West Nile Virus (WNV)

Dallas County Health and Human Services - 2377 N. Stemmons Freeway, Dallas, TX 75207
Telephone: 214-819-2000

West Nile Virus (WNV) is a flavivirus historically found in Africa, West Asia, and the Middle East. It is closely related to St. Louis encephalitis virus found in the United States. The virus can infect humans, birds, mosquitoes, horses and some other animals.


Generally, WNV is spread by the bite of an infected mosquito. Mosquitoes are WNV carriers that become infected when they feed on infected birds. Infected mosquitoes can then spread WNV to humans and other animals when they bite.

WNV is not spread through casual contact such as touching or kissing a person with the virus.

In a very small number of cases, WNV has spread through blood transfusions, organ transplants, breastfeeding and even during pregnancy from mother to baby.


Most infected people will show no symptoms. Symptoms typically develop between 3 to 14 days after a mosquito bite.

Mild Infection

Up to 20% of the people who become infected will display:

  • mild flu-like symptoms, including headache and body aches
  • nausea
  • vomiting
  • swollen lymph glands
  • a skin rash on the chest, stomach and back

Symptoms typically last a few days. This is known as West Nile fever.

Serious Infection

About one in 150 people infected with WNV will develop severe illness, such as West Nile Encephalitis, West Nile Meningitis or West Nile Meningoencephalitis. The symptoms can include:

  • headache as well as neck stiffness
  • confusion
  • high fever

These symptoms may last several weeks or even months. The neurological effects may be permanent.

WNV is known to cause death in rare circumstances.


Diagnosis of WNV requires a special blood test. Anyone who experiences symptoms of severe WNV illness should see a physician as soon as possible.


There is no specific treatment for WNV infection. Patients receive supportive medical care and rehabilitation if needed.


DCHHS advises the public to use the 4Ds to help reduce the chance of being bitten by a mosquito.

  • DEET All Day, Every Day:  Whenever you’re outside, use insect repellents that contain DEET or other EPA approved repellents and follow instructions.
  • DRESS:  Wear long, loose, and light-colored clothing outside.
  • DRAIN: Remove all standing water in and around your home.
  • DUSK & DAWN:  Limit outdoor activities during dusk and dawn hours when mosquitoes are most active.

In addition to the 4Ds, travelers can protect themselves by doing the following:

  • Choose a hotel or lodging with air conditioning or screens on windows or doors.
  • Sleep under a mosquito bed net if you are outside or in a room that is not well-screened.

Downloadable Information

DCHHS WNV Fact Sheet in Various Languages


More Information

For the Media and News Updates


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