West Nile Virus Symptoms, Treatment and Prevention

West Nile virus is an infection spread mainly through bites from infected mosquitoes^1,2. Most infected people never develop signs of illness^2,3. Some people develop fever, while a much smaller group develops severe nerve disease.

This guide explains common symptoms, warning signs, care, and ways to avoid bites. It also explains when you should get medical help.

No licensed human vaccine or specific antiviral drug is available. Care aims to ease symptoms and support the body^3,9. Prevention depends on personal bite protection and local mosquito control.

West Nile virus can cause fever or disease involving the nervous system^3,4. The nervous system includes the brain, spinal cord, and nerves.

Anyone living in or visiting an affected area may become infected. Cases have occurred across the mainland United States. The virus also occurs in Africa, Europe, the Middle East, and parts of Asia³.

Birds are natural hosts for the virus³. This means birds help keep the virus active in nature.

Mosquitoes often become infected after feeding on infected birds^2,3. The virus then moves into the mosquito’s saliva glands³. A later bite can pass the virus to a person or animal³.

People do not get the infection directly from birds. Avoid barehanded contact with dead birds. Local health offices may ask residents to report dead birds for tracking.

In the United States, infections occur during mosquito season from summer through fall. That season often runs from June through October. Diagnosed cases tend to peak in late August or early September.

All mainland states have reported West Nile cases. People who travel to places with virus activity may also face exposure.

West Nile virus does not spread through routine contact, such as touching or kissing. It also does not pass through sexual contact.

Rare spread can happen in the following settings:

The risk from blood transfusions or organ transplants is very low. People should not avoid these lifesaving procedures because of that risk.

About 80% of infected people develop no West Nile virus symptoms^3,4. About 20% develop an illness marked by fever³. Doctors may call this illness West Nile fever.

Less than 1% develop disease involving the brain or spinal cord. This is called neuroinvasive disease. It is the most serious form of the infection.

The incubation period is the time between infection and the first symptoms. It generally lasts from 2 to 14 days.

Symptoms often begin from 2 to 6 days after an infected bite. Onset may take longer in people with weak immune systems.

A mild illness can cause several flu-like symptoms. Possible symptoms include the following:

Most people with mild illness recover fully. Symptoms may last from a few days to several weeks^1,7. Tiredness or weakness can remain for weeks or months.

These signs can look like other viral illnesses⁵. Our influenza guide may help you prepare questions for your doctor.

Neuroinvasive disease affects the central nervous system. This system includes the brain and spinal cord.

Severe forms include meningitis, encephalitis, and acute flaccid myelitis. Meningitis is swelling in the tissues around the brain and spinal cord¹. Encephalitis is swelling within the brain¹.

Acute flaccid myelitis affects the spinal cord and can cause limb weakness. It can also harm the nerves used for movement and breathing.

Get emergency care now if any of these warning signs occur:

About 10% of people with neuroinvasive disease die from the illness. Death risk is higher among older people and those with weak immune systems.

A healthcare professional will ask about symptoms and possible mosquito exposure. The review may include recent blood transfusions or organ transplants.

The clinician will also perform a physical exam¹. Routine blood test results may not clearly show this infection.

Diagnosis uses the illness history, exposure history, exam, and lab tests^5,8. Testing may use blood or cerebrospinal fluid.

Cerebrospinal fluid is the liquid around the brain and spinal cord. A clinician may test this fluid when severe nerve disease is suspected.

Special labs can look for signs of the virus or related antibodies. Antibodies are proteins made by the immune system after an infection.

Doctors may also test for other infections that cause similar symptoms. West Nile meningitis can look like other forms of viral meningitis. This makes lab testing important in serious cases.

Anyone with the infection can develop severe illness. However, the risk rises with age.

The following factors are linked with greater risk:

If symptoms develop during pregnancy or breastfeeding, contact a healthcare professional. Mother-to-baby spread is possible, but the risk is low^1,6.

Most infected mothers have delivered babies without signs of infection or health problems. However, a clinician should still review possible symptoms and exposure.

No specific antiviral medicine is available for West Nile virus treatment^9,10. Antibiotics do not treat the illness because they do not act against viruses^5,8.

Care can include supportive care for severe illness to help reduce the risk of complications^3,9,5. The type of care depends on how serious the illness becomes.

People with mild illness can often manage their symptoms at home. The following supportive steps may help:

Do not use antibiotics for this virus^5,8. A clinician may still prescribe them for a separate bacterial infection.

Ask a clinician before using a medicine if you have health concerns. This includes questions about other drugs or existing conditions.

People with severe illness often need hospital care and close monitoring. Hospital teams may provide several forms of support:

Severe muscle weakness can affect the muscles needed for breathing. This problem may develop fast and require long-term breathing support.

No drug tested for West Nile disease has shown clear benefit so far. Hospital care therefore focuses on symptoms, breathing, fluids, and other needs.

Most people recover fully from mild illness or West Nile meningitis. However, low energy and weakness may last for weeks or months.

Recovery from severe disease may take several weeks or months. Some effects may not go away.

People who survive brain or spinal cord disease may have lasting problems. These may affect movement, thinking, or daily tasks.

Reported effects include memory loss, hearing loss, weakness, and trouble walking^7,10. Some people need rehabilitation or long-term care after leaving the hospital.

Follow-up care can help address ongoing needs. Your care team can explain which services fit your symptoms and daily tasks.

No licensed vaccine or preventive medicine is available for people. West Nile virus prevention therefore focuses on avoiding infected mosquito bites.

Personal protection and community mosquito control both play a role. Using several steps can reduce mosquito exposure.

EPA-registered insect repellents can help prevent mosquito bites^1,6. Follow the label each time you use a repellent^1,6.

Use these steps when you spend time outdoors:

Repellents with EPA-registered ingredients are considered safe during pregnancy and breastfeeding. Always use these products as directed.

Window and door screens help keep mosquitoes from entering the home^1,10. Repair damaged screens and close gaps around them.

Keep unscreened windows and doors closed. Use a mosquito net around beds when good screens are unavailable^3,7.

When sleeping outside, place a mosquito net around the sleeping area. Check that the net covers the full space before going to sleep.

Mosquitoes can breed in standing water around homes and yards^5,10. Check outdoor areas often for collected water.

These common items may need regular care:

Community mosquito programs can also help reduce mosquito numbers. These efforts may include tracking mosquitoes and removing breeding sites³.

Many mosquitoes are active around dawn and dusk^1,6. Limiting outdoor time during these periods may reduce exposure.

If you go outside, wear protective clothes and use repellent. Use the same steps when visiting places with West Nile activity.

Local health offices may share news about mosquito activity and control work. Follow their advice during mosquito season.

Some health offices track reports of dead birds^3,7. Follow local instructions before handling or reporting a dead bird.

If mild symptoms follow possible mosquito exposure, you may contact a healthcare professional. A clinician can decide whether you need testing or follow-up¹⁰.

If symptoms develop during pregnancy or breastfeeding, contact a clinician. People with weak immune systems should also discuss possible symptoms with a healthcare professional.

Knowing West Nile virus symptoms, treatment, and prevention supports safer choices. Most infections cause no illness, while some cause fever and other mild symptoms^2,3.

Less than 1% of infections lead to severe nervous system disease. This disease can cause lasting health problems or death.