Colorado’s first human death of West Nile virus for 2023

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State health officials seeing concerning trends of West Nile virus

STATEWIDE (August 4, 2023) — The Colorado Department of Public Health and Environment confirmed the first human death of West Nile virus this year in a person from Weld County. So far this season, state health officials have confirmed 12 human cases of West Nile virus in residents of eight counties. This is a significant increase from the three cases reported last week and is more cases than we would typically see at this time of year.  Additionally, West Nile virus has been found in mosquitoes in eight of the 11 counties that have tested mosquitoes this season, including Adams, Arapahoe, Boulder, Delta, Denver, Larimer, Pueblo, and Weld counties. 

These increased human cases of West Nile virus follow reports of unusually high levels of infection in the Culex mosquitoes that carry the virus. The abundance of Culex mosquitoes this season is likely due to the unusual amount of precipitation this winter and spring. 

“The trends we are seeing in our West Nile virus tracking data are unprecedented,” said Dr. Rachel Herlihy, state epidemiologist, CDPHE. “The number of West Nile virus-infected mosquitoes we've detected this season is the highest we've seen in years. This is especially concerning now that August is here and September is just around the corner, as this is usually when human cases peak in Colorado." 

While most people infected with West Nile virus don’t have symptoms, some can develop a serious, potentially deadly illness. In 2022, Colorado had 206 reported human cases of West Nile virus, including 20 deaths. People aged 60 years and older and those with certain medical conditions are at greater risk of serious illness. Talk with a health care provider if you develop a fever with severe headaches or confusion.

West Nile virus is most commonly spread to people by the bite of an infected mosquito. To protect yourself:

  • Use insect repellents when you go outdoors. Repellents containing DEET, picaridin, IR3535, oil of lemon eucalyptus, or para-menthane-diol products provide the best protection. For more information about insect repellents, visit the EPA’s information webpage. Always follow label instructions.
  • Limit outdoor activities at dusk and dawn, when mosquitoes are most active.
  • Wear long pants, long-sleeved shirts, and socks in areas where mosquitoes are active. Spray clothes with insect repellent for extra protection.
  • In addition to eliminating standing water around your home weekly, you can also mosquito-proof your home by installing or repairing screens on windows and doors.

You can find additional data going back to 2003 on CDPHE’s West Nile virus webpage, which is updated weekly throughout the season.