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Virus del Nilo Occidental

About West Nile Virus (WNV)


Although mosquito-borne disease is rare in Illinois, mosquitoes can carry at least three encephalitis viruses that can cause human disease. In 2001, West Nile virus (WNV) was detected for the first time in Illinois in birds, horses, and mosquitoes. West Nile virus causes encephalitis primarily in older adults.

Encephalitis is an inflammation of the brain that can be caused by arboviruses (viruses carried by arthropods, such as mosquitoes and ticks) or by other types of viruses. In Illinois, arboviruses are primarily transmitted to humans by the bites of infected mosquitoes. Most individuals who are bitten by an infected mosquito will experience no symptoms of the disease or will have only very mild symptoms. Approximately 1 to 2 percent will develop recognizable symptoms. Some persons may have mild symptoms, such as a fever and headache. Severe infection may cause rapid onset of severe headache, high fever, muscle aches, stiffness in the back of the neck, problems with muscle coordination, disorientation, convulsions and coma. Symptoms usually occur 5 to 15 days after the bite of an infected mosquito. Not all viruses that cause encephalitis are carried by mosquitoes.


Frequently Asked Questions about West Nile Virus:


How can I help protect my family and myself from mosquitoes and the diseases they may carry?

During the summer, mosquitoes can develop in any standing water that lasts more than 7 to 10 days. Consequently, you can begin protecting your family from mosquitoes by reducing the amount of standing water available for mosquito breeding around your home:

  • Dispose of discarded tires, cans, plastic containers, ceramic pots or other unused similar water-holding containers that have accumulated on your property. Do not overlook containers that have become overgrown by vegetation

  • At least once per week, empty standing water from containers on your property, such as tire swings, or bird baths.

  • Fill in tree rot holes and hollow stumps that hold water.

  • Drill holes in the bottom of recycling containers that are left outdoors. Drainage holes drilled in the sides of containers allow sufficient water to collect in which mosquitoes may breed.

  • Clean clogged roof gutters, particularly if the leaves from surrounding trees have a tendency to plug up the drains. Flooded roof gutters are easily overlooked but can produce hundreds of mosquitoes each season.

  • Turn over plastic wading pools when not in use. A wading pool becomes a mosquito producer if it is not used on a regular basis.

  • Turn over wheelbarrows and do not allow water to stagnate in bird baths. Change water in bird baths and wading pools on a weekly basis.

  • Store boats covered or upside down, or remove rainwater weekly.

  • Aerate ornamental pools or stock them with fish. Water gardens are fashionable but become major mosquito producers if they are allowed to stagnate.

  • Clean and chlorinate swimming pools that are not being used. A swimming pool that is left untended by a family that goes on vacation for a month can produce enough mosquitoes to result in neighborhood-wide complaints. Be aware that mosquitoes may breed in the water that collects on swimming pool covers.

  • Keep drains, ditches and culverts free of grass clippings, weeds and trash so water will drain properly.

  • Fill in low areas on your property to eliminate standing water. Ponds or streams where fish are present or the water is disturbed by current or wave action do not produce many mosquitoes.

  • Report possible mosquito breeding sites to your local mosquito control agency if one exists in your community.

Should We Stay Indoors?

It is not necessary to limit outdoor activities unless there is evidence of mosquito-borne disease in your area. However, you can, and should, try to reduce the risk of being bitten by mosquitoes.

  • Minimize time spent outdoors between dusk and dawn when mosquitoes are most active.

  • Be sure door and window screens are tight-fitting and in good repair.

  • Wear shoes, socks, long pants and a long-sleeved shirt when outdoors for long periods of time, or when mosquitoes are most active. Clothing should be light colored and made of tightly woven materials to keep mosquitoes away from the skin.

  • Use mosquito netting when sleeping outdoors or in an unscreened structure and to protect small babies when outdoors.

  • Consider the use of mosquito repellent according to label directions when it is necessary to be outdoors.

  • Generally, repellents with about 25 percent – 35 percent DEET work best for adults; use lower concentrations for children. Do NOT use products containing DEET on infants.

  • Insect light electrocutors (“bug zappers”) or sound devices do little to reduce biting mosquitoes in an area.

  • Spraying your backyard with an insecticide fog or mist is effective only for a short time. Mosquitoes will return when the effect of the spray has ended.

  • Installing bird or bat houses to attract these insect-eating animals has been suggested as a method of mosquito control. However, there is little scientific evidence that this significantly reduces the mosquito population around homes.

Can pets and livestock get WNV infection?

Horses can become infected with WNV if bitten by mosquitoes that carry the virus. There is a published report of West Nile virus isolated from a dog in southern Africa (Botswana) in 1982 West Nile virus has been isolated from several dead cats in 1999 and 2000. A survey of the blood of dogs and cats in the epidemic area showed a low infection rate.

What signs of infection should I look for in domestic animals?

West Nile virus and other mosquito-borne viruses can cause encephalitis in domestic animals. Sick animals may have a fever, weakness, poor muscle coordination, muscle spasms and signs of a neurological disease, such as change in temperament or seizures.

What should I do if I suspect my pet has WNV?

If your animal is sick, contact your veterinarian. The veterinarian will evaluate your animal, provide treatment and forward samples for laboratory testing to rule out other possible diseases. The Illinois Department of Agriculture can help veterinarians determine if WNV is the cause once the illness is reported.

Can you get WNV directly from birds, game or domestic animals?

The risk to humans and domestic animals is from the bite of WNV-infected mosquitoes. Although there is no evidence of human infection from handling infected live or dead animals, the U.S. Centers for Disease Control and Prevention recommends that anyone handling sick or dead animals avoid bare-handed contact. Hunters should use gloves when cleaning game animals and persons disposing of dead birds should use a shovel, gloves or double plastic bags to place carcasses in a garbage can. After disposing of the carcass, thoroughly wash your hands with soap and warm water. Veterinarians should use normal veterinary infection control precautions when caring for a horse suspected to have this or any other infection.

Is there a vaccine for pets and livestock for WNV?

A vaccine is available to protect horses from WNV infection; vaccines for other domestic animals are not available currently.

How is WNV infection in domestic animals treated?

As in people, there are no specific treatments for WNV infection in domestic animals. Treatment is primarily supportive to lessen the severity of the symptoms.

How can I protect pets and livestock from WNV infection?

You can reduce the risk of WNV infection in animals by minimizing their exposure to infected mosquitoes.

Where can I get more information on WNV?

Call your local health department or the Illinois Department of Public Health at the telephone numbers listed below.

You may also want to visit the Illinois Public Health Department’s web site or the U.S. Centers for Disease Control and Prevention’s West Nile virus site.

-Vector Control and Arbovirus Surveillance Program
-Division of Environmental Health
-For technical questions regarding mosquitoes and mosquito control measures,
call 217-782-5830 or visit

-Division of Infectious Diseases
-Office of Health Protection

For information about WNV infections in people, call 217-782-2016 or visit

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