Overview of West Nile Virus

Q: What is West Nile virus?
A. West Nile Virus is a flavivirus commonly found in Africa, West Asia, and the Middle East. It is closely related to St. Louis encephalitis virus which is also found in the United States. The virus can infect humans, birds, mosquitoes, horses and some other mammals

Q. What are West Nile encephalitis, West Nile meningitis and “neuroinvasive disease” and West Nile fever?
A. 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 be as short as a few days, even healthy people have been sick for several weeks.

Q. Historically, where has West Nile encephalitis occurred worldwide?
A. See the map describing distribution of flaviviruses, including West Nile virus:

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 CDC scientists believe the virus has probably been in the eastern U.S. since the early summer of 1999, possibly longer.

Q. I understand West Nile virus was found in "overwintering" mosquitoes in the New York City area in early 2000. What does this mean?
A. One of the species of mosquitoes found to carry West Nile virus is the Culex species which survive through the winter, or "overwinter," in the adult stage. That the virus survived along with the mosquitoes was documented by the widespread transmission the summer of 2000.

Q. Is West Nile virus now established in the Western Hemisphere?
A. The continued expansion of West Nile virus in the United States indicates that it is permanently established in the Western Hemisphere.

Q. Is the disease seasonal in its occurrence?
A. In the temperate zone of the world (i.e., between latitudes 23.5° and 66.5° north and south), West Nile encephalitis cases occur primarily in the late summer or early fall. In the southern climates where temperatures are milder, West Nile virus can be transmitted year round.

West Nile virus vaccine

Q. Is there a vaccine available to protect humans from West Nile virus?

A. No. Currently there is no WNV 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.

Q. Should people take the West Nile virus vaccine that is licensed for use in horses?

A. 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.

Prevention

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

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

Protect yourself from mosquito bites:

• Apply insect repellent 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. DEET concentrations higher than 50% do not increase the length of protection. 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 DEET since mosquitoes may bite through thin clothing. Do not apply repellents containing permethrin directly to exposed skin. If you spray your clothing, there is no need to spray repellent containing DEET on the skin under your clothing.
• When possible, wear long-sleeved 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. In 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.

New! Kids can learn how to protect themselves from mosquito bites on "The Buzz-z-z-z on West Nile Virus" (on BAM!, the CDC site for kids).

Q. What can be done to prevent outbreaks of West Nile virus?
A. 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).

Q. Is there a vaccine against West Nile encephalitis?
A. No, but several companies are working towards developing a vaccine.

Q. Where can I get information about the use of pesticide sprays that are being used for mosquito control?
A. The federal agency responsible for pesticide evaluation is the Environmental Protection Agency (EPA). See the EPA Web site for detailed answers to the questions about pesticides used for mosquito control.

Transmission

Q. How do people get infected with West Nile virus (WNV)?
A. 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. See Blood Transfusions and Transmission: Questions and Answers.

There is one reported case of transplacental (mother-to-child) WNV transmission. This case is detailed in MMWR Dec 20, 2002. 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 MMWR Dec 20, 2002.

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

Q. Can you get West Nile encephalitis from another person?
A. 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.

Q. Is a woman's pregnancy at risk if she gets infected with West Nile virus?
A. 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 acure WNV infection, should undergo appropriate diagnostic testing.

See MMWR Dec 20, 2002 for more information.

Q. Can West Nile virus be transmitted through blood transfusions?
A. Please refer to Blood Transfusions and Transmission: Questions and Answers.

Q. Besides mosquitoes, can you get West Nile virus directly from other insects or ticks?
A. 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.

Q.  How many types of animals have been found to be infected with West Nile virus?  
A.  Although the vast majority of infections have been identified in birds, WN virus has been shown to infect horses, cats, bats, chipmunks, skunks, squirrels, and domestic rabbits.

Q. Can you get West Nile virus directly from birds?
A. 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.

Q. Can you get infected with West Nile virus by caring for an infected horse?
A. 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.

Q. Can you get WNV from eating game birds or animals that have been infected?
A. There is no evidence that WNV 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.

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. How long does the West Nile virus remain in a person’s body after they are infected?
A. 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 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 laboratories testing is performed. Both antibodies and “memory” T-lymphocytes provide future protection from the virus.

Q. If a person contracts West Nile virus, does that person develop a natural immunity to future infection by the virus?
A. It is assumed that immunity will be lifelong; however, it may wane in later years.

Insect Repellent Use and Safety

Q. Why should I use insect repellent?
A. Insect repellents help people reduce their exposure to mosquito bites that may carry potentially serious viruses such as West Nile virus, and allow them to continue to play and work outdoors.

Q. When should I use mosquito repellent?
A. Apply repellent when you are going to be outdoors and will be at risk for getting bitten by mosquitoes.

Q. What time of day should I wear mosquito repellent?
A. Many of the mosquitoes that carry the West Nile virus are especially likely to bite around dusk and dawn. If you are outdoors around these times of the day, it is important to apply repellent. In many parts of the country, there are mosquitoes that also bite during the day, and these mosquitoes have also been found to carry the West Nile virus. The safest decision is to apply repellent whenever you are outdoors.

Q. How often should repellent be reapplied?
A. Follow the directions on the product you are using in order to determine how frequently you need to reapply repellent. Sweating, perspiration or getting wet may mean that you need to re-apply repellent more frequently. If you are not being bitten, it is not necessary to re-apply repellent. Repellents containing a higher concentration of active ingredient (such as DEET) provide longer-lasting protection.

Q. Should I wear repellent while I am indoors?
A. Probably not. If mosquitoes are biting you while you are indoors, there are probably better ways to prevent these bites instead of wearing repellent all the time. Check window and door screens for holes that may be allowing mosquitoes inside. If your house or apartment does not have screens, a quick solution may be to staple or tack screening (available from a hardware store) across the windows. In some areas community programs can help older citizens or others who need assistance.

Q. How does mosquito repellent work?
A. 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. Many repellents contain a chemical, N,N-diethyl-m-toluamide (DEET), which repels the mosquito, making the person unattractive for feeding. DEET does not kill mosquitoes; it just makes them unable to locate us. Repellents are effective only at short distances from the treated surface, so you may still see mosquitoes flying nearby. As long as you are not getting bitten, there is no reason to apply more DEET.

Q. Which mosquito repellent works the best?
A. The most effective repellents contain DEET (N,N-diethyl-m-toluamide), which is an ingredient used to repel pests like mosquitoes and ticks. DEET has been tested against a variety of biting insects and has been shown to be very effective. 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. DEET concentrations higher than 50% do not increase the length of protection.

Q. How does the percentage of DEET in a product relate to the amount of protection it gives?
A. Based on a 2002 study:

• A product containing 23.8% DEET provided an average of 5 hours of protection from mosquito bites.
• A product containing 20% DEET provided almost 4 hours of protection
• A product with 6.65% DEET provided almost 2 hours of protection
• Products with 4.75% DEET and 2% soybean oil were both able to provide roughly 1 and a half hour of protection.

Choose a repellent that provides protection for the amount of time that you will be outdoors. A higher percentage of DEET should be used if you will be outdoors for several hours while a lower percentage of DEET can be used if time outdoors will be limited. You can also re-apply a product if you are outdoors for a longer time than expected and start to be bitten by mosquitoes. (For more information, see Table 1: Fradin and Day, 2002. See Publications page.)

Q. Why does CDC recommend using DEET?
A. DEET is the most effective and best-studied insect repellent available. (Fradin, 1998). Studies using humans and mosquitoes report that among those tested only products containing DEET offer long-lasting protection after a single application. (Fradin and Day, 2002. See Publications page.)

Q. Are non-DEET repellents effective (e.g. Skin-So-Soft, plant-based repellents)?
A. Some non-DEET repellent products which are intended to be applied directly to skin also provide some protection from mosquito bites. However, studies have suggested that other products do not offer the same level of protection, or that protection does not last as long as products containing DEET. A soybean-oil-based product has been shown to provide protection for a period of time similar to a product with a low concentration of DEET (4.75%) (Fradin and Day, 2002. See Publications page.).

People should choose a repellent that they will be likely to use consistently and that will provide sufficient protection for the amount of time that they will be spending outdoors. Product labels often indicate the length of time that protection that can be expected from a product. Persons who are concerned about using DEET may wish to consult their health care provider for advice. The National Pesticide Information Center (NPIC) can also provide information through a toll-free number, 1-800-858-7378 or npic.orst.edu.

Q. I'm confused. Which products contain "DEET"?
A. Most insect repellents that are available in stores are now labeled with the word "DEET" as well as labeled with the chemical name. Look for N,N-diethyl-m-toluamide or, sometimes, N,N-diethly-3-methylbenamide. Choose a repellent that offers appropriate protection for the amount of time you will be outdoors (see above). A higher percentage of DEET should be used if you will be outdoors for several hours while a lower percentage of DEET can be used if time outdoors will be limited.

Using Repellents Safely

Q. What are some general considerations to remember in order to use products containing DEET safely?
A. 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.
• Do not spray aerosol or pump products in enclosed areas.
• Do not apply aerosol or pump products directly to your face. Spray your hands and then rub them carefully over the face, avoiding eyes and mouth.

Q. How should products containing DEET be used on children?
A. 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 the product recommendations. The American Academy of Pediatrics (AAP) Committee on Environmental Health has recently updated their recommendation for use of DEET products on children, citing: "Insect repellents containing DEET (N,N-diethyl-m-toluamide, also known as N,N-diethyl-3-methylbenzamide) 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."

The AAP and other experts suggest that it is acceptable to apply repellent with low concentrations of DEET to infants over 2 months old. Other guidelines cite that it is acceptable to use repellents containing DEET on children over 2 years of age.

Repellent products that do not contain DEET are not likely to offer the same degree of protection from mosquito bites as products containing DEET. Non-DEET repellents have not necessarily been as thoroughly studied as DEET, and may not be safer for use on children.

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. Persons who are concerned about using DEET or other products on children may wish to consult their health care provider for advice. The National Pesticide Information Center (NPIC) can also provide information through a toll-free number, 1-800-858-7378 or npic.orst.edu.

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 to skin under clothing. If repellent is applied to clothing, wash treated clothing before wearing again.

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 (don t use permethrin 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 the mosquitoes.

Q. Is DEET safe for pregnant or nursing women?
A. There are no reported adverse events following use of repellents containing DEET in pregnant or breastfeeding women.

Q. Are there any risks due to using repellents containing DEET?
A. Use of these products may cause skin reactions in rare cases. If you suspect a reaction to this product, discontinue use, wash the treated skin, and call your local poison control center. There is a new national number to reach a Poison Control Center near you: 1-800-222-1222.

If you go to a doctor, take the product with you. Cases of serious reactions to products containing DEET have often been related to occasions where the product was not used as directed, such as swallowing, using over broken skin, or using for multiple days without washing skin in between use, for example. Always follow the instructions on the product label.

Insect Repellents and Sunscreen

Q. Can I use an insect repellent containing DEET and a product containing sunscreen at the same time?
A. Yes. People can and should use both sunscreen and DEET when they are outdoors to protect their health. Follow the instructions on the package for proper application of each product. Apply sunscreen first, followed by repellent containing DEET.

To protect from sun exposure and insect bites, you can also wear long sleeves and long pants. You can also apply insect repellent containing DEET or permethrin to your clothing, rather than directly to your skin.

Q. Has CDC changed its recommendations for use of DEET and sunscreen?
A. No. Based on available research, CDC believes it is safe to use both products at the same time. Follow the instructions on the package for proper application of each product. Apply sunscreen first, then insect repellent containing DEET, to be sure that each product works as specified.

Q. Should I use a combination sunscreen/DEET-based insect repellent?
A. Because the instructions for safe use of DEET and safe use of sunscreen are different, CDC does not recommend using products that combine DEET with sunscreen.

In most situations, DEET does not need to be reapplied as frequently as sunscreen. DEET is safe when applied correctly. The rare adverse reactions to DEET have generally occurred in situations where people do not follow the product instructions. Sunscreen often requires frequent reapplication, so using a combined product is not recommended. You do not need to reapply insect repellent every time you reapply sunscreen. Follow the instructions on the package for each product to get the best results.

Q. I heard about a study saying that there may be some type of interaction between repellents containing DEET and sunscreen. Is this true?
A. There has been attention to a study concerning the chemicals in DEET and sunscreen presented at a scientific meeting. This is an in vitro study, which means that it is a laboratory study that did not include human or animal testing. The goal of the study was to examine absorption of these chemicals, and it did not evaluate or make conclusions about health effects related to this issue. The study authors stated that further evaluation of the interaction of these chemicals should be conducted. The study has not yet been published (as of July 2003).

Evaluation by the EPA, which regulates products such as DEET, indicates that it is safe to use insect repellents containing DEET and sunscreen at the same time. CDC recommends using two separate products because sunscreen requires frequent applications while DEET should be used sparingly. Follow the directions on the package for each product, and consult your physician or pharmacist if you have questions. CDC's recommendations for the safe use of insect repellents on children and adults remain unchanged.

More information

Q. Where can I get more information about repellents?
A. For more information about using repellents safely, please consult the Environmental Protection Agency (EPA) Web site 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 1-800-858-7378.

Who's at Risk for West Nile Virus

Q. Who is at risk for getting West Nile encephalitis?
A. 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 WNV disease.

Symptoms of West Nile Virus

Q. What are the symptoms of West Nile virus infection?
A. Most people who are infected with the West Nile virus will not have any type of illness.

It is estimated that about 20% of the people who become infected will develop West Nile fever: the symptoms include fever, headache, tiredness, and body aches, occasionally with a skin rash on the trunk of the body and swollen lymph glands.

The symptoms of severe infection (West Nile encephalitis or meningitis) 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 disease.

Q. What is the incubation period in humans (i.e., time from infection to onset of disease symptoms) for West Nile encephalitis?
A. Usually 3 to 14 days.

Q. How long do symptoms last?
A. Symptoms of West Nile fever will generally last a few days, although even some healthy people report having been sick for several weeks. Symptoms of severe disease (encephalitis or meningitis) may last several weeks, although neurological effects may be permanent.

Q. 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 (WNF in 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.

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

Some people may develop a brief, WNF-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.

Testing and Treating West Nile Virus in Humans

Q. I think I have symptoms of West Nile virus. What should I 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 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. People who live in or traveled to areas where West Nile virus activity has been identified are at risk of getting West Nile encephalitis; persons older than 50 years of age have the highest risk of severe disease. 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 are human cases of WNV diagnosed?
A. West Nile virus (WNV) infection can be suspected in a person based on clinical symptoms and patient history. Laboratory testing is required for a confirmed diagnosis.

The most commonly used WNV laboratory test measures antibodies that are produced very early in the infected person. These antibodies, called IgM antibodies, can be measured in blood or cerebrospinal fluid (CSF), which is the fluid surrounding the brain and spinal cord. This blood test may not be positive when symptoms first occur; however, the test is positive in most infected people within 8 days of onset of symptoms.

A test for WNV IgM-antibody is used by CDC, state and local public health labs and increasingly at private laboratories. When testing is conducted at private laboratories, the health department or CDC will often confirm results in their own laboratories before officially reporting WNV cases.

In some instances, health departments may conduct or request additional testing from CDC before officially reporting a case to CDC's ArboNET Surveillance System. The state or CDC reference laboratory may repeat the initial IgM-antibody testing.

A state may also perform or ask CDC to perform an additional, different test on a specimen. This latter test (plaque reduction neutralization test [PRNT]) is usually performed when:

• the state finds its initial case(s) of human WNV illness,
• IgM results are not definitive due to equivocal laboratory testing results or insufficient specimens,
• the patient might have been exposed to other closely related viruses (like St. Louis encephalitis virus) which may result in a "false" positive laboratory test for WNV.

These additional tests require growth of the virus and may take a week or longer (plus shipping time) to conduct. The results from the PRNT are often needed before CDC considers a human WNV infection confirmed.

Q. How does CDC decide when to report a case of WNV?
A. CDC reports a case of WNV once a state officially reports and verifies that case to CDC.

The timing of the official report to CDC, relative to onset of symptoms in a person, is variable and depends on when an individual first seeks medical care and the extent of the laboratory testing, as described above, that the state determines is necessary before reporting.

At any given time, in addition to the official case count reported by CDC, there may be additional suspect cases under investigation or in various stages of testing, including supplemental or confirmatory laboratory testing.

Q. How many of the human WNV cases are being confirmed by the CDC laboratories?
A. When WNV was first found in the United States in 1999, the CDC reference laboratory confirmed all human cases of WNV. Through a comprehensive, CDC-sponsored laboratory training program, most states are now able to perform the initial blood tests to identify IgM-antibody in the blood or CSF of suspect human WNV infections, and many state laboratories are also able to perform the more involved PRNT. The CDC reference lab is called upon for confirmatory testing by fewer and fewer states; although the increased activity of WNV still requires that many tests be performed at the CDC reference laboratory.

Q. How is West Nile encephalitis treated?
A. There is no specific treatment for West Nile virus infection. In more severe cases, intensive supportive therapy is indicated, often involving hospitalization, intravenous fluids, airway management, respiratory support (ventilator), prevention of secondary infections (pneumonia, urinary tract, etc.), and good nursing care.

Questions about Commercial Laboratories New!

Q.What role do commercial laboratories play in diagnosing people with West Nile virus infection?

A.When a person goes to see a health care provider, and has symptoms of a West Nile illness a specimen may be sent to a commercial laboratory to determine if the person has been infected by West Nile virus. The tests used in commercial labs check for antibodies to the virus (the body’s response to infection). The results of the test will be sent to the doctor and the state health department will be informed if the results are positive. There is no specific treatment available for West Nile virus infection, so the diagnosis will not necessarily change the way the person is being treated but it will let the doctor know that he/she does not have to investigate another cause of illness, and it will help the health department know where the virus is active in order to focus prevention measures.

The state health department may choose to accept the positive results from the commercial lab, or they may choose to test the sample again in the state health department laboratory for confirmation of the infection. The state health department will report the case to CDC.

Q.How accurate are the tests used in commercial labs?

A.The tests used in commercial labs are modeled on the tests created by CDC and used at CDC and in state public health laboratories. This is the first year that many of these tests have been widely used in commercial labs, and laboratories are learning more about the specific measurements used in each test. Often, a second test will be done to confirm the infection. State health departments, the FDA (which licenses and regulates medical tools such as these tests), the association of Public Health Laboratories and CDC are all engaged in monitoring new commercial tests, and are committed to working with industry to make these tests as accurate and useful as possible.

Q.If a test is a “false positive” what does that mean?

A.A “false positive” occurs when an initial tests indicates that a person does have a West Nile infection, but a later, more specific tests indicates that the person does not actually have the infection. While it is important to health department and CDC to get an accurate idea of where people are being infected in order to focus prevention and control efforts, the result does not have a great impact on the individual person. There is no specific treatment that the person would receive due to West Nile virus infection. The person may want to work with their physician to see if another cause of the illness needs to be identified.

West Nile Virus, Pregnancy and Breast-feeding

Q. What risk does WNV illness during pregnancy present to a fetus?
A. Based on the limited number of cases studied so far, it is not yet possible to determine what percentage of WNV infections during pregnancy result in infection of the fetus or medical problems in newborns.

In 2002, one case of transplacental (mother-to-child) transmission of WNV was reported to CDC. In this case, the infant was born with WNV infection and severe medical problems. It is unclear, however, whether WNV infection caused these problems or whether they were due to other causes (see MMWR Dec 20, 2002).

After the report of this case, CDC and state and local health departments formed a registry to follow birth outcomes among women with WNV illness in pregnancy. Three additional pregnancies in which the expectant mother became infected with WNV were detected and evaluated in 2002; none of these 3 resulted in fetal infection. In one additional case it remains unclear whether the fetus was infected; appropriate testing was not done.

In 2003, the registry identified 74 women who acquired WNV illness while pregnant. Preliminary findings regarding outcomes of these pregnancies were first presented at the Fifth Annual National West Nile Virus Conference in Denver CO on February 2, 2004.

As of May 10, 2004, 62 of these women had delivered live infants, 2 had had elective abortions, 5 miscarried in the first trimester and 5 had not yet delivered.

In 2004, CDC is continuing to gather clinical and laboratory information on outcomes of pregnancies of women with WNV illness during pregnancy. Pregnant women who think they may have become infected with WNV should contact their private health care providers. Clinicians who are aware of WNV infections of pregnant women are encouraged to report such cases by calling their state or local health departments, or by contacting CDC, telephone 970-221-6400. For more information see the section on Clinical Guidance.

Because of ongoing concerns that mother-to-child WNV transmission can occur with possible adverse health effects, pregnant women should take precautions to reduce their risk for WNV and other mosquito-borne infections by avoiding mosquitoes, using protective clothing, and using repellents containing DEET. Repellents with DEET are safe for pregnant women, and there are other options as well such as a soybean oil based repellent that provides good, though quite limited, protection, as judged by a study published in the new England Journal of Medicine.

Pregnant women who become ill should see their health care provider, and those who have an illness consistent with acute WNV infection should undergo appropriate diagnostic testing.

Additional clinical information on intrauterine WNV can be found in these recent publications:

• Hayes EB and O'Leary DR. West Nile virus infection: a pediatric perspective. Pediatrics. 5 May 2004; 113(5): 1375-1381.
• Alpert SG, Fergerson J, Noel LP. Intrauterine West Nile virus: ocular and systemic findings. Am J Ophthalmol. 2003 Oct;136(4):733-5.
• Chapa et al. West Nile Virus Encephalitis During Pregnancy. Obstetrics and Gynocology. 2003 Aug; 102(2):229-231.
  

Q. Where can I get more detailed clinical information about WNV in pregnancy?
A. More information on issues that may be helpful to clinicians working with WNV can be found on the Clinical Guidance page.

Q. Are infants at higher risk than other groups for illness with West Nile virus?
A. No. West Nile virus illnesses in children younger than 1 year old are infrequent. During 1999-2001, no cases in children younger than one year of age were reported to CDC. In 2002, 2,500 total West Nile Virus disease cases were reported to CDC, and only six occurred in children less than one year of age. The number of children infected with WNV during 2003 will be updated when data are finalized.

Breastfeeding

Q. Can West Nile virus be transmitted through breast milk?
A. Based on a 2002 case in Michigan, it appears that West Nile virus can be transmitted through breast milk. A new mother in Michigan contracted West Nile virus from a blood transfusion shortly after giving birth. Laboratory analysis showed evidence of West Nile virus in her breast milk. She breastfed her infant, and three weeks later, her baby's blood tested positive for West Nile virus. Because of the infant's minimal outdoor exposure, it is unlikely that infection was acquired from a mosquito. The infant was most likely infected through breast milk. The child is healthy, and does not have symptoms of West Nile virus infection.

Q. Should I continue breast-feeding if I am symptomatic for West Nile virus?
A. Because the health benefits of breast-feeding are well established, and the risk for West Nile virus transmission through breast-feeding is unknown, the new findings do not suggest a change in breast-feeding recommendations.

Lactating women who are ill or who are having difficulty breast-feeding for any reason should, as always,consult their physicians.

Q. Should I continue breast-feeding if I live in an area of WNV transmission?
A. Yes. Because the health benefits of breast-feeding are well established, and the risk for West Nile virus transmission through breast-feeding is unknown, the new findings do not suggest a change in breast-feeding recommendations.

Q. If I am breast-feeding, should I be tested for West Nile virus?
A. No. There is no need to be tested just because you are breast-feeding.

Additional Information About West Nile Virus

• Publications on West Nile Virus
• West Nile Virus and Kids

Source is the CDC guide to the West Nile Virus

	Web keepkidshealthy.com

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