West Nile virus (WNV) is a virus carried by mosquitoes. Most people infected with the virus do not have symptoms or have only a mild illness. Less 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) or 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 from an infected bird, only from the bite of an infected mosquito.
The risk of West Nile virus is seasonal in the northern states of the US and begins 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. 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 recommend that you continue 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 have no symptoms. Children are more likely to have symptoms. 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 people (and even fewer children) develop encephalitis or meningitis. Symptoms of these less common but severe illnesses include:
Some people develop a poliolike syndrome with sudden weakness and paralysis.
Symptoms usually appear 2 to 15 days after your child is bitten by an infected mosquito.
Your child's health care provider will ask about the symptoms and examine your child. Your child may have the following tests:
There is no specific medicine to treat West Nile virus. If the infection is serious, your child may need to stay at the hospital. At the hospital, your child may be given intravenous (IV) fluids, pain relievers, or other treatments.
Most people infected with WNV, including nearly all children, do not get seriously ill, and they recover fully.
If you have a serious infection, you may be ill for weeks. You may have some injury to the nervous system and brain. 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.
Yes, WNV can be prevented. Take precautions to avoid exposure to mosquitoes:
Repellent products containing either DEET or picaridin as active ingredients have been proven to provide longer-lasting protection than others. Oil of lemon eucalyptus, a plant-based repellent, hasn't been as well tested. But in some studies it provided as much protection as repellents with low concentrations of DEET. Oil of lemon eucalyptus should not be used on children under age 3.
Note: Vitamin B and ultrasonic devices DO NOT 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.