Facts About West Nile Virus

How is West Nile Virus Transmitted?

West Nile virus is transmitted to a human by a mosquito that has first bitten an infected bird. A person who is bitten by an infected mosquito may show symptoms from 3 to 15 days after the bite.

Culex mosquitoes, which can carry the West Nile virus, breed in places like ditches, open septic systems, discarded tires, unused wading pools, and other assorted containers, particularly if they are in the shade. In urban areas, many sewer catch basins can be found holding not only water, but also thousands of Culex larvae and pupae.

How Can I Protect Myself?

  • When possible, avoid places and times when mosquitoes bite.
  • Use an insect repellant containing DEET (N,N-diethyl-m-toluamide).
  • Wear shoes, socks, long pants and a long-sleeved shirt when outdoors for long periods of time, or from dusk to dawn, when mosquitoes are most active. Clothing should be light colored and made of tightly woven materials to keep mosquitoes away from the skin.
  • Make sure all windows and doors have screens, and that all screens are in good repair.
  • Use mosquito netting when sleeping outdoors or in an unscreened structure.

Mosquitoes breed in standing water. Even a small bucket that has stagnant water in it for seven days can become home to up to 1,000 mosquitoes. 

You can also protect your family and your community from biting mosquitoes by:

  • Eliminating areas of standing water available for mosquito breeding in or near your property.
  • Repairing failed septic systems.
  • Drilling holes in the bottom of recycling containers that are left out of doors. Drainage holes that are located on the container sides collect enough water for mosquitoes to breed in.
  • Keeping grass cut short and shrubbery trimmed.
  • Disposing of old tires, tin cans, plastic containers, ceramic pots or other unused containers that can hold water. Do not overlook containers that have become overgrown by aquatic vegetation. 
  • Cleaning clogged roof gutters, particularly if leaves tend to plug up the drains. Roof gutters are easily overlooked but can produce millions of mosquitoes each season.
  • Aerating ornamental pools, or stocking them with predatory fish. Water gardens are fashionable but become major mosquito producers if they are allowed to stagnate. 
  • Turning over plastic wading pools when not in use. A wading pool becomes a mosquito producer if it is not used on a regular basis.
  • Turning over wheelbarrows and not allowing water to stagnate in birdbaths. Both provide breeding habitat for domestic mosquitoes.
  • Cleaning and chlorinating 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 even breed in the water that collects on swimming pool covers.
  • Using landscaping to eliminate standing water that collects on your property. Mosquitoes will develop in any puddle that lasts for more than four days.

Electromagnetic and ultrasound devices and Vitamin B are not effective in preventing mosquito bites.

How Serious is West Nile Disease?

Most people who get infected with West Nile virus will have either no symptoms or mild symptoms. A few individuals will have a more severe form of the disease, encephalitis (inflammation of the brain) or meningitis (inflammation of the tissues that cover the brain and spinal cord). Health officials say that individuals over age 50 are at higher risk for serious illness.

There is no specific treatment for West Nile virus, and no vaccine is available for humans. In severe cases, intensive supportive therapy including intravenous fluids, airway management, respiratory support, prevention of secondary infections (pneumonia, urinary tract, etc.) and good nursing care are indicated.


You should see a doctor immediately if you develop symptoms such as the following:

  • high fever,
  • severe headache,
  • neck stiffness,
  • muscle weakness or paralysis,
  • nausea, vomiting,
  • sore joints, or
  • confusion.

In humans, symptoms generally occur three to 15 days following the bite of an infected mosquito.

Mild cases of West Nile encephalitis may cause a slight fever, rash, swollen lymph nodes, conjunctivitis (irritation of the eye), or headache. Patients with mild symptoms are likely to recover completely and do not require any specific medication or laboratory testing.

More severe infections are marked by rapid onset of a high fever with head and body aches, stiff neck, muscle weakness, disorientation, tremors, convulsions and, in the most severe cases, coma or paralysis. In some individuals, West Nile Virus can cause permanent neurological damage or death.

Who is at Risk?

Most people have very mild disease. Although the virus has been reported in people ranging in age from nine months to 99 years old, severe disease has been most often present in individuals over 50 years old or those with weakened immune systems.

The case fatality rate ranges from 3 percent to 15 percent. A 1999 survey of residents in the most affected area of New York City showed that about 3 percent of residents had been infected with West Nile Virus, but either had no illness or only a mild illness.

West Nile Virus in Animals

Wild and domestic animals, such as horses, can also be infected with West Nile Virus. Sick animals may have fever, weakness, poor muscle coordination, muscle spasms and neurological disorders such as seizures or a change in temperament. If your animal is sick, contact a veterinarian. The veterinarian will evaluate the animal, provide treatment and collect samples for laboratory analysis to determine the cause.

Although there is no evidence of human infection from handling animals infected with West Nile Virus, the federal Centers for Disease Control and Prevention recommends that anyone handling sick or dead animals avoid bare-hand 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, the hands must be thoroughly washed with soap and warm water. Veterinarians should use normal veterinary infection control precautions when caring for a horse suspected to have West Niles Virus. Proper cooking kills West Nile Virus, so there is no danger from eating wild game that may be infected.