West Nile virus

West Nile virus (WNV) is primarily a disease of birds, spread by infected mosquitoes to people; it is not transferred from person to person.

Female Culex tarsalis mosquito, the species that transmits the virus, usually start emerging in late April or early May and continue transmitting the virus until the first hard frost, which usually is in September along the Front Range. 
Protect yourself and your family from mosquito bites, especially from dusk (evening) to dawn (morning) when the mosquitoes that spread WNV are more active.


No symptoms in most people

Most people (70-80%) who become infected with WNV do not develop any symptoms.

Mild symptoms in some people

About 1 in 5 people who are infected will develop a fever with other symptoms such as headache, fatigue, body aches, joint pains, vomiting, diarrhea, or rash. Symptoms begin between 2 to 14 days after being bitten. People with milder symptoms typically recover on their own, although some symptoms may last for several weeks.

Severe symptoms in a few people

Less than 1% of people who have mild symptoms will develop serious neurologic illness such as encephalitis or meningitis (inflammation of the brain or surrounding tissues).
  • The symptoms of neurologic illness can include headache, high fever, neck stiffness, disorientation, coma, tremors, seizures, or paralysis.
  • Serious illness can occur in people of any age. However, people over 60 years of age and/or people with certain medical conditions, such as cancer, diabetes, hypertension, kidney disease, and people who have received organ transplants, are at greater risk for serious illness.
  • Recovery from severe disease may take several weeks or months. Some of the neurologic effects may be permanent.
  • About 10% of people who develop neurologic infection due to WNV will die.


See your healthcare provider if you think you have symptoms of WNV. There is no treatment, cure, or human vaccination for WNV, but health care providers can treat symptoms to help patients feel better and possibly recover more quickly.


Practice the Four D's

  1. Drain standing water around the house since that's where mosquitoes breed. Be sure to empty old tires, cans, flowerpots, clogged rain gutters, rain barrels, and toys where puddles can occur.
  2. Dusk and Dawn are when mosquitoes that carry the virus are most active, so limit outdoor activities or take precautions to prevent mosquito bites.
  3. DEET is an effective ingredient to look for in insect repellents. Always follow label instructions carefully.
  4. Dress in long sleeves and pants to keep mosquitos from biting.


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