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West Nile Virus

Last Updated August 2023 | This article was created by editorial staff and reviewed by Leisa Bailey, MD

What is West Nile virus?

West Nile virus is an infectious disease that can cause flu-like symptoms. It’s usually spread by infected mosquitos. It first appeared in the United States in 1999. Most people infected with the virus have mild symptoms or no symptoms. Rarely, the virus can enter the brain and cause life-threatening complications.

You may have a higher risk for infection by the virus if you live in an area where it has been found in humans, birds, horses, or mosquitoes. However, even in areas where the virus has been reported, it’s very unlikely that a person will get sick from a mosquito bite.

Symptoms of West Nile virus

Most people infected with West Nile virus have no symptoms. However, about 1 in 5 people may have mild, flu-like symptoms, which include:

  • Skin rash
  • Fever
  • Headache
  • Nausea
  • Vomiting
  • Diarrhea
  • Loss of appetite
  • Swollen lymph nodes (lymph glands)
  • An achy feeling in the back and muscles

Symptoms usually occur 3 to 14 days after a person is bitten by an infected mosquito. Symptoms can last from a few days up to several weeks. They usually go away on their own. Contact your doctor if you have any of these symptoms and have recently been bitten by a mosquito.

About 1 in 150 people who are infected with West Nile virus develop a more severe illness. This happens when the virus enters the brain. There it can cause encephalitis. This is inflammation of the brain. It can also cause meningitis. This is inflammation of the membranes that surround the brain and the spinal cord. Symptoms of these illnesses include:

  • A sudden high fever (above 102°F)
  • Severe headache
  • Stiff neck
  • Feeling disorientated or confused
  • Tremors or muscle jerks
  • Seizures
  • Coma
  • Weakness or partial paralysis

These illnesses can be life-threatening. If you have any of these symptoms, get medical help right away.

What causes the virus?

West Nile virus is most often spread by mosquitoes. Mosquitoes become infected by biting birds that carry the virus. People can get the virus when an infected mosquito bites them. This happens most often in the warm-weather months of spring, summer, and early fall. You can’t get the virus from another person or from your pet.

In a small number of cases, people have gotten the virus through blood transfusions and organ transplants. However, the risk of getting the virus in these ways is very low.

A few cases have also been reported of the virus being passed from a pregnant or breastfeeding woman to her baby. These cases are extremely rare.

How is West Nile virus diagnosed?

Your doctor will do a physical examination and ask about your symptoms. They will want to know if you’ve been bitten by a mosquito recently. They may order blood tests to confirm if you have West Nile virus. Sometimes your doctor may order a spinal tap to check for the virus. This involves collecting a sample of the fluid that surrounds your spinal cord and brain. If you’re having severe symptoms, your doctor may order a CT scan or an MRI to look for inflammation in the brain.

If you’re infected with West Nile virus, you may have a higher risk of developing severe illness if you:

  • Are more than 60 years of age
  • Have certain medical conditions, such as cancer, diabetes, hypertension, or kidney disease
  • Are a transplant recipient

Less than 1% of the people who do get infected with the West Nile virus develop severe symptoms or complications. Almost all of those people fully recover. Of the small number of people who do get severely ill from the virus, about 10% will die.

Can the virus be prevented or avoided?

There is no vaccine to prevent West Nile virus in humans. The best way to prevent infection is to avoid getting bitten by a mosquito.

The following are a few things you can do:

  • Get rid of standing water. Mosquitos breed in standing water. Dump water from birdbaths, flowerpots, wading pools, old tires, or other places where it can collect. Garden supply stores carry products to keep mosquitoes from breeding in small ponds.
  • Repair screens. Fix any tears in window, porch, and patio screens to keep mosquitoes out of your home.
  • Stay indoors. Mosquitoes are most active during dawn, dusk, and early evening hours. Try to stay indoors at these times.
  • Dress appropriately. If you go outdoors when mosquitos are active, wear shoes and socks, long pants, and a long-sleeved shirt. Loose-fitting, light-colored clothing is best.
  • Use insect repellant with DEET. If you’re going to be outdoors, use an insect repellent that contains 20% to 30% DEET. Apply it according to the directions on the label. Talk to your doctor before you use insect repellent on your child.

West Nile virus treatment

There is no specific treatment for West Nile virus infection. People who experience mild symptoms usually get better after a few days. You can take over-the-counter pain relievers to help reduce fever and to help with any pain you may be having.

People who have severe illness may be hospitalized and given intravenous (IV) fluids. They may need to be on a machine called a ventilator to help them breathe. Their doctor also will try to keep them from getting other infections, such as pneumonia.

Living with West Nile Virus

West Nile virus usually only causes mild symptoms, if any. Most people recover in a few days. Some symptoms may last a few weeks, such as weakness or fatigue. But they will go away on their own.

Most people who develop severe illness from the virus recover. But your recovery could take weeks or months. It’s possible that some of the effects the virus has on your central nervous system might never go away. You could have long-term illness, brain damage, or permanent disability. But remember that less than 1% of people who get sick from West Nile virus develop severe illness.

Questions for Your Doctor

  • Is West Nile virus common in this area?
  • Am I at risk of contracting the virus?
  • What can I do to protect myself from the virus?
  • What treatment is best for me?
  • Will cold or flu medicines help?
  • Can my child get the virus from me?
  • What kind of insect repellent should I use?
  • If I start feeling worse, when should I seek medical help?
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