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 U.S. Centers for Disease Control and Prevention (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. In a few cases, West Nile can be fatal.
About 80 out of 100 people
who have West Nile have no symptoms and only about 1 out of 150 people infected develops serious illness.2 When symptoms do
appear, they begin 3 to 14 days after the mosquito bite. Mild symptoms
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:
- A high fever.
- A stiff neck or paralysis.
- Reduced attention to surroundings.
convulsions, or muscle weakness.
- A coma.
In rare cases, West Nile virus can cause death.