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In this Aug. 26, 2019, file photo, Salt Lake City Mosquito Abatement District biologist Nadja Reissen examines a mosquito in Salt Lake City. (AP Photo/Rick Bowmer)

Colorado is in the midst of the worst season for West Nile virus in the country.

As of Wednesday, when the state posted its most recent data, 435 people had been diagnosed with a West Nile infection, including 262 who had been hospitalized. That is more than double the number of infections reported from all of last year’s season — which itself was the worst per capita in the country.

Colorado’s case total so far is more than double that of any other state. In older data published by the federal Centers for Disease Control and Prevention, only one other state — California — had seen more than 100 cases this year.

There have been 21 reported deaths in Colorado, one more than last year. But the true number might be higher because mortality data typically takes a while to filter in.

The current season still has another month or more to run before a hard freeze kills off the mosquitoes that spread the virus to humans. But Natalie Marzec, a West Nile specialist with the Colorado Department of Public Health and Environment, said mosquito activity has begun to slow across the state as the weather cools. Accounting for the virus’s incubation time, though, Marzec there are still more reported infections to come.

“Even though the mosquito activity does appear to be calming down, we do expect to see a good number of cases continuing through the month,” she said.

West Nile virus is transmitted through the bite of an infected mosquito, and it does not spread person-to-person. About 80% of those infected suffer no symptoms. But, in the most severe cases, the virus can attack the central nervous system, causing what are known as “neuroinvasive cases” that can be fatal. Nonfatal neuroinvasive cases can still leave people with long-term physical and mental disabilities.

Colorado’s agricultural hubs — Weld and Larimer counties, especially — often see the most West Nile cases. And they have seen a lot this year, too. But the largest number of cases has occurred in Denver, with Arapahoe and Adams counties also in the top five.

“You see a lot of it in that I-25 corridor, in the northern part, but it’s all over the state,” Marzec said.

To Marzec, that geographic distribution sends a message: No matter where you are, you should take precautions against West Nile. There is no vaccine against the virus for humans. (There is one, though, for horses, which can also be affected.)

So that means people should try to avoid being out at dawn or dusk when mosquitoes are most active, wear long pants and sleeves and use bug spray, especially one that contains DEET. People should also drain and dry pools of standing water on their property where mosquitoes lay their eggs.

This West Nile season is exactly the kind of year experts feared when a wet spring and early summer sent mosquito populations skyrocketing. But it’s not just moisture that leads to bad West Nile years in Colorado. Over the past several years, in seasons wet and dry, Colorado has emerged as one of the nation’s worst hot spots for West Nile.

Metropolitan State University of Denver professor Bob Hancock shows a case of nearly 1,000 mosquitoes obtained from a light trap in Weld County from 2014. (Olivia Sun, The Colorado Sun via Report for America)

According to Marzec and other experts, West Nile’s presence here is also a function of how we move and store water, how we irrigate crops and landscaping, how we manage stormwater runoff in new developments, and how we build parks, recreation trails and new housing subdivisions.

In other words, we are creating the conditions that make the West Nile virus a menace to humans.

The Sun will be exploring this idea more — as well as new solutions that may be on the horizon to controlling mosquito populations — at SunFest on Sept. 29 on the Auraria Campus in Denver. The panel will feature famed Metropolitan State University of Denver entomologist Bob Hancock and longtime Colorado mosquito-control specialist Doc Weissmann.

For more information on SunFest, including a full lineup of panels and information on how to buy tickets, go to

John Ingold is a co-founder of The Colorado Sun and a reporter currently specializing in health care coverage. Born and raised in Colorado Springs, John spent 18 years working at The Denver Post. Prior to that, he held internships at...