West Nile Virus

Overview

What is West Nile virus?

West Nile virus is an infectious disease caused by microscopic germs (called a virus) that can make you sick. Mosquitoes infected with the West Nile virus can give it to people or animals, such as horses, when they bite the skin.

In most people, West Nile virus causes minor or no symptoms. In rare instances, West Nile virus can cause a dangerous neurological infection (an infection in your nerves and brain). A neurological infection can pose a serious threat to your health.

Where did West Nile virus come from?

In 1937, scientists first identified West Nile virus in Africa, in the West Nile region of Uganda. Today, it can be found on many continents and throughout most states in the U.S., except Hawaii and Alaska.

How common is West Nile virus?

Lots of people get bitten by mosquitoes. Few ever get sick from West Nile virus. Only one in five people who contract West Nile virus have any symptoms. Most people who do have symptoms experience minor aches and pains that mirror influenza (the flu). In less than 1% of cases, West Nile virus causes a sometimes life-threatening infection that can make people very sick.

Symptoms and Causes

How do you get West Nile virus?

You are most likely to get West Nile virus from a mosquito bite, after being bitten by a mosquito with the virus.

Mosquitoes usually contract West Nile virus by biting an infected bird. When an infected mosquito bites you (or an animal), they transmit the virus through their saliva.

In a very few cases, West Nile virus has spread:

Is West Nile virus contagious?

Mosquitoes spread West Nile virus. You can’t get West Nile virus from being near someone who has it.

What does West Nile virus do?

In most cases, West Nile virus causes few or no symptoms. One in five people gets mild, flulike symptoms that typically go away without treatment.

In one out of 150 cases, West Nile virus is a more serious problem. It can cause a dangerous infection that affects the brain and nervous system. This infection can cause lasting or permanent side effects, such as muscle weakness. Some people die. Seek prompt medical care for any West Nile virus symptoms you have that don’t go away or get worse.

What are the symptoms of West Nile virus?

Most people who get West Nile virus don’t get sick. Some have mild symptoms that tend to go away on their own. Doctors sometimes call this West Nile fever. It often feels like the flu.

Minor symptoms of West Nile virus include:

Less than 1% of people who get West Nile virus experience more severe symptoms that affect the central nervous system. West Nile virus can cause encephalitis or meningitis (dangerous inflammation in the brain or spinal cord). This inflammation can be life-threatening if not treated promptly. It’s important to call your doctor if you have any severe symptoms of West Nile virus, such as:

  • Intense, very painful headache.
  • High fever (above 103° F or 39.5° C).
  • Stiff neck.
  • Confusion (problems connecting routine thoughts).
  • Muscle weakness or movements you can’t control (tremors or convulsions).
  • Coma.
  • Paralysis.

Can West Nile virus kill you?

It’s not common, but West Nile virus can be fatal in certain circumstances. One in 150 people who get West Nile virus experiences more severe symptoms. Of that group, an estimated 10% of cases lead to death. Call your healthcare provider if you’re concerned about any symptoms that may be due to West Nile virus.

When do West Nile virus symptoms start?

Most people with West Nile virus never feel sick from it. If you have symptoms, you will probably start to feel sick between three days and two weeks after getting infected.

When is West Nile viral infection more likely to occur?

You’re more likely to get West Nile virus when mosquitoes are most active. Mosquitoes like warmth. In the U.S., mosquitoes are most active during the summer months (between June and September).

You may notice more mosquitoes on the hottest summer days or at certain times of the day, such as dawn and dusk.

Diagnosis and Tests

How is West Nile virus diagnosed?

A simple blood test can confirm a West Nile virus diagnosis. This test looks for West Nile virus antibodies (proteins that show your body is fighting the virus) in your blood.

If your symptoms are severe, your healthcare provider may recommend a spinal tap. During this specialized procedure, your provider uses a needle to carefully remove a small sample of fluid from your spine. This test checks for signs of serious infection.

Management and Treatment

How is West Nile virus treated?

No medication or therapy can rid your body of West Nile virus. Still, most people get better on their own. Over-the-counter medicines, such as acetaminophen (Tylenol®) or ibuprofen (Advil® or Motrin®), may relieve minor aches or discomfort.

For the most serious cases of West Nile virus, you’ll receive hospital care. Therapies — such as intravenous fluids and breathing support — focus on supporting your body while it fights the virus.

Prevention

Who is at risk for West Nile virus?

Since West Nile virus usually spreads through mosquito bites, anyone who spends time outside is at risk. Still, most mosquitoes don’t have West Nile virus.

The more mosquitoes you are near, the higher your risk of getting West Nile virus. You can reduce your risk by not going outside at certain times or taking extra mosquito precautions.

Mosquitoes tend to be most active:

  • During warm weather, between June and September in the U.S.
  • At dusk and dawn, when mosquitoes tend to be out in high numbers.
  • Near standing or slow-moving water, where mosquitoes like to lay eggs.

Your body’s natural defenses against infection can usually fight off West Nile virus with little issue. People 60 and older may be more likely to have more serious West Nile virus symptoms. Having a weakened immune system also puts you more at risk. Some people have a lowered immune system due to an autoimmune disease or medical conditions such as cancer.

In rare cases, West Nile virus has spread through a blood transfusion or organ transplant. Hospitals now screen blood and organ donors for West Nile virus to minimize this risk.

Can West Nile virus be prevented?

No vaccine can prevent West Nile virus. The best way to reduce your risk is to protect yourself against mosquito bites.

You can:

  • Avoid high-risk times: Stay inside or be extra careful when mosquitoes are most active, especially at dawn (early morning) and dusk (when the sun sets).
  • Use insect repellent: Spray an effective insect repellent on exposed skin or clothes before going outdoors. Insect repellent ingredients proven safe and effective include DEET, picardin, IR3535, oil of lemon eucalyptus, para-menthane-diol and 2-undecanone. (Keep insect repellent away from children. Never put repellent on a child’s hands or near their mouth or eyes.)
  • Cover skin: Wear lightweight, light-colored clothes, such as pants or a long-sleeved shirt, to cover more of your body when you’re outdoors.
  • Drain standing water: Regularly empty and clean any areas that may collect water, such as a birdbath or clogged rain gutters, where mosquitoes could breed.
  • Keep mosquitoes outside: Keep doors and windows screened or closed to keep mosquitoes from entering your home.
  • Don’t touch dead birds: It’s unlikely you could contract West Nile from a dead bird. Still, alert your state health department to any dead or dying birds. Wear gloves if you need to have any contact with a dead animal.

Outlook / Prognosis

What is the prognosis (outlook) for people with West Nile virus?

Most people with West Nile virus either don’t get sick or fully recover from mild, flulike symptoms. After a more severe case of West Nile virus, some symptoms may linger for months. West Nile virus can sometimes cause long-lasting effects on the body’s nervous system, leading to fatigue (being overly tired for no known reason) and muscle weakness.

Living With

When should I call the doctor?

Call your healthcare provider if you are concerned about symptoms that may be due to West Nile virus. In rare cases, West Nile virus can cause a dangerous brain or spinal cord infection.

Call your provider now if you experience:

  • Stiff neck.
  • Intense headache.
  • High fever.
  • Muscles giving out (weakening) suddenly.

A note from Cleveland Clinic

Mosquitoes commonly spread West Nile virus throughout most U.S. states and many parts of the world. Most mosquito bites won’t cause a West Nile virus infection. If you do get West Nile virus, you probably won’t get sick. Most people who do get sick recover without seeing a doctor. In rare cases, West Nile virus can lead to a dangerous infection that can pose a serious threat to your health. If you think you have West Nile virus and are concerned about your symptoms, talk to your healthcare provider today. Taking steps to protect yourself against mosquito bites when spending time outdoors is the best way to avoid getting West Nile virus.

Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy

Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy

Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy