West Nile virus infection

Definition

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

Causes

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 US.

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 the 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:

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

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:

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 one half of people with West Nile virus infection may have a rash.

Tests to diagnose West Nile virus include:

Treatment

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

When to Contact a Medical Professional

Call your health care provider if you have symptoms of West Nile virus infection, particularly 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.

Prevention

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

Community spraying for mosquitoes may also reduce mosquito breeding.


Review Date: 12/1/2018
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, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.

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