West Nile Virus

What is the West Nile virus?

The West Nile virus is spread by mosquitoes. The West Nile virus can infect humans, birds, mosquitoes, horses, and some other mammals. Very rarely, the virus can spread in transfused blood, a transplanted organ, or through the placenta to a fetus.

West Nile virus occurs in mid- to late summer and early fall in mild zones. It can also occur year-round in southern climates. Most often, the West Nile virus causes no symptoms or a mild, flu-like illness. But the virus can cause life-threatening illnesses, such as:

  • Inflammation of the brain (encephalitis)
  • Inflammation of the lining of the brain and spinal cord (meningitis)
  • Inflammation of the brain and its surrounding membrane (meningoencephalitis) 

What causes West Nile virus?

West Nile virus is spread to humans
through the bite of an infected female mosquito. The mosquitoes get the virus when they
bite an infected bird. Crows and jays are the most common birds linked to the virus. But
at least 110 other bird species also have the virus.

West Nile virus isn’t spread
between humans. But in a few cases it has spread through organ transplant. Health
officials think the organ donor acquired the virus through a blood transfusion. All
blood is screened for the virus. The risk for getting West Nile virus from blood is much
lower than the risk of not having any procedure that would call for a blood

Who is at risk for West Nile virus?

Certain things can increase the
risk for getting West Nile virus. You are more likely to get the virus if you are
exposed to mosquito bites during the summer months.

Most people who are infected have a
minor illness and recover fully. But older people and those with weak immune systems are
more likely to get a serious illness from the infection.

What are the symptoms of West Nile virus?

Most people infected with West Nile
virus have no symptoms. They won’t know they had the infection.

About 1 in 5 people who become
infected will develop West Nile fever. This is a flu-like illness. These are the most
common symptoms of West Nile fever:

  • Fever
  • Headache
  • Body aches
  • Skin rash on trunk of body
  • Swollen lymph glands

The more severe form of the West
Nile virus affects mostly older adults. It occurs when the virus crosses the blood-brain
barrier and can cause:

  • Headache
  • High fever
  • Neck stiffness
  • A state of impaired consciousness,
    extreme lethargy, and reduced reactivity to outside stimuli (stupor)
  • Disorientation
  • Coma
  • Tremors
  • Convulsions
  • Muscle weakness
  • Paralysis

These symptoms may look like other
health problems. Always see your healthcare provider for a diagnosis.

How is West Nile virus diagnosed?

Your healthcare provider will order a blood test to check for
antibodies to the West Nile virus. He or she may also do a lumbar puncture to test
cerebrospinal fluid for signs of infection.