Aug 7, 2023 - Climate

"Unprecedented" outbreak of West Nile virus alarms Colorado health officials

A specimen of a Culex mosquito, which carries the West Nile virus. Photo: Jason Wambsgans/Chicago Tribune via Getty Images

The number of mosquitos infected with West Nile virus this season is the highest Colorado health officials have seen in years.

Driving the news: A Weld County resident recently died from the virus, marking the state's first West Nile-related fatality in 2023, officials announced this month.

What they're saying: "The trends we are seeing in our West Nile virus tracking data are unprecedented," Colorado epidemiologist Rachel Herlihy said in a statement Friday.

  • "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," Herlihy added.

By the numbers: State data shows at least 12 human cases of West Nile virus confirmed across eight counties as of Aug. 4, more cases than typical at this time of year.

  • The virus has been found in mosquitoes in eight of the 11 counties that have tested the insects this season: Adams, Arapahoe, Boulder, Delta, Denver, Larimer, Pueblo and Weld.

Flashback: 20 people died from West Nile in Colorado last year, and more than 200 were infected.

The big picture: Although this season appears to be bringing mosquito mayhem, "mosquito days" — ones with the hot and humid weather the flying insects crave — have trended downward in Denver over the past several decades.

  • Experts suspect long-term drought conditions across the region could be at play because mosquitoes rely on moisture to thrive.

Be smart: Most cases in humans are asymptomatic, and about 1 in 5 people develop a fever with flu-like symptoms, per the CDC.

  • About 1% of infected people develop neuroinvasive infection, which can be fatal, and people aged 60 years and older are at greater risk of serious illness.
  • The CDC suggests talking with a health care provider if you suspect the virus.

What to do: The best way to avoid bites is to cover exposed skin while outdoors — particularly at dusk and dawn; drain any standing water; and wear insect repellent.


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