What Is West Nile Virus?

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West Nile infection is caused by a mosquito-borne virus. Most people infected with it have either no symptoms or only a mild illness. However, in a small proportion of cases, West Nile virus can cause severe, life-threatening meningitis (inflammation of the spinal cord) or encephalitis (inflammation of the brain). These neurological complications have made the West Nile virus a cause of concern across the globe.

West Nile Virus Prevention Tips
Verywell / Emily Roberts 

West Nile Virus Symptoms

West Nile Fever

Sixty to 80 percent of people infected with the West Nile virus have no signs or symptoms of any disease. Roughly 20 percent will develop what is called West Nile fever.

West Nile fever is a self-limited illness that is pretty much indistinguishable from many other viral infections.

Symptoms often include:

  • Fever
  • Muscle aches
  • Headache
  • Sore throat
  • Nausea and vomiting
  • Fatigue 
  • Skin rash 

These typical viral symptoms usually improve rapidly after a few days—just a “summer cold”—and most people (and their doctors) never realize they had a West Nile virus infection.


In a small number of infected people—thought to be well under 1 percent—a serious neurological infection can occur. People who develop West Nile meningitis or encephalitis may experience:

  • Very high fever
  • Stiff neck
  • Disorientation or confusion
  • Paralysis
  • Seizures
  • Coma 

West Nile meningitis or encephalitis can be fatal, even with aggressive medical care. Many who recover have lingering neurological symptoms for a year or longer, and some may be left with permanent neurological deficits.

The neurological complications from West Nile virus are more likely in older people and in people who have cancer. There is some evidence that hypertension, alcohol abuse, and diabetes may also predispose to serious illness with West Nile virus.


West Nile virus is an RNA virus that is now found all over the world, including Europe, Africa, Asia, Australia, and North and South America. While the virus itself is not new, it was far more localized to Africa and the Middle East until a few decades ago. And scientists first associated it with severe neurological illness only in the 1990s.

The primary hosts for the West Nile virus are birds. Mosquitoes pass the virus from bird to bird, allowing the virus to multiply and spread. When a mosquito carrying the virus “bites” a person, the virus can enter the bloodstream and cause a West Nile virus infection. The virus can also be spread from infected people who donate blood.

In the United States, West Nile virus was first detected in 1999, with a major outbreak of illness in New York City. It has now been identified in every one of the 48 contiguous states. Up to 3000 cases of neurological infections from West Nile virus have been seen annually in the United States in recent years.

In the northern hemisphere, infections with the West Nile virus are seen from late May or early June through September, when mosquitoes are active. The risk of infection tends to peak in late summer.


There are two tests that may be used to formally diagnose West Nile virus infection:

  • Polymerase chain reaction (PCR) test: This is a blood test that can identify the virus itself. However, this has limited usefulness because of the virus's short lifespan in humans. It's possible to have been infected with West Nile and test negative.
  • ELISA testing: This blood test (which is done in two parts) can detect IgM antibodies—proteins your body generates in reaction to the virus, if present.

These tests are usually only performed if a doctor feels it's imperative to formally diagnose West Nile, such as in cases where an individual is seriously ill. Again, most people with the infection experience no or only flu-like symptoms that resolve on their own. While those with mild cases might want to officially know if it's West Nile that is affecting them, a test-confirmed diagnosis won't affect treatment recommendations. The PCR and ELISA tests are also quite costly.


There is no specific therapy for West Nile virus, so treatment is primarily “supportive.” Those with typical West Nile fever (the vast majority of whom never learn they are infected with the West Nile virus) generally treat themselves with usual measures—rest, fluid, and analgesics—and their illnesses resolve in a few days.

West Nile Virus Doctor Discussion Guide

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In people hospitalized with serious illness from West Nile virus, measures are taken to keep the fever down and keep the vital signs as stable as possible. While antiviral medication and intravenous immunoglobulin are often used to treat West Nile infection in hospitalized patients, real evidence is lacking that such measures help with recovery.

Mortality with serious neurological West Nile infection, even with aggressive medical care, has been reported as 2-7% in the United States, mostly in patients who developed neuroinvasive disease as a result. Rates may be even higher in patients with West Nile virus encephalitis.


Because there are no good treatments for this infection, prevention is very important.

Avoiding mosquito-infested areas, clearing living spaces of any standing water where mosquito larvae may develop, and using insect repellant, are important measures. Screening donated blood for West Nile virus has significantly reduced the risk of transmission by blood transfusion.

Vaccines against West Nile virus are being developed. While vaccines for horses have been licensed for use in the United States, no vaccines for human use have yet entered clinical trials.

A Word From Verywell

West Nile virus is a mosquito-borne infection that is very widespread across the United States and the world. While most people infected with West Nile virus have relatively mild illnesses and recover completely, those who develop neurological infections can become seriously ill, have an appreciable risk of death, and can suffer a very prolonged recovery. Because there is no specific treatment for West Nile virus, preventing infection is extremely important.

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Article Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. New York State. Department of Health. West Nile Virus - Frequently Asked Questions

  2. Centers for Disease Control and Prevention. West Nile virus. Symptoms, Diagnosis, & Treatment

  3. Centers for Disease Control and Prevention. West Nile virus. Transmission

  4. World Health Organization. West Nile virus

  5. Centers for Disease Control and Prevention. Treatment & Prevention

  6. Debiasi RL, Tyler KL. West Nile virus meningoencephalitis. Nat Clin Pract Neurol. 2006;2(5):264-75. doi:10.1038/ncpneuro0176

  7. National Institute of Allergy and Infectious Diseases. West Nile Virus Vaccines

Additional Reading
  • Loeb M, Hanna S, Nicolle L, Et Al. Prognosis After West Nile Virus Infection. Ann Intern Med 2008; 149:232.
  • Murray Ko, Garcia Mn, Rahbar Mh, Et Al. Survival Analysis, Long-Term Outcomes, And Percentage Of Recovery Up To 8 Years Post-Infection Among The Houston West Nile Virus Cohort. Plos One 2014; 9:E102953.
  • O'leary Dr, Marfin Aa , Montgomery Sp, Et Al. The Epidemic Of West Nile Virus In The United States, 2002. Vector Borne Zoonotic Dis 2004; 4:61.