consult a health care provider for advice or contact the National Pesticide Information Center (NPIC) through their toll-free number, (800) 858-7378 or npic.orst.edu.
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.
Always follow the recommendations appearing on the product label when using repellent:
- When using repellent on a child, apply it to your own hands and then rub them on your child. Avoid children’s eyes and mouth and use it sparingly around their ears.
- Do not apply repellent to children’s hands. (Children may tend to put their hands in their mouths.)
- Do not allow young children to apply insect repellent to themselves; have an adult do it for them.
- Keep repellents out of reach of children.
- Do not apply repellent under clothing. If repellent is applied to clothing, wash treated clothing before wearing again. (May vary by product, check label for specific instructions.)
There is no evidence West Nile virus can be transmitted to humans through consuming infected birds or animals. In keeping with overall public health practice, and due to the risk of known food-borne pathogens, people should always follow procedures for fully cooking meat from either birds or mammals.
Using repellents on the skin is not the only way to avoid mosquito bites. Children (and adults) can wear clothing with long pants and long sleeves while outdoors. DEET or other repellents such as permethrin can also be applied to clothing (but is not registered for use on skin), as mosquitoes may bite through thin fabric.
Mosquito netting can be used over infant carriers.
Finally, it may be possible to reduce the number of mosquitoes in the area by getting rid of containers with standing water that provide breeding places for mosquitoes.
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.
Other than the routine precautions noted earlier, the EPA does not recommend any additional precautions for using registered repellents on pregnant or lactating women. Consult your health care provider if you have questions.
There is no scientific evidence indicating that people can be chronically infected with West Nile virus. What remain in a person’s body for long periods of time are antibodies and “memory” white blood cells (T-lymphocytes) that the body produces to fight the virus. These antibodies and T-lymphocytes last for years, and may last for the rest of a person’s life. Antibodies are what many diagnostic tests look for when clinical laboratory testing is performed. Both antibodies and “memory” T-lymphocytes provide future protection from the virus.
Insect Repellents containing DEET and Sunscreen
Yes. People can, and should, use both a sunscreen and an insect repellent when they are outdoors. Follow the instructions on the package for proper application of each product. In general, the recommendation is to apply sunscreen first, followed by repellent.
It is recommended not to use a single product that combines insect repellent containing DEET and sunscreen, because the instructions for use of insect repellents and use of sunscreen are different.
In most situations, insect repellent does not need to be reapplied as frequently as sunscreen. While no recommendations are available at this time regarding products that combine other active ingredients and sunscreen, it is important to always follow the label on whatever product you are using.
To protect from sun exposure and insect bites, you can also wear long sleeves and long pants. You can also apply insect repellent to your clothing, rather than directly to your skin.
It is assumed that immunity will be lifelong; however, it may wane in later years.
For more information access the CDC Question and Answer section of its website at http://www.cdc.gov/ncidod/dvbid/westnile/q&a.htm.
For more information about using repellents, please consult the Environmental Protection Agency (EPA) website or consult the National Pesticide Information Center (NPIC), which is cooperatively sponsored by Oregon State University and the U.S. EPA. NPIC can be reached at: npic.orst.edu or (800) 858-7378.
Updated on: August 5, 2019