Denver joins a slew of other cities and counties across the U.S. that are reporting an earlier season for West Nile virus.

What's happening?

The city reported that it had discovered its first West Nile-positive mosquito for the 2025 season in early July. The Daily Camera reported the news, saying that while West Nile virus season was starting early in Colorado's Front Range, experts weren't certain that this would translate into a heavy season for infections.

Why is this announcement concerning?

West Nile is typically asymptomatic, but more severe cases can result in fever, headache, body aches, vomiting, diarrhea, and rash, according to the Centers for Disease Control and Prevention. However, the virus can also turn deadly. Colorado reported 634 cases and 51 deaths from West Nile in 2023 and 207 cases and 20 deaths in 2022, per data from the state's Department of Public Health and Environment.

"I have seen healthy 20- and 30-year-olds with severe West Nile disease and permanent neurological damage," Dr. Daniel Pastula, chief of neuroinfectious diseases and global neurology at the University of Colorado School of Medicine, told the Daily Camera. "It is much better to avoid getting this."

Though West Nile has been in the U.S. since 1999, an overheating planet is driving increases in mosquito-borne illnesses. That's because these insects are taking advantage of warmer and wetter conditions, which are helping them expand their ranges, remain active for longer seasons, and move into higher altitudes. 

For instance, Massachusetts residents are advised to stay vigilant after the state discovered its first West Nile-carrying mosquitoes two weeks earlier than last year. Likewise, Larimer County, Colorado, reported its first West Nile-carrying mosquito in late June — several weeks earlier than usual — according to the Daily Camera. And Connecticut officials are warning residents that a mosquito that can carry several mosquito-borne illnesses, including dengue, is expected to spread farther and faster than usual across the state in 2025.

What's being done about mosquito-borne illnesses?

The CDPHE recommends that people protect themselves from being bitten by mosquitoes by using insect repellent when spending time outdoors, limiting outdoor activities at dusk and dawn, wearing long clothing and spraying it with insect repellent, draining standing water, and installing or repairing screens on windows and doors at home.

Many communities are also looking for ways to reduce mosquito populations in an effort to curb the transmission of mosquito-borne illnesses. For instance, officials in one Florida county are using X-rays to kill invasive mosquitoes, and one Manila neighborhood is paying people to turn over any mosquitoes they can capture.

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