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West Nile virus



West Nile virus is a disease spread by mosquitos. 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 United States in the summer of 1999 in New York. Since then, the virus has spread throughout the United States.

The West Nile virus is a type of virus known as a flavivirus. Researchers believe West Nile virus is spread when a mosquito bites an infected bird and then bites a person.

Mosquitos carry the highest amounts of virus in the early fall, which is why the rate of the disease increases in late August to early September. The risk of disease decreases as the weather becomes colder and mosquitos die off.

Although many people are bitten by mosquitos that carry West Nile virus, most do not know they've been exposed. Few people develop severe disease or even notice any symptoms at all.

Mild, flu-like illness is often called West Nile fever. More severe forms of disease, which can be life threatening, may be called West Nile encephalitis or West Nile meningitis, depending on what part of the body is affected.

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

  • Conditions that weaken the immune system, such as HIV, organ transplants, and recent chemotherapy
  • Older 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.


Mild disease, generally called West Nile fever, has some or all of the following symptoms:

  • Abdominal pain
  • Back pain
  • Diarrhea
  • Fever
  • Headache
  • Lack of appetite
  • Muscle aches
  • Nausea
  • Sore throat
  • Vomiting

These symptoms usually last for 3 - 6 days.

With more severe disease, the following symptoms can also occur, and need prompt attention:

Signs 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 to diagnose West Nile virus infection.

About 20 - 50% of patients may have a rash. Muscle weakness with other related symptoms are signs of a West Nile virus infection.

Tests to diagnose West Nile virus include:

The most accurate way to diagnose this infection is with a serology test, which checks a blood or CSF sample for antibodies against the virus.

Rarely, a sample of blood or CSF may be sent to a lab to be cultured for the presence of West Nile virus. The virus can also be identified in body fluids using a technique called polymerase chain reaction (PCR). However, these methods can provide false negative results and are not commonly used.

Support Groups

Expectations (prognosis)

In general, the likely outcome of a mild West Nile virus infection is excellent.

For patients with severe cases of West Nile virus infection, the outlook is more uncertain. West Nile encephalitis or meningitis may lead to brain damage and death. Approximately 10% of patients with brain inflammation do not survive.

Calling your health care provider

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

If you have been bitten by an infected mosquito, there is no treatment to avoid getting West Nile virus infection. People in good general health generally do not develop a serious illness, even if they are bitten by an infected mosquito.


Complications from mild West Nile virus infection are extremely rare.

Complications from severe West Nile virus infection include:

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

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

Research trials are under way to determine whether ribavirin, an antiviral drug used to treat hepatitis C, may be helpful.


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 mosquitos may also prevent mosquito breeding.

Testing of donated blood and organs is currently being evaluated. There are currently no guidelines.


Tsai TF, Vaughn DW, Solomon T. Flaviviruses (yellow fever, dengue, dengue hemorrhagic fever, Japanese encephalitis, West Nile encephalitis, St. Louis encephalitis, tick-borne encephalitis). In: Mandell GL, Bennett JE, Dolin R, eds. Principles and Practice of Infectious Diseases. 6th ed. Philadelphia, Pa: Elsevier Churchill Livingstone; 2005:chap 149.

Naides SJ. Arthropod-borne viruses causing fever and rash syndromes. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 405.

Bleck TP. Arthropod-borne viruses affecting the central nervous system. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 406.

Review Date: 9/28/2008
Reviewed By: Linda Vorvick, MD, Seattle Site Coordinator, Lecturer, Pathophysiology, MEDEX Northwest Division of Physician Assistant Studies, University of Washington School of Medicine; and Jatin M. Vyas, PhD, MD, Instructor in Medicine, Harvard Medical School, Assistant in Medicine, Division of Infectious Disease, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.
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