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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 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.

    Mosquitoes 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 mosquitoes die off.

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

    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 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
    • Diarrhea
    • Fever
    • Headache
    • Lack of appetite
    • Muscle aches
    • Nausea
    • Rash
    • Sore throat
    • Swollen lymph nodes
    • Vomiting

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

    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. The following symptoms can occur, and need prompt attention:

    • Confusion or change in ability to think clearly
    • Loss of consciousness or coma
    • Muscle weakness
    • Stiff neck
    • Weakness of one arm or leg

    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. However, up to half of patients with West Nile virus infection may have a rash.

    Tests to diagnose West Nile virus include:

    • Complete blood count (CBC)
    • Head CT scan
    • Head MRI scan
    • Lumbar puncture and cerebrospinal fluid (CSF) testing

    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. More rapid techniques using polymerase chain reaction (PCR) may be used.


    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.

    Outlook (Prognosis)

    In general, the 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. About 1 in 10 patients 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.

    Thereis 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.


    Beckham JD, Tyler KL. Encephalitis. In: Mandell GL, Bennett JE, Dolin R, eds. Principles and Practice of Infectious Diseases. 7th ed. Philadelphia, Pa: Elsevier Churchill Livingstone; 2009:chap 87.

    Bleck TP. Arthropod-borne viruses affecting the central nervous system. In: Goldman L, Schafer AI, eds. Cecil Medicine. 24th ed. Philadelphia, Pa: Saunders Elsevier; 2011:chap 391.


    • Mosquito, adult feeding ...


    • Mosquito, adult


    • Mosquito, egg raft


    • Mosquito, pupa


    • Meninges of the brain


      • Mosquito, adult feeding ...


      • Mosquito, adult


      • Mosquito, egg raft


      • Mosquito, pupa


      • Meninges of the brain


      A Closer Look

      Self Care

        Tests for West Nile virus

          Review Date: 10/6/2012

          Reviewed By: David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine. 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. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M. Health Solutions, Ebix, Inc.

          The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition. A licensed medical professional should be consulted for diagnosis and treatment of any and all medical conditions. Links to other sites are provided for information only -- they do not constitute endorsements of those other sites. © 1997- A.D.A.M., Inc. Any duplication or distribution of the information contained herein is strictly prohibited.

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