What is West Nile virus?

West Nile virus causes an infection that is spread by certain kinds of mosquitoes . Most often, mosquitoes get infected when they bite infected birds. Then the mosquitoes spread the virus when they bite people or other animals, such as horses. West Nile cannot spread from these animals to people or from person to person through casual contact.

West Nile can spread through an organ transplant or a blood transfusion. So all donated blood in the United States is screened to see if the virus is present. Some evidence suggests that West Nile can spread from a mom to her baby during pregnancy, at birth, or through breast milk. But the CDC still recommends that women breast-feed, because the risk of spreading the virus to babies is unclear and the benefits of breast-feeding are known.¹

Most people who have West Nile have no symptoms. Or the symptoms may be so mild that people may not even realize that they have the virus. In rare cases, West Nile can lead to swelling of the brain (encephalitis), swelling of the spinal cord (myelitis), or swelling of the tissues around the brain and spinal cord (meningitis). But very few people with West Nile will get a severe illness that affects the brain or spinal cord.²

Anyone who is bitten by a mosquito may get West Nile. Most of the time people fully recover from it. But permanent problems such as seizures, memory loss, and brain damage can occur, especially in children and older people. As you get older, you have a higher risk for getting encephalitis and other serious problems from West Nile. Of the people who have serious problems, those older than age 70 have the biggest risk of dying from them.³ In a few cases, West Nile can be fatal.

What are the symptoms?

About 80 out of 100 people who have West Nile have no symptoms.² When symptoms do appear, they begin 3 to 14 days after the mosquito bite. Mild symptoms include:

• A fever.
• Headaches, body aches, or pain in your eyes.
• A rash, usually on the chest, back, and arms.
• Feeling very tired.
• Not feeling hungry.
• Feeling sick to your stomach or throwing up.
• Swollen glands (lymph nodes), in rare cases.

In mild cases of West Nile, symptoms usually last for 3 to 6 days. If you get a more severe case of West Nile, symptoms can last for weeks or months. Severe cases that involve problems with the brain and spinal cord are rare, but they may cause:

• Headaches.
• A high fever.
• A stiff neck or paralysis.
• Confusion.
• Reduced attention to surroundings.
• Tremors, convulsions, or muscle weakness.
• A coma.

In rare cases, West Nile virus can cause death.

How is West Nile virus diagnosed?