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What Is West Nile Virus?

West Nile virus is primarily transmitted to humans via mosquito bites; it also affects birds and mammals like horses.

The virus is the leading cause of mosquito-transmitted disease in the United States,

 and around 80 percent of people infected with the disease do not experience any symptoms.

In about 20 percent of infected people, West Nile virus evolves into a febrile illness (sometimes called West Nile fever), with symptoms including fever, headache, body aches, vomiting, diarrhea, and rash. In rarer cases (about 1 in 150 infected people), West Nile virus can lead to severe and potentially fatal neurological diseases like encephalitis or meningitis. Elderly and immunocompromised people are most at risk for severe disease.

There is no vaccine or medicine to treat West Nile virus in humans. The best way to prevent infection is to protect yourself against mosquito bites. Treatment for symptomatic West Nile virus typically involves hospital care.

The 1 in 5 people who develop symptoms from West Nile virus experience one of two types: febrile illness and, less commonly, severe neurological illness that can take multiple forms.

The symptoms of West Nile virus depend on the form the disease takes when it occurs. Those with febrile illness may experience the following symptoms in addition to fever:

  • Headache
  • Body aches
  • Joint pain
  • Vomiting
  • Diarrhea
  • Rash

Additionally, people with febrile illness may experience fatigue or weakness for an extended period — up to weeks or even months — following recovery.

Those who develop neurological disease, which takes the form of brain inflammation (encephalitis) and inflammation of the membranes surrounding the brain and spinal cord (meningitis), among others — may experience the following symptoms:

  • High fever
  • Headache
  • Stiffness in the neck
  • Stupor
  • Disorientation
  • Convulsions
  • Tremors
  • Muscle weakness
  • Loss of vision
  • Numbness
  • Paralysis
  • Coma

In the vast majority of cases, West Nile virus is caused by bites from mosquitos that picked up the virus by feeding on infected birds.

In very rare instances, the virus is spread through exposure in a lab, blood transfusions, organ transplants, or from mother to infant via the placenta or breastfeeding. It is not spread through coughing, sneezing, or touching infected people or animals, and you can’t get it from eating infected animals.

Anyone can be bitten by a mosquito and infected, but some people have factors that increase their risk of severe illness. These include:

A healthcare provider may diagnose West Nile virus on the basis of a person’s signs, symptoms, and history of possible exposure to infected mosquitos.

 The provider will also perform a blood test and in some cases a lumbar puncture, sometimes called a spinal tap.

There are no vaccines, medicines, or other specific treatments for West Nile virus in humans. In generally healthy people with mild febrile illness, rest, fluids, and over-the-counter medications may be enough to relieve symptoms until the infection runs its course.

Treatment in more severe cases typically involves a hospital stay and supportive therapies, such as:

  • Intravenous fluids
  • Breathing support such as a ventilator
  • Monitoring and prevention of secondary infections, such as pneumonia
  • Nursing care

There are no medical options to prevent infection, but you can protect yourself by taking precautions against mosquito bites. Recommendations include:

  • Insect repellent
  • Long-sleeved, loose-fitting clothing
  • Treat clothing and outdoor gear with permethrin, an insecticide that kills or repels mosquitos
  • Screens and other devices for preventing exposure to mosquitoes in indoor and outdoor spaces

Febrile illness from West Nile virus typically lasts a few weeks, though weakness and fatigue can continue for weeks or months afterward. Symptoms such as paralysis, motor impairments, hearing loss, and other neurological conditions may last permanently in more serious cases. West Nile virus is fatal in about 1 in 1,500 people who experience neurological symptoms.

People with febrile illness may experience fatigue or weakness for weeks or months after initial onset. Possible long-term effects of a neurological infection include:

  • Memory loss
  • Hearing loss
  • Gait disorders or difficulty walking
  • Weakness
  • Abnormal reflexes
  • Depression

There were 1,466 total human cases of West Nile virus illness in the United States in 2024.

 In the past 10 years, the number of cases in the country has varied from as low as 731 in 2020 to as high as 2,911 in 2021. The states with the highest number of reported cases between 1999 and 2023 are California, Colorado, Texas, Nebraska, and Arizona.

  • Four out of five people infected with West Nile virus never experience any symptoms.
  • The best way to prevent infection is to protect yourself against mosquito bites.
  • Most cases of the virus are febrile, with flu-like symptoms.
  • Neurological illness from the West Nile virus is rare but may cause permanent side effects or be fatal in extremely rare cases.

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