What is West Nile virus?

West Nile virus causes an infection that is spread by certain species of mosquitoes that become infected when they bite infected birds. The mosquitoes spread the virus to people. Most West Nile virus infections cause either no symptoms or symptoms that are so mild that people do not realize they have been infected. Mild or moderate symptoms include fever, tiredness, headache, and body aches, occasionally with skin rash and swollen lymph nodes.

In rare cases, it can lead to inflammation of the brain (encephalitis), spinal cord (myelitis), or the tissues surrounding the brain and the spinal cord (meningitis).

Very few people who are bitten by an infected mosquito will become severely ill with an illness affecting the brain and spinal cord.¹

Birds are a reservoir for West Nile virus, meaning that the virus multiplies in them. Infected mosquitoes then may spread the virus to humans and animals, such as horses. However, the virus cannot be spread by these animals to people or from person to person through casual contact.

The U.S. Centers for Disease Control and Prevention (CDC) reported 3,830 cases of West Nile virus infection and 119 West Nile virus deaths in the United States in 2006, as of November 7, 2006. Updated figures for 2006 are available online at http://www.cdc.gov/ncidod/dvbid/westnile/surv&controlCaseCount06_detailed.htm.¹

The virus can be spread through an organ transplant or a blood transfusion. Therefore, all blood and blood products donated for transfusion in the United States are screened for the virus. Some evidence suggests that West Nile virus can be spread from a mother to her baby during pregnancy, at birth, or through breast milk. However, the CDC still recommends that women breast-feed, because the benefits of breast-feeding are known and the risk of spreading the virus to babies is unclear.²

Anyone exposed to mosquitoes can become infected.

People usually recover fully from West Nile virus infection, but permanent problems may develop, especially children and older people. They may have seizures, memory loss, personality changes, brain damage, paralysis, tremors, walking or balance problems, or symptoms similar to Parkinson's disease. Some of these symptoms may last a long time. In a few cases, the infection can be fatal.

Older people infected by West Nile virus have the greatest risk for developing encephalitis and other complications. People ages 50 to 59 are 10 times more likely to develop complications than people younger than age 20, and people 80 or older are 43 times more likely to develop complications. Of the people who develop complications, those older than age 70 have the greatest risk of death.³

What are the symptoms?

About 80% of people who are infected with West Nile virus have no symptoms.¹ When symptoms do appear, they develop 3 to 14 days after infection and include fever, tiredness, headache, and body aches, occasionally with skin rash and swollen lymph nodes. Symptoms usually last 3 to 6 days in people who have a mild infection. With more severe infections, symptoms can last from weeks to months.