It's an unusually muggy morning at the CSU Environmental Learning Center on the east side of Fort Collins with bird chirping and twittering penetrating the stifling air.
A white-tailed doe browses as its spotted fawn plays among the tall grasses under a canopy of cottonwoods, paying little attention to Tyhan Tearman and Travis Wells.
The two agree seeing wildlife is a major perk of their job, but the task this July 1 day is looking for mosquito larvae — and avoiding ticks.
They work in the West Nile virus capital of Colorado , and they serve a pivotal role in raising the alarm regarding the virus that can kill and debilitate. It's not a matter of "if'' the virus will be found in Larimer County they patrol, but when.
Tearman, a field technician, and Wells, a field supervisor, work for the Loveland office of Vector Disease Control International, which has mosquito control contracts for much of Larimer County.
They are joined by a cadre of fellow field workers who regularly patrol designated areas around the county, looking for the mosquito larvae and treating sites to reduce the chance of the virus starting.
The pair walk the ELC's earthen paths that wind among tall grasses, stopping to dip long-handled cups into pools and channels of the Poudre River searching for the most miniscule of mosquito larvae.
After several dippings that come up with damsel flies and water fleas, but no mosquito larvae, Wells peers into his cup and spots one.
“Yep, there’s larvae in there,'' he says, his eyes hovering just above the cup straining to identify barely identifiable wriggly things among sediment. "It's super small. I'm not sure I can even point it out. There, he floated right up to the top. He's wriggling by my finger.''
He identifies it as a first-instar mosquito larvae, which are larvae shortly after hatching and are approximately 2 mm long. Second-instar larvae grow to around 10 mm and third-instar larvae grow to between 15 mm and 20 mm before wandering off as pre-pupae.
The area where Wells finds the larvae is a stagnant pool off a channel of running water, left after a rain or spring runoff.
"This is mosquito habitat right here,'' he says. "There's a few centimeters of water, which is more than enough for the mosquitoes to breed. The eggs float down the river and the eggs end up in these pockets, hatch and breed. That's probably why we are seeing so many mosquitoes in this one area. They probably just hatched and already made more eggs in there. ''
In each water hole where they find mosquito larvae, the two toss handfuls of BTI (Bacillus thuringiensis israelensis) pellets into the water. BTI is a naturally occurring soil bacterium that can effectively kill mosquito larvae in the water without harming other insects and wildlife.
"The one- to three-instar mosquito larvae feed on the pellets, causing them to bloat and basically explode,'' Tearman says matter-of-factly. "If we're doing our job really well, we don’t see pupae except for the start of the season.''
Other workers check 53 traps around the city of Fort Collins weekly, collect any mosquitoes and take them back to the lab to identify what species of mosquitoes they trapped.
Those identified as Culex mosquitoes, which are the sole mosquito species to carry the virus, are sent to Colorado State University's Center for Vector-Borne Infectious Diseases. There, it is determined if the mosquitoes are infected with West Nile virus.
That report is then disseminated to municipalities and the Larimer County Health Department to assess the threat of West Nile virus in people.
West Nile virus outlook for Larimer County
As of July 1, there has been one human case of West Nile virus in Colorado, that one coming in the last week in Arapahoe County.
Will Schlatmann, Northern Colorado program manager for Vector Disease Control International, said there have been no West Nile positive mosquitoes found in traps this year in Larimer County. Surveillance trapping started the first week of June and usually goes to the end of August, he said.
But he knows inevitably that will change because West Nile has become endemic in the state.
"So far this season, it's been very hot and dry and so we have had a fraction of the mosquitoes we saw last year when we had record rain,'' he said. "But there is so much that goes into it, which makes it hard to predict the factors that determine how much West Nile we find.''
In 2023, when near-record to record rain fell over wide swaths of Colorado, there were 634 human cases and 51 deaths . Both of those numbers were the highest recorded since the first year the Colorado Department of Health and Environment kept records in 2003. That year, the state saw 2,948 cases and 66 deaths.
He said his company has sprayed for mosquitoes in Loveland and parts of Windsor and Johnstown this year but not in Fort Collins.
Why Larimer County has the most West Nile cases in Colorado
Last year, Larimer County tied Weld County with 52 human cases of West Nile.
Since 2003, Larimer County has had 1,064 West Nile reported cases in humans, by far the most of any county in Colorado. Weld County is second with 913 cases.
According to previous Coloradoan reporting , Larimer and Weld counties have higher rates of West Nile than other counties because irrigation is prevalent, and where it gets used for agricultural purposes, mosquitoes thrive in the canals and flooded fields and anywhere else water pools.
Ways to prevent mosquito bites and contracting West Nile virus
- Culex mosquitoes can lay eggs in as little as 2 inches of water.
- Drain items such as bird baths, containers and kiddie pools.
- Use DEET or other effective mosquito repellent.
- Avoid exposure during peak Culex mosquito feeding times, which are dusk through dawn.
- Wear long sleeves and pants to reduce skin exposure.
For more information, visit the Larimer County Health Department's website .
This article originally appeared on Fort Collins Coloradoan: West Nile virus continues to plague Colorado. Here's what is being done to prevent it
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