West Nile Virus Infection and Symptoms

Adapted from the Centers for Disease Control and Prevention

Aedes aegypti female mosquito sucking blood from a man's arm
Close-up of an Aedes aegypti female mosquito sucking blood from a man's arm. Only the female mosquitos bite. James Gathany, CDC, Atlanta, GA

Experts believe West Nile virus (WNV) is established as a seasonal epidemic in North America that flares up in the summer and continues into the fall.

Symptoms of West Nile Virus

Approximately 80% of people (about 4 out of 5) who are infected with West Nile Virus will not show any symptoms at all. People typically develop symptoms between 3 and 14 days after they are bitten by the infected mosquito.

Up to 20% of the people who become infected have mild to moderate symptoms. Symptoms can last for as short as a few days, though even healthy people have become sick for several weeks:

  • fever
  • headache
  • body aches
  • nausea & vomiting
  • sometimes swollen lymph glands or a skin rash on the chest, stomach, and back

About one in 150 people infected with West Nile Virus will develop severe illness. These symptoms may last several weeks, and neurological effects may be permanent. These severe symptoms can include:

  • high fever
  • headache
  • neck stiffness
  • stupor
  • disorientation
  • coma
  • tremors
  • convulsions (seizures)
  • muscle weakness
  • vision loss
  • numbness and paralysis

People over the age of 50 are more likely to develop serious symptoms of West Nile Virus if they do get sick and should take special care to avoid mosquito bites.

Spreading West Nile Virus

Infected Mosquitoes. Most often, West Nile Virus is spread by the bite of an infected mosquito.

Mosquitoes become infected when they feed on infected birds. Infected mosquitoes can then spread West Nile Virus to humans and other animals when they bite. Being outside means you're at risk. The more time you're outdoors, the more time you could be bitten by an infected mosquito. Pay attention to avoiding mosquito bites if you spend a lot of time outside, either working or playing.​​

Transfusions, Transplants, and Mother-to-Child. In a very small number of cases, West Nile Virus also has been spread through blood transfusions, organ transplants, breastfeeding and even during pregnancy from mother to baby. Risk through medical procedures is very low. All donated blood is checked for West Nile Virus before being used. The risk of getting West Nile Virus through blood transfusions and organ transplants is very small, and should not prevent people who need surgery from having it. If you have concerns, talk to your doctor.

Pregnancy and nursing do NOT increase risk of becoming infected with WNV. The risk that West Nile Virus may present to a fetus or an infant infected through breastmilk is still being evaluated. Talk with your care provider if you have concerns. West Nile Virus is NOT spread through casual contact such as touching or kissing a person with the virus.

Preventing West Nile Virus

The easiest and best way to avoid West Nile Virus is to prevent mosquito bites.

  • When you are outdoors, use insect repellent containing an EPA-registered active ingredient. Follow the directions on the package.
  • Many mosquitoes are most active at dusk and dawn. Be sure to use insect repellent and wear long sleeves and pants at these times or consider staying indoors during these hours.
  • Make sure you have good screens on your windows and doors to keep mosquitoes out.
  • Get rid of mosquito breeding sites by emptying standing water from flower pots, buckets, and barrels. Change the water in pet dishes and replace the water in bird baths weekly. Drill holes in tire swings so water drains out. Keep children's wading pools empty and on their sides when they aren't being used.

Treating West Nile Virus Infection

There is no specific treatment for West Nile Virus infection. In cases with milder symptoms, people experience symptoms such as fever and aches that pass on their own, although even healthy people have become sick for several weeks.

In more severe cases, people usually need to go to the hospital where they can receive supportive treatment including intravenous fluids, help with breathing and nursing care.

Milder West Nile Virus illness improves on its own, and people do not necessarily need to seek medical attention for this infection though they may choose to do so. If you develop symptoms of severe West Nile Virus illness, such as unusually severe headaches or confusion, seek medical attention immediately. Severe West Nile Virus illness usually requires hospitalization. Pregnant women and nursing mothers are encouraged to talk to their doctor if they develop symptoms that could be WNV. Additional Information on West Nile Virus If you find a dead bird: Don't handle the body with your bare hands. Contact your local health department for instructions on reporting and disposing of the body. They may tell you to dispose of the bird after they log your report.

For more information call the CDC public response hotline at (888) 246-2675 (English), (888) 246-2857 (Español), or (866) 874-2646 (TTY)

Adapted from the Centers for Disease Control and Prevention (CDC) Fact Sheet on West Nile Virus

Continue Reading