West Nile virus (WNV) is a virus carried by mosquitoes. Most people infected with the virus don’t have symptoms or have only a mild illness. Fewer than 1 in 100 people who are infected with the virus develop serious illness. Serious forms of illness caused by WNV include encephalitis (inflammation of the brain) and meningitis (inflammation of the membranes that cover the brain and spinal cord).
Wild and domestic birds, mainly crows, carry the West Nile virus. Mosquitoes become carriers of the virus when they bite infected birds. Humans can get the virus when they are bitten by infected mosquitoes. There are no known cases of a human getting WNV directly from an infected bird.
The risk of West Nile virus is seasonal in the northern states of the US and usually starts in the spring. The peak time for infection is mid to late August. In milder southern climates, the infection may occur year-round. The risk of severe infection is greatest for people who are over 70 years old or who have a weakened immune system.
West Nile virus may be spread from person to person through blood transfusions and organ transplants. However, blood centers and hospitals screen for West Nile Virus in donated blood and organs. The virus might also be transmitted through breast milk. However, the risk of transmission of the virus to the baby is believed to be very low. If you are breast-feeding and you have a WNV infection, the American Academy of Pediatrics and the CDC say that it’s OK to keep breast-feeding.
The infection is not spread by normal person-to-person contact like touching, kissing, or caring for someone who is infected.
About 4 out of 5 infected people don’t have symptoms. Children are more likely to have symptoms than adults. When symptoms do occur, they are usually mild and last a few days. Symptoms of WNV infection may include:
A WNV infection usually does not involve the brain. However, a few infected adults and children develop encephalitis or meningitis. Symptoms of these severe illnesses include:
Some people develop a polio-like syndrome with sudden weakness and paralysis.
Symptoms usually appear 3 to 14 days after you are bitten by an infected mosquito.
Your healthcare provider will ask about your symptoms and examine you. Tests you may have are:
There is no medicine that cures West Nile virus. If your symptoms are mild, they will go away on their own. Most people with mild symptoms can care for themselves at home. If you have a serious infection, you may need to stay at the hospital. You may be given intravenous (IV) fluids and pain relievers. For severe or life-threatening infection, you may need treatment in an intensive care unit.
Most people infected with WNV, including nearly all children, don’t get seriously ill, and they recover fully. Symptoms usually last 3 to 6 days, but they can last as long as several weeks or months.
If you have a serious infection, you may be ill for weeks or months. Your nervous system or brain may be injured. The injury is sometimes permanent.
If you get West Nile virus, you will probably be immune to future infection by the virus, but your immunity might decrease over time.
If you are older or live alone, you may need someone to check to make sure you don’t develop serious symptoms, such as confusion and coma.
WNV can be prevented by taking precautions to avoid exposure to mosquitoes:
Note: Vitamin B and ultrasonic devices don’t help prevent mosquito bites.
A vaccine is available to protect horses from West Nile virus. No vaccine is available for humans yet, but several companies are working to develop a human vaccine.