West Nile Virus: Overview and More

West Nile infection is caused by a virus transmitted by mosquitoes. Most people infected with this virus have no symptoms or only mild illness. However, in a small number of cases, West Nile virus can cause severe and life-threatening meningitis (inflammation of the spinal cord) or encephalitis (inflammation of the brain). These neurological complications have made the West Nile virus a worldwide concern.

Get Medical Information / Emily Roberts

West Nile virus symptoms

An estimated 60-80% of people infected with West Nile virus show no signs or symptoms of any illness. About 20% will develop West Nile fever.

Symptoms of West Nile fever can include:

  • Hot
  • Muscle pain
  • Headache
  • Throat pain
  • Nausea and vomiting
  • Fatigue
  • Acne

These typical viral symptoms usually resolve within a few days and are generally considered just a "summer cold." Most people and their doctors never realize they have a West Nile virus infection.

West Nile fever is a self-limited disease that is virtually indistinguishable from many other viral infections .

Meningitis / encephalitis

A small number of those infected, estimated to be less than 1%, can develop a serious neurological infection.

People who develop meningitis or West Nile encephalitis may experience:

  • High fever
  • Stiff neck muscles
  • Disorientation or confusion
  • Paralysis
  • Seizures
  • Eat

West Nile meningitis or encephalitis can be fatal even with comprehensive medical care. Many convalescents have neurological symptoms for a year or more, and some may have permanent neurological deficits.

Neurological complications from West Nile virus are more likely in older people with cancer and in people who have had organ transplants from a West Nile virus-infected donor. There is some evidence that high blood pressure, alcohol abuse, and diabetes can also predispose to serious West Nile virus disease.


West Nile virus is an RNA virus found throughout the world, including Europe, Africa, Asia, Australia, and the Americas. While the virus itself is not new, it was much more localized in Africa and the Middle East until a few decades ago. And scientists first linked this to a serious neurological disease only in the 1990s.

Birds are the main hosts of the West Nile virus. Mosquitoes transmit the virus from one bird to another, allowing the virus to multiply and spread. When a mosquito carrying the virus bites a person, the virus can enter the bloodstream and cause an infection.

The virus can also be transmitted from infected people who donate blood.

In the United States, West Nile virus was first detected in 1999 when a major outbreak occurred in New York City. It has now been identified in each of the 48 neighboring states. In recent years, up to 3,000 cases of West Nile virus neurological infections have been reported annually in the United States.

In the Northern Hemisphere, West Nile virus infection occurs from late May or early June through September, when mosquitoes are active. The risk of infection usually peaks in late summer.


There are two tests that can be used to detect West Nile viral infection:

  • Polymerase chain reaction (PCR) test: This is a blood test that can identify the virus itself. However, this is of limited utility due to the short lifespan of the virus in humans. It is possible to become infected with West Nile and test negative.
  • ELISA test: This blood test (performed in two parts) can detect IgM antibodies, proteins that your body makes in response to a virus. This test can detect evidence of immunity to the virus even if the virus is no longer present in your body.

PCR and ELISA tests are expensive. These tests are usually only done if the doctor deems it necessary to formally diagnose West Nile infection, for example, in cases where a person is seriously ill.

Most people with the infection have no or only mild flu symptoms that go away on their own. While people with mild cases may want to formally find out if West Nile is affecting them, a test-confirmed diagnosis will not affect treatment recommendations.

Watch out

There is no specific therapy for West Nile virus, so the main goal of treatment is to reduce symptoms. Those with typical West Nile fever (the vast majority of them never know they are infected with West Nile virus) are generally treated with the usual means of comfort (rest, drinks, and pain relievers) and their illness clears within a few days.

West Nile Virus Discussion Guide

Get our printable guide to your next doctor's appointment to help you ask the right questions.

People hospitalized with severe West Nile virus illness are treated to control fever and keep vital signs stable. Although antiviral drugs and intravenous immune globulin are often used to treat West Nile infection in hospitalized patients, there is no real evidence that these treatments aid recovery.

The West Nile death rate from severe neurological infection, even with optimal medical care, in the United States is 2% to 7%.


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

Avoiding mosquito-infested areas, cleaning homes of standing water in which mosquito larvae can develop, and using insect repellants are important preventive measures. Testing donated blood for West Nile virus has significantly reduced the risk of transmission through blood transfusions.

Vaccines against West Nile virus are being developed. Although equine vaccines have been licensed for use in the United States, no human vaccine has yet been clinically tested .

Get the word of drug information

West Nile virus is a mosquito-borne infection that is widespread in the United States and around the world. While most people infected with West Nile virus have relatively mild illness and make a full recovery, those who develop neurological infections can become seriously ill and have a long recovery, as well as risk death. Since there is no specific treatment for West Nile virus, it is important to take steps to prevent infection by protecting yourself and your loved ones from mosquitoes whenever possible.

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