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West Nile virus


West Nile virus is a disease spread by mosquitoes. The condition ranges from mild to severe.


West Nile virus was first identified in 1937 in Uganda in eastern Africa. It was first discovered in the U.S. in the summer of 1999 in New York. Since then, the virus has spread throughout the U.S.

Researchers believe West Nile virus is spread when a mosquito bites an infected bird and then bites a person.

Mosquitoes carry the highest amounts of virus in the early fall, which is why more people get the disease in late August to early September. As the weather becomes colder and mosquitoes die off, there are fewer cases of the disease.

Although many people are bitten by mosquitoes that carry West Nile virus, most do not know they have been infected.

Risk factors for developing a more severe form of West Nile virus include:

  • Conditions that weaken the immune system, such as HIV/AIDS, organ transplants, and recent chemotherapy
  • Older or very young age
  • Pregnancy

West Nile virus may also be spread through blood transfusions and organ transplants. It is possible for an infected mother to spread the virus to her child through breast milk.


Symptoms may occur 1 to 14 days after becoming infected. Mild disease, generally called West Nile fever, may cause some or all of the following symptoms:

  • Abdominal pain
  • Fever, headache, and sore throat
  • Lack of appetite
  • Muscle aches
  • Nausea, vomiting, and diarrhea
  • Rash
  • Swollen lymph nodes

These symptoms usually last for 3 to 6 days, but may last a month.

More severe forms of disease are called West Nile encephalitis or West Nile meningitis, depending on what part of the body is affected. The following symptoms can occur, and need prompt attention:

Exams and Tests

Signs of West Nile virus infection are similar to those of other viral infections. There may be no specific findings on a physical examination. About half of people with West Nile virus infection may have a rash.

Tests to diagnose West Nile virus include:


Because this illness is not caused by bacteria, antibiotics do not treat West Nile virus infection. Supportive care may help decrease the risk of complications in severe illness.

Outlook (Prognosis)

People with mild West Nile virus infection do well after treatment.

For those with severe infection, the outlook is more uncertain. West Nile encephalitis or meningitis may lead to brain damage and death. 1 in 10 people with brain inflammation do not survive.

Possible Complications

Complications from mild West Nile virus infection are very rare.

Complications from severe West Nile virus infection include:

  • Brain damage
  • Permanent muscle weakness (sometimes similar to polio)
  • Death

When to Contact a Medical Professional

Call your health care provider if you have symptoms of West Nile virus infection, especially if you may have had contact with mosquitoes. If you are very sick, go to an emergency room.

There is no treatment to avoid getting West Nile virus infection after a mosquito bite. People in good health generally do not develop a serious West Nile infection.


The best way to prevent West Nile virus infection is to avoid mosquito bites:

  • Use mosquito-repellant products containing DEET
  • Wear long sleeves and pants
  • Drain pools of standing water, such as trash bins and plant saucers (mosquitos breed in stagnant water)

Community spraying for mosquitoes may also reduce mosquito breeding.


Fischer M, Staples JE, Campbell GL. West Nile virus. In: Magill AJ, Hill DR, Solomon T, Ryan ET, eds. Hunter's Tropical Medicine and Emerging Infectious Diseases. 9th ed. Philadelphia, PA: Elsevier; 2013:chap 34.5.

Review Date: 12/7/2014
Reviewed By: Jatin M. Vyas, MD, PhD, Assistant Professor in Medicine, Harvard Medical School; Assistant in Medicine, Division of Infectious Disease, Department of Medicine, Massachusetts General Hospital, Boston, MA. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.
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