Frequently Asked Questions (FAQ) 

West Nile Virus Overview: Frequently Asked Questions (FAQ)

Information provided by the Centers for Disease Control and Prevention (CDC) and the Texas Department of State Health Services (DSHS)

Q: What is West Nile virus?

A. West Nile Virus is a virus 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 the differences between West Nile neuroinvasive disease (WNND) and West Nile fever (WNF)?

A. The most severe type of disease due to a person being infected with West Nile virus is called “neuroinvasive disease” because it affects a person’s nervous system. It is characterized by high fever, headache, neck stiffness, stupor, disorientation, and other serious symptoms. Symptoms can last weeks to months, and can even result in death. WNND only infects approximately 1% of people who have been bitten by a WNV-carrying mosquito. 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. WNF infects approximately 20% of people who have been bitten by a WNV-carrying mosquito.

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. It is now established as a seasonal epidemic in North America.

Q. How long has West Nile virus been in Denton County?

 A. Denton County reported its first case of WNV in 2002.

Q. How long is the West Nile virus season in the U.S.?

A. Typically, the season lasts from the summer months and into the fall.

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. What is the incubation period in humans (i.e., time between bite from infected mosquito to appearance of symptoms) for West Nile disease?

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 the illness last for several weeks. The symptoms of severe disease (West Nile neuroinvasive disease) may last several weeks or months, although neurological effects may be permanent.

Q: I think I have West Nile virus - do I need to report it to the Health Department?

 A. If you believe you have WNV, see your health care provider. He/she will do a clinical exam, and will run blood tests or spinal fluid tap if necessary. If a case is suspected by your health care provider, he/she is required to report that information to the Health Department. The Health Department will then investigate every case, and will contact you if necessary. There are 78 diseases that health care providers are required to report to their county health department, and West Nile virus is one of them.

Q: I've been bitten by a mosquito. Should I be tested for West Nile virus?

A: No. Illnesses related to mosquito bites are still uncommon, and only 20% of people bitten by a mosquito infected with WNV will develop symptoms. However, you should see a doctor immediately if you develop symptoms such as high fever, confusion, muscle weakness, severe headaches, stiff neck, or if your eyes become sensitive to light. Patients with mild symptoms should recover completely, and do not require any specific medication or laboratory testing.

Q: My doctor told me that I have West Nile virus. How do I report it to the Health Department?

A: No. Your health care provider is required to report that information to the Health Department. The Health Department will then investigate every case, and will contact you if necessary. There are 78 diseases that health care providers are required to report to their county health department, and West Nile virus is one of them.

Q. Can a human get the virus twice?

 A. We don't think so. It is assumed that a person would develop a natural immunity to future infection by the virus, and that this immunity would be lifelong. However, this immunity may wane in later years.

Q. Is there a human vaccine?

A. No, but the CDC is working with several partners towards developing a vaccine.

Q. What kind of laboratory tests are done to identify the West Nile virus in humans?

A. Various tests can be done. The type of test also depends on the kind of samples available (blood serum, cerebrospinal fluid, brain tissue, etc.). Samples may be tested to find antibodies to West Nile virus, or there may be an attempt to isolate virus particles from the sample. Tests that can be done include Hemagglutination-Inhibition, IgM-Capture, Plaque-Reduction Neutralization, virus isolation, and PCR.

Q. How many human cases have there been in the United States? How many deaths?

A. In 1999, 62 cases of severe disease, including 7 deaths, occurred in the New York area. In 2000, 21 cases were reported, including 2 deaths in the New York City area. In 2001, there were 66 human cases of severe disease and 9 deaths. In 2012, there were 2,734 cases of severe disease and 243 deaths. No reliable estimates are available for the number of cases worldwide of West Nile encephalitis, the disease caused by the West Nile virus. For the latest up-to-date information, visit the Centers for Disease Control and Prevention website on West Nile virus »

Q. How many human cases have there been in Texas? How many deaths?

A. The first human death in Texas occurred on August 16th, 2002. In 2012, Texas reported 86 human deaths from West Nile virus. For the latest up-to-date information on human cases in Texas, see the DSHS West Nile Virus home page on the Texas Health and Human Services website »

Q. What is the risk of someone becoming infected with West Nile?

A. The risk is very low. 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. In addition to being infected by the West Nile virus, what else can cause bird "die-offs" ?

A. Chemical spills, pesticides, drought, severe weather, and other diseases can cause die-offs in birds. To report a die-off of birds, contact the Texas Parks and Wildlife Department's Kills and Spills Team (KAST) at (512) 389-4848 or visit the Texas Parks and Wildlife Department's Kills and Spills Team website »

Note: the Texas Department of State Health Services (DSHS) does not test birds for West Nile virus. Please do not contact DSHS if you find a dead bird. If you wish to dispose of a bird carcass, wear gloves and dispose of it in the garbage. As a reminder, whenever you handle an animal, you should wash your hands.

Q. Can dogs, cats and other pets get the West Nile virus?

A. Yes. But they rarely, if ever, get sick. No cases of West Nile disease have been confirmed in dogs and cats. The virus can infect many species of animals, but few actually get the disease. Most infections have been identified in birds, but West Nile virus has been shown to infect dogs, cats, horses, and domestic rabbits, as well as bats, chipmunks, skunks, and squirrels.

Q. Is there a vaccine for dogs and cats? A. No.

Myths and Rumors FAQ

Information provided by the Texas Department of State Health Services (DSHS) and Texas A&M AgriLife Extension

Q. I heard that putting up purple martin houses and bat houses will help prevent West Nile virus because those animals eat mosquitoes. Is this true?

While it is true that certain types of birds and bats eat mosquitoes, putting up bird and bat houses on your property will not necessarily help prevent the transmission of West Nile virus. Several agencies have tried to control mosquitoes by using birds, bats, dragonflies and frogs. However, according to the American Mosquito Control Association, there is no proof that bats, purple martins, or other animals that eat insects are able to eat enough adult mosquitoes to make a difference. One reason for this is because purple martins fly and eat during the day and most mosquitoes are active at night. In addition, most bats eat June bugs and moths, but do not eat mosquitoes. Also, bats can transmit the rabies virus and encouraging them to live in your yard could pose a health risk to your family and neighbors.

Q. My community won't spray for mosquitoes. Therefore, my town has no mosquito control and the citizens are at risk for West Nile virus.

Mosquito control can be divided into two areas of responsibility - individual and public. Public spraying to control mosquitoes is just one of many pest control methods used for effective long-term mosquito control. The reduction, elimination, or treatment of mosquito breeding areas is the best and most cost-effective technique for mosquito control. The most important things you and the citizens of your community can do to reduce the risk of exposure to West Nile virus are to eliminate mosquito-breeding areas in your environment and limit your exposure to feeding mosquitoes. Many female mosquitoes can lay 100-300 eggs on the surface of fresh or stagnant water every third night during its life span. Here are some simple things you can do to eliminate potential mosquito breeding sites in your environment: 

arrowDo not allow water to accumulate in the saucers of flowerpots, urns, kiddie pools, tires, tin cans, or in pet dishes for more than 2 days.
arrowConsistently check areas that collect water and drain them.
arrowGet rid of tin cans, old tires, buckets, unused kiddie pools or other containers that collect and hold water.
arrowClean debris from rain gutters, remove standing water from flat roofs, and repair leaks around faucets and air conditioners.
arrowChange the water in birdbaths and wading pools at least every 3-4 days.
arrowFill or drain puddles, ditches and swampy areas.
arrowClean and chlorinate swimming pools, outdoor saunas and hot tubs.
arrowCheck for trapped water in plastic or canvas tarps used to cover boats or pools, and arrange the tarp to drain the water.
arrowIf ditches do not flow and contain stagnant water for a week or longer, report this problem to your town or city.
arrowClean out livestock water troughs on a weekly basis or stock them with the mosquitofish, Gambusia affinis, which feed on mosquito larvae in the water.
arrowIf you are unable to drain an area with standing water, such as an abandoned swimming pool, animal watering troughs, or small, shallow ponds, consider the use of a larvicide product with Bacillus thuringiensis israelensis (BTI). These products are known as mosquito “dunks” or “donuts,” and the BTI bacteria kill mosquito larvae, but will not harm plants or animals. Follow all product information for safe handling and use.
arrowBe a good neighbor: help those who are unable to drain standing water on their property, to eliminate breeding sites in your neighborhood.

Q. My neighbor says you can get West Nile virus from handling dead birds. Is this true?

No. West Nile virus is spread to humans mainly through the bite from an infected mosquito. There is no proof that West Nile virus can be spread from person to person or from animal to person.

Q. I found a dead bird in my yard today and the Department of State Health Services will not test it. I know my family has been exposed to West Nile virus from this bird. How can we be tested for this deadly disease?

A. West Nile virus transmission to people only occurs from the bite of an infected mosquito. Keep in mind, most people who become infected with West Nile virus will have no symptoms at all. About 20% of people infected with West Nile virus may develop mild flu-like symptoms. Less than 1% of those bitten by infected mosquitoes become severely ill. To be tested for West Nile virus, you should contact your health-care provider. If necessary, a blood sample will be taken and sent to a laboratory for testing. Some states and jurisdictions are no longer collecting dead birds because they have sufficiently established that the virus is in an area, and additional testing will not reveal any more information. Shifting resources away from testing of dead birds allows those resources to be devoted elsewhere in surveillance and control.

Q. My neighbor's bug zappers keep me awake all night. He says he got several zappers to protect his family from West Nile virus because they kill mosquitoes. I say they don't work. Who is right?

A. You are right. Studies show that they actually attract mosquitoes into your yard. In addition, most insects killed by bug zappers include moths, beetles and other harmless bugs - not mosquitoes. Mosquito control products are big business. Americans have invested billions of dollars in zappers, repellers, and other products that claim they will keep pesky mosquitoes from biting. In almost every case, the merits of the product are rarely backed with scientific testing. All products should be thoroughly researched before you purchase them. Your best bet would be to use a proven method for keeping your home and property mosquito-free.

Q. Why do some areas stop collecting dead birds?

A. Some states and jurisdictions are no longer collecting dead birds because they have sufficiently established that the virus is in an area, and additional testing will not reveal any more information. Shifting resources away from testing of dead birds allows those resources to be devoted elsewhere in surveillance and control.

West Nile Virus in Denton County FAQ

Information provided by the Denton County Health Department (DCHD)

Q. How long has West Nile virus been in Denton County?

A. Denton County reported its first case of WNV in 2002. 

Q: What is the County’s role during the 2013 West Nile virus season?

A. During and after the 2012 West Nile virus season, DCHD immediately started working on plans for the 2013 season. The goal was to continue with what was done well, and take lessons learned from 2012 to make improvements next year. Denton County’s plans include: 

arrowDCHD developed a Vector Control Plan based on CDC standards. Within the plan are Risk Levels, which indicate actions that will be considered at each particular level.
arrowPlans were put into place to begin testing in unincorporated areas of the county. Mosquito traps were purchased, Environmental Health employees have been trained, and the Health Department arranged to have mosquitoes shipped and tested by the state (DSHS) lab. This testing will begin May 1, 2013. DCHD will also ship mosquitoes from municipality traps, if the municipality has no means to test.
arrowVolunteers from Denton County Retired and Senior Volunteer Program (RSVP) and Medical Reserve Corps (MRC) will be deployed to conduct educational presentations and outreach, both as a prevention measure, and if positive mosquito pools are found in unincorporated areas with a high population density.
arrowDCHD hosted a WNV Partner meeting in February and invited all municipalities in Denton County to discuss information sharing, and improve communication.
arrowNew education materials were developed based on feedback and frequently asked questions from 2012. More emphasis will be placed on “source reduction” in 2013.
arrowDCHD’s West Nile virus website has been redesigned to make it more “user friendly.” Visit the Denton County's website homepage » 

Q: How does DCHD investigate a case of West Nile virus?

A. Case investigations are an everyday job function at the Health Department, and our investigation procedures are dictated by DSHS and the CDC. There are 78 diseases that health care providers are required to report to their county health department, and West Nile virus is one of them. If a case is reported, the Health Department will then investigate, and will contact the patient and/or health care provider if necessary. In some cases, contact with DSHS is necessary for clarification before a case can be ruled out or confirmed as WNV. If a case is confirmed, DCHD contacts the city of residency first as a courtesy. No patient information is ever released, in order to protect the patient’s privacy. The main source of current Denton County case information is on our WNV webpage »

Q: What is my city/town’s role during West Nile virus season?

 A. Each municipality (city or town) in Denton County set up their own mosquito control program. Please contact your municipality to find out what program they have established. The Denton County Health Department is responsible for the unincorporated areas of Denton County.

Mosquito Control FAQ

Information provided by the Centers for Disease Control and Prevention (CDC)

Q. What are "larvicides" and "adulticides"?

A. Larvicides are products used to kill immature mosquitoes before they become adults. 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 burden by limiting the number of new mosquitoes that are produced. An example of a larvicide is Bacillus thuringiensis israelensis (BTI), which comes in products known as mosquito “dunks” or “donuts.” 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 US Environmental Protection Agency. 

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

A. 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: (1) surveillance (monitoring levels of mosquito activity, and where virus transmission is occurring), (2) reduction of mosquito breeding sites, (3) community outreach and public education, and (4) 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. 

Q. How are mosquitoes controlled during outbreaks?

A. Adult mosquito control may be undertaken to combat an outbreak of mosquito-borne disease or a very heavy nuisance infestation of mosquitoes in a community. Pesticides registered for this use are known as adulticides and are applied either by aircraft or on the ground employing truck-mounted sprayers. State and local municipalities commonly use the organophosphate insecticides malathion and naled and the synthetic pyrethroid insecticides permethrin, resmethrin, and sumithrin for adult mosquito control. Check with your municipality to determine the exact adulticides used, if any. Mosquito adulticides are applied as ultra-low volume (ULV) sprays. ULV sprayers dispense very fine aerosol droplets that stay aloft and kill flying mosquitoes on contact. ULV applications involve small quantities of pesticide active ingredient in relation to the size of the area treated, typically less than 3 ounces per acre, which minimizes exposure and risks to people and the environment. Adulticides can be used for public health mosquito control programs without posing unreasonable risks to the general population or to the environment when applied according to the pesticide label. More information on pesticides commonly-used in public health mosquito control programs is available at the EPA's Pesticides website » 

Q. Are pesticides harmful to people?

A. Effect on human health is one of the primary factors considered in 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 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 &"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.&" This article can be viewed online. For more information on pesticides and health, consult the US 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, 1-800-858-7378 or online.

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

A. 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.

Q. How does pesticide spraying affect the environment?

A. 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 the US 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, 1 (800) 858-7378 or online.

Q. What training is required for workers who apply pesticides?

A. Each state has mandated training and experience requirements that must be met before an individual can commercially apply pesticides. 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 U.S. EPA.

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

A. Questions concerning specific pesticides can be directed to the U.S. Environmental Protection Agency, as this agency has responsibility for registration of pesticides. Many issues are addressed on the EPA's Mosquito Control Web site. The National Pesticide Information Center (NPIC) provides pesticide information and questions about the impact of pesticide use on human health. NPIC is cooperatively sponsored by Oregon State University and the U.S. Environmental Protection Agency. NPIC can be reached online or toll-free: 1 (800) 858-7378.

Q. How can I find out what types of pesticides are being used in my area?

A. Your city or town may or may not be using larvicides and adulticides for mosquito control. Check with your municipality to be sure. Another resource to learn more about mosquito control is the American Mosquito Control Association.