West Nile Virus


West Nile virus (WNV) is an infection that is most commonly spread by a mosquito bite. In rare cases, WNV can lead to serious complications, or death.This infection is found worldwide. In the United States, it tends to occur during late summer and early fall.


West Nile is caused by a virus. The virus is passed through the bite of an infected mosquito. The virus can also be passed through a blood transfusion with infected blood, but this is rare.

Risk Factors

The greatest risk factors for WNV are spending time in areas where mosquitoes are present and not using insect repellent. The risk of complications with WNV is highest in people:
  • Over 50 years old
  • With a weakened immune system
Mosquito Bite
Mosquito bite
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Most people with WNV have no symptoms. About 20% of the people who become infected with WNV develop flu-like symptoms such as:
  • Fever and chills
  • Headache
  • Muscle pain
  • Nausea and vomiting
  • Diarrhea
  • Rash
  • Swollen lymph nodes
  • Eye inflammation—conjunctivitis
Symptoms may appear within 2-6 days and can last from a few days to several weeks.A small number of people with WNV will develop serious, neurological symptoms. These symptoms may include:
  • High fever
  • Headache
  • Stiff neck
  • Disorientation
  • Stupor
  • Tremors
  • Vision loss
  • Severe muscle weakness
  • Paralysis
  • Coma
These serious symptoms need immediate medical care.


In addition to taking your medical history and doing a physical exam, your doctor will ask you:
  • What kind of symptoms you are experiencing
  • Where you have been living or traveling to
  • Whether you have been exposed to mosquitoes
A blood test is commonly used to confirm the diagnosis of WNV. Depending on the symptoms that you have, your doctor may order other tests, such as:
  • MRI scan
  • Cerebrospinal fluid tests to evaluate the fluid that surrounds the brain and spinal cord

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