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West Nile Virus

Topic Overview

What is West Nile virus?

West Nile virus causes an infection that is spread by certain kinds of mosquitoesClick here to see an illustration.. Most often, mosquitoes get infected when they bite infected birds. Then the mosquitoes spread the virus when they bite people or other animals, such as horses. West Nile cannot spread from these animals to people or from person to person through casual contact.

West Nile can spread through an organ transplant or a blood transfusion. So all donated blood in the United States is screened to see if the virus is present. Some evidence suggests that West Nile can spread from a mom to her baby during pregnancy, at birth, or through breast milk. But the U.S. Centers for Disease Control and Prevention (CDC) still recommends that women breast-feed, because the risk of spreading the virus to babies is unclear and the benefits of breast-feeding are known.1

Most people who have West Nile have no symptoms. Or the symptoms may be so mild that people may not even realize that they have the virus. In rare cases, West Nile can lead to swelling of the brain (encephalitis), swelling of the spinal cord (myelitis), or swelling of the tissues around the brain and spinal cord (meningitis). But very few people with West Nile will get a severe illness that affects the brain or spinal cord.2

Anyone who is bitten by a mosquito may get West Nile. Most of the time people fully recover from it. But permanent problems such as seizures, memory loss, and brain damage can occur, especially in children and older people. As you get older, you have a higher risk for getting encephalitis and other serious problems from West Nile. In a few cases, West Nile can be fatal.

What are the symptoms?

About 80 out of 100 people who have West Nile have no symptoms and only about 1 out of 150 people infected develops serious illness.2 When symptoms do appear, they begin 3 to 14 days after the mosquito bite. Mild symptoms include:

  • A fever.
  • Headaches, body aches, or pain in your eyes.
  • A rash, usually on the chest, back, and arms.
  • Feeling very tired.
  • Not feeling hungry.
  • Feeling sick to your stomach or throwing up.
  • Swollen glands (lymph nodes), in rare cases.

In mild cases of West Nile, symptoms usually last for 3 to 6 days. If you get a more severe case of West Nile, symptoms can last for weeks or months. Severe cases that involve problems with the brain and spinal cord are rare, but they may cause:

  • Headaches.
  • A high fever.
  • A stiff neck or paralysis.
  • Confusion.
  • Reduced attention to surroundings.
  • Tremors, convulsions, or muscle weakness.
  • A coma.

In rare cases, West Nile virus can cause death.

How is West Nile virus diagnosed?

If your doctor thinks that you may have West Nile, he or she will ask questions to find out when you were bitten by a mosquito and what symptoms you have.

If you have symptoms of encephalitis, your doctor will test your blood for antibodies to the virus. If you have these antibodies in your blood, your doctor will know that you have West Nile. In that case, your doctor may test your blood 2 weeks later to see if the antibodies are increasing.

Your doctor may also do a test called a spinal tap (lumbar puncture) to look for antibodies in the fluid that surrounds your brain and spinal cord.

To help find out if you have encephalitis, you may have an MRI scan so your doctor can see pictures of your brain.

How is it treated?

There is no treatment for West Nile. Your body just has to fight the infection on its own. If you have a mild case, you can recover at home. But if West Nile is severe, you may need to stay in a hospital while you get treatment to help your body fight the illness. You may get IV (intravenous) fluids, help with breathing (using a ventilator), and help preventing other illnesses such as pneumonia. You also may get medicine to help with pain or fever.

How can you prevent infection?

You can contact your local health department for the latest information on the virus in your area. It's also a good idea to take steps to lower your risk of getting a mosquito bite:

  • Use insect repellent when you go outdoors in the late spring, summer, and early fall.
  • Wear long-sleeved shirts and long pants if you know that you will be in areas with lots of mosquitoes or where you know West Nile virus has been found.
  • Do not leave puddles or open containers of water near your house, because mosquitoes breed in standing water.
  • Stay indoors at dawn, dusk, and in the early evening when mosquitoes are the most active.

A West Nile virus vaccine is available for horses only. If you own horses, you may think about getting the vaccine for them. The death rate from West Nile is much higher in horses than in humans.

Researchers are working to create a vaccine that prevents West Nile virus in humans.

Frequently Asked Questions

Learning about West Nile virus:

Being diagnosed:

Getting treatment:


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