West Nile Virus Found in Denver Mosquito Sample
Published on July 03, 2025
The Denver Department of Public Health & Environment (DDPHE), in collaboration with the Colorado Department of Public Health and Environment (CDPHE), has detected West Nile virus in a Denver mosquito sample.
In the continental United States, West Nile virus is the most common mosquito-borne infection and circulates in nature between birds and mosquitoes. West Nile virus can be transmitted anytime mosquitoes are active. In Colorado, mosquitoes can be active as early as May through the first hard freeze of the year, October or November.
Denver conducts routine testing of adult mosquitoes for West Nile virus from mid-June through mid-September. The mosquito monitoring program involves weekly testing of mosquito traps from five locations across the city. Alongside virus surveillance, DDPHE takes proactive measures to mitigate mosquito populations by reducing breeding sites and applying larvicide to decrease larval mosquitoes in Denver.
While everyone is at risk of contracting West Nile virus, individuals over 50 years old or with weakened immune systems are at higher risk of developing serious illness. Common symptoms include fever, extreme fatigue, headache, and body aches. In some cases, symptoms may also include skin rashes and swollen lymph nodes. These symptoms generally appear three to 14 days after being bitten by an infected mosquito. While most infections are mild, severe cases can lead to encephalitis (inflammation of the brain) or meningitis (inflammation of the brain's lining), which can cause loss of vision, paralysis, coma, tremors, convulsions, and even death.
Anyone experiencing these symptoms should consult a doctor and get tested. There is currently no treatment, cure, or vaccine for West Nile virus, but medical professionals can provide supportive care to alleviate symptoms and aid recovery.
DDPHE encourages residents to remember the 4Ds to avoid getting infected with the virus:
- Defend- Use effective insect repellants when you are outdoors. Repellants that contain DEET, picaridin, IR3535, and oil of lemon eucalyptus provide effective protection against mosquitoes. If you are also wearing sunscreen, use the sunscreen first followed by the insect repellant. Apply insect repellant to your clothes in addition to any exposed skin. Always follow the labels instructions when using insect repellants.
- Dress- Wear closed-toe shoes, long sleeves, and pants to cover as much exposed skin as possible. Loose-fitting clothes are effective at preventing mosquito bites.
- Drain- Drain any standing water around your property at least once a week to eliminate mosquito breeding grounds. Keep your yard clear of debris that can hold water.
- Dusk & Dawn- Mosquito activity typically peaks at dusk and dawn. To reduce your exposure to mosquitoes, limit any outdoor activities at these times. During the day, mosquitoes tend to linger around cool and shaded areas.
For more information about West Nile virus in Colorado or mosquito activity data visit,CDPHE’s website. Learn more about Denver’s mosquito management program onDDPHE's Environmental What If podcast.