West Nile virus

Learn more about the symptoms, treatment and prevention of this mosquito-borne illness.

A mosquito-transmitted virus causes most cases of West Nile infection. Most people infected with West Nile virus either don't develop signs or symptoms or have only minor ones, such as a fever and mild headache. However, some people develop a life-threatening illness that includes inflammation of the spinal cord or brain.

Mild signs and symptoms of a West Nile virus infection generally go away on their own. But severe signs and symptoms — such as a severe headache, fever, disorientation or sudden weakness — need immediate attention.

Exposure to mosquitoes where West Nile virus exists increases your risk of getting infected. You can lower your risk by protecting yourself from mosquitoes by using mosquito repellent and wearing clothing that covers your skin.

Most people infected with the West Nile virus have no signs or symptoms.

Mild infection signs and symptoms

About 20% of people develop a mild infection called West Nile fever. Common signs and symptoms include:

• Fever
• Headache
• Body aches
• Vomiting
• Diarrhea
• Fatigue
• Skin rash

Serious infection signs and symptoms

In less than 1% of infected people, the virus causes a serious nervous system (neurological) infection. This may include inflammation of the brain (encephalitis) or of the membranes surrounding the brain and spinal cord (meningitis).

Signs and symptoms of neurological infections include:

• High fever
• Severe headache
• Stiff neck
• Disorientation or confusion
• Coma
• Tremors or muscle jerking
• Seizures
• Partial paralysis or muscle weakness
• Vision loss
• Numbness

Signs and symptoms of West Nile fever usually last a few days. But signs and symptoms of encephalitis or meningitis can linger for weeks or months. Certain neurological effects, such as muscle weakness, can be permanent.

When to see a doctor

Mild symptoms of West Nile fever usually resolve on their own. Seek medical attention right away if you have signs or symptoms of serious infection, such as severe headaches, a stiff neck, disorientation or confusion. A serious infection generally needs hospitalization.

West Nile virus generally spreads to humans and animals from bites of infected mosquitoes. Mosquitoes get infected and carry the virus after biting infected birds. You can't get infected from casual contact with an infected person or animal.

Most West Nile virus infections happen during warm weather, when mosquitoes are active. The incubation period — the period between when you're bitten by an infected mosquito and the appearance of signs and symptoms of the illness — generally ranges from four to 10 days.

West Nile virus has appeared in Africa, Asia, Europe and the Middle East. It appeared in the United States in the summer of 1999, and since then has been reported in every state — except Hawaii and Alaska — as well as in Canada.

Other possible routes of transmission

In a few cases, West Nile virus might have spread through other routes, including organ transplants and blood transfusions. However, blood donors are screened for the virus, greatly reducing the risk of infection from blood transfusions.

There also have been reports of some transmission of the virus from mother to child during pregnancy or breastfeeding or exposure to the virus in a lab, but these are rare.

Most cases of West Nile virus in the United States occur June through September. Cases have been reported in all 48 lower states.

Risk of serious infection

Even if you're infected, your risk of developing a serious West Nile virus-related illness is very small. Less than 1% of people who are infected become severely ill. And most people who do become sick recover fully. You're more likely to develop a severe or fatal infection based on:

• Age. Being older puts you at higher risk.
• Certain medical conditions. Certain diseases, such as cancer, diabetes, high blood pressure (hypertension) and kidney disease, increase your risk. So does receiving an organ transplant.

Your best bet for preventing West Nile virus and other mosquito-borne illnesses is to avoid exposure to mosquitoes and remove standing water, where mosquitoes breed.

• Unclog roof gutters.
• Empty unused swimming pools or empty standing water on pool covers.
• Change water in birdbaths and pet bowls regularly.
• Remove old tires or unused containers that might hold water and serve as a breeding place for mosquitoes.
• Install or repair screens on windows and doors.

To reduce your exposure to mosquitoes:

• Avoid unnecessary outdoor activity when mosquitoes are most common, such as at dawn, dusk and early evening.
• Wear long-sleeved shirts and long pants when outdoors.
• Apply mosquito repellent containing an Environmental Protection Agency-registered insect repellent to your skin and clothing. Choose the concentration based on the hours of protection you need. The higher the percentage (concentration) of the active ingredient, the longer the repellent will work. Follow the directions on the package, paying special attention to recommendations for use on children.
• When outside, cover your infant's stroller or playpen with mosquito netting.

Besides performing a physical exam, your doctor can confirm the presence of West Nile virus or a West Nile-related illness, such as meningitis or encephalitis, by performing one of the following tests:

• Lab tests. If you're infected, a blood test may show a rising level of antibodies to the West Nile virus. Antibodies are immune system proteins that attack foreign substances, such as viruses. A blood test may not show antibodies at first; another test may need to be done a few weeks later to show the rising level of antibodies.
• Spinal tap (lumbar puncture). The most common way to diagnose meningitis is to analyze the cerebrospinal fluid surrounding your brain and spinal cord. A needle inserted between the lower vertebrae of your spine is used to remove a sample of fluid for analysis in a lab. The fluid sample may show an elevated white blood cell count — a signal that your immune system is fighting an infection — and antibodies to the West Nile virus. If the sample doesn't show antibodies, another test may be done a few weeks later.
• Brain tests. In some cases, doctors may order electroencephalography (EEG) — a procedure that measures your brain's activity — or an MRI scan to help detect brain inflammation.

Most people recover from West Nile virus without treatment. Most people who are severely ill need supportive therapy in a hospital with intravenous fluids and pain medication.

For mild cases, over-the-counter pain relievers can help ease mild headaches and muscle aches. Use caution when giving aspirin to children or teenagers. Children and teenagers recovering from chickenpox or flu-like symptoms should never take aspirin. This is because aspirin has been linked to Reye's syndrome, a rare but potentially life-threatening condition, in such children.

Interferon therapy

Scientists are investigating interferon therapy — a type of immune cell therapy — as a treatment for encephalitis caused by West Nile virus. Some research shows that people who receive interferon recover better than those who don't receive the drug, but further study is needed.

If you have signs and symptoms of infection of the brain or spinal cord — high fever, severe headache, stiff neck, confusion or sudden muscle weakness — see your doctor right away or go to an urgent care center.

Here's some information to help you get ready for your appointment.

What you can do

Take with you a list of the following:

• Your symptoms, including any that seem unrelated to why you're seeing a doctor
• Key personal information, including recent activities or travel to an area where West Nile virus is prevalent
• All medications, vitamins or other supplements you take, including the doses
• Questions to ask the doctor

Take a family member or friend along, if possible, to help you remember the information you're given.

For West Nile virus, some basic questions to ask your doctor include:

• What's likely causing my symptoms?
• What are other possible causes for my symptoms?
• What tests do I need?
• Is my condition likely temporary or chronic?
• What's the best course of action?
• What are the alternatives to the primary approach you're suggesting?
• I have other health conditions. How can I best manage them together?
• Are there restrictions I need to follow?
• Should I see a specialist?
• Are there brochures or other printed material I can have? What websites do you recommend?

Don't hesitate to ask other questions.

What to expect from your doctor

Your doctor is likely to ask you several questions, such as:

• When did your symptoms begin?
• Have your symptoms been continuous or occasional?
• How severe are your symptoms?
• What, if anything, seems to improve your symptoms?
• What, if anything, appears to worsen your symptoms?

Last Updated:

December 24th, 2020