West Nile fever is a mosquito-borne illness caused by West Nile virus, a member of the same virus family as dengue and Zika. About 80% of people infected never develop any symptoms at all. Of those who do get sick, most experience a flu-like illness that resolves on its own, but less than 1% develop severe, potentially life-threatening complications involving the brain and spinal cord.
How the Virus Spreads
West Nile virus cycles between Culex species mosquitoes and birds. Infected birds can develop high levels of virus in their bloodstream, and mosquitoes pick it up during a blood meal. When one of those mosquitoes bites a person, the virus can transfer. Humans are considered “dead-end” hosts, meaning our blood doesn’t carry enough virus for a mosquito to pick it up and pass it along further.
The virus first appeared in the United States in 1999 and has since become the most common mosquito-borne illness in the country. In 2024 alone, 1,808 human cases were reported to the CDC. Transmission peaks during summer and early fall, when mosquitoes are most active.
Symptoms of Mild West Nile Fever
About 20% of infected people develop what’s formally called non-neuroinvasive disease, or simply West Nile fever. Symptoms typically include fever, fatigue, headache, muscle pain, joint pain, and sometimes a temporary rash. Vomiting and diarrhea can also occur. The illness feels a lot like the flu, and most people recover fully within days to a few weeks without any specific treatment.
When the Virus Affects the Brain
In less than 1% of infections, the virus crosses into the central nervous system. This is called neuroinvasive disease, and it takes three main forms:
- Meningitis: Inflammation of the membranes surrounding the brain and spinal cord, causing fever, severe headache, sensitivity to light, and neck stiffness.
- Encephalitis: Inflammation of the brain itself, which can cause confusion, altered mental status, tremors, and movement problems resembling Parkinson’s disease. This is the most dangerous form.
- Acute flaccid myelitis: Damage to the spinal cord that can cause sudden limb weakness, paralysis, and in severe cases, respiratory failure. It can develop alongside meningitis or encephalitis.
Severe symptoms like high fever, neck stiffness, disorientation, convulsions, muscle weakness, vision loss, or numbness require immediate medical attention. Of the 2024 hospitalizations reported to the CDC, 1,263 were for neuroinvasive disease.
Who Faces Higher Risk
Anyone bitten by an infected mosquito can contract West Nile virus, but not everyone faces the same odds of severe illness. Older adults, particularly those over 60, are significantly more likely to develop neuroinvasive disease. People with weakened immune systems, including organ transplant recipients and those on immunosuppressive medications, are also at elevated risk. Conditions like diabetes, high blood pressure, and kidney disease have been associated with worse outcomes.
How It’s Diagnosed
Diagnosing West Nile fever relies on a blood test that detects antibodies your immune system produces in response to the virus. These antibodies typically become detectable 3 to 8 days after symptoms begin and can persist for 30 to 90 days, sometimes longer. If blood is drawn too early in the illness, the test may come back negative and need to be repeated.
For suspected neuroinvasive disease, a spinal fluid sample may also be tested. Because West Nile virus belongs to the same family as dengue and Zika, initial antibody tests can sometimes cross-react with those viruses. A more specialized follow-up test can distinguish which virus is actually responsible.
Treatment Options
There are no antiviral medications that treat West Nile virus, and no vaccine is available for humans. Treatment is entirely supportive, meaning it focuses on managing symptoms while your body fights the infection.
For mild cases, that means over-the-counter pain relievers for fever and headache, staying hydrated, and resting. People with severe meningitis symptoms often need medication for pain and nausea, along with IV fluids. Encephalitis patients require close monitoring for dangerous complications like brain swelling, seizures, or breathing problems. Recovery from hospitalization can take weeks to months.
Long-Term Effects After Severe Illness
People who recover from mild West Nile fever generally return to normal without lasting problems. The picture is different for those who had neuroinvasive disease. Between 30% and 60% of survivors experience lingering symptoms, with the most common being depression, memory loss, fatigue, and difficulty with movement.
Encephalitis carries the highest risk of long-term effects. Up to 86% of encephalitis patients still show abnormal neurological exams three years after their infection. People who had meningitis alone tend to recover more completely. Those who experienced paralysis may regain some function, but full recovery is not guaranteed. These lasting effects can include ongoing fatigue, difficulty concentrating, and muscle weakness that persists for years or, in some cases, permanently.
Prevention Through Mosquito Protection
Since there’s no vaccine, avoiding mosquito bites is the only way to prevent West Nile fever. The EPA registers several active ingredients for skin-applied insect repellents, with DEET being the most widely available (found in over 500 products). Picaridin, oil of lemon eucalyptus, and IR3535 are also effective alternatives.
Beyond repellent, practical steps make a real difference. Wear long sleeves and pants during dawn and dusk, when Culex mosquitoes are most active. Eliminate standing water around your home, since even a bottle cap’s worth can serve as a breeding site. Make sure window and door screens are intact. If you live in an area with active West Nile virus transmission, these habits are especially important from late spring through early fall.