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The Mosquito Man enters his lab with the energy of a kid bounding into a Chuck E. Cheese.
All his friends are inside. There in one small screen-and-plexiglass enclosure is Sabethes cyaneus, a mosquito with an iridescent blue body and feathery paddles — what one researcher has called the “Hollywood showgirls of the mosquito world.” They float inside the box like dandelion seeds in the breeze.
But the mosquito that has his attention for the moment is Sabethes chloropterus, a colony of which he derived from mosquitoes he first captured on an island in Panama as a graduate student. It has been a few days since this colony has eaten, he explains. They must be hungry.
The Mosquito Man — Metropolitan State University of Denver biology professor Bob Hancock — rolls up his sleeve and sticks his right arm through a shrouded hole into their plexiglass home. The mosquitoes latch onto his hand. One. Two. Five. Fifteen. Forty.
“Oh, this is a good feed,” he says, as the mosquitoes linger on his hand, growing sluggish the more of him they drink in.
For Hancock, who studies mosquitoes, bed bugs and other insects most people would rather not spend time around, this is just another Wednesday morning. And the blood sacrifice is just part of the job for a scientist whose CV contains research titles like “Analysis of a complex vertical copulatory-courtship display in the yellow fever vector Sabethes chloropterus.”
But sitting to Hancock’s right is something extraordinary that could lead to some of the most important research of his career. It is a large enclosure that Hancock hopes will one day hold a colony of a Colorado mosquito called Culex tarsalis. Tarsalis is notable for its zebra-striped belly and for what Hancock says are “among the most beautiful mosquito larvae I know.”
They are also notable for another reason in Colorado: They are the primary vector for West Nile virus. And Colorado has just endured two of its worst West Nile years on record.
In fact, the state has become one of the nation’s hot spots for West Nile virus, a disease that can attack the central nervous system and be fatal or cause permanent damage. Colorado recorded the highest-number of West Nile cases in the country in 2022, as well as the highest number of neuroinvasive cases, the most severe form of the disease, which causes swelling in the brain and spinal cord. In 2021, Colorado ranked second behind Arizona.
But, despite West Nile virus being the leading cause of mosquito-borne disease in the United States, shockingly little is known about the dynamics of its spread. It takes mosquitoes, sure. But migratory birds, water, agriculture, climate change, the timing of snowmelt runoff, suburban development patterns, and human behavior all play a role, too.
“We have extensive understanding of so much,” Hancock says, “and also extensive ignorance of so much.”
So that is where the Mosquito Man and other researchers in Colorado come in.
Tracking West Nile’s history in Colorado
As its name indicates, West Nile virus was discovered in Africa — the first known case in a human occurred in Uganda in 1937. The virus is a flavivirus, a single-strand strip of RNA that hails from the same family that gave the world Zika, dengue and yellow fever.
West Nile did not pop up in North America until 1999, when a handful of cases were found in New York. By 2002, it had made its way to Colorado. But 2003 was the year that West Nile really announced its presence in the state. There were nearly 3,000 confirmed cases that year, including 622 neuroinvasive infections, 146 hospitalizations and 66 deaths.
No year has come close to that since — and part of that has to do with a slowly building layer of protection in the state.
We have extensive understanding of so much and also extensive ignorance of so much.
— Bob Hancock, Metropolitan State University of Denver biology professor
“We had no immunity in 2003, which is why the numbers were so high that year,” said Jennifer House, the deputy state epidemiologist and the state public health veterinarian at the Colorado Department of Public Health and Environment.
But, several thousand infections later, House said Colorado isn’t anywhere close to the kind of population immunity levels that could significantly dampen West Nile transmission. And that makes the increase in the past two years all the more alarming. Colorado still has plenty of people who could potentially get very sick with the virus.
The state saw 92 neuroinvasive West Nile infections and 11 deaths in 2021 — at the time more than in any year since 2003. But 2022 topped that with 132 neuroinvasive cases and 19 deaths.
Those numbers suggest that this year’s West Nile outbreak was much, much larger. House said only about one in five people who are infected with West Nile will show symptoms. Of those with symptoms, only a fraction will develop neuroinvasive disease or need to be hospitalized.
Historically, Colorado’s West Nile seasons — which run from about the beginning of June until the end of October — “sort of fluctuate all over the place,” House said. But 2021 and 2022 had the most West Nile hospitalizations of any two-year period in the state’s history.
And that makes finding out what causes bad West Nile years all the more urgent.
Unraveling the West Nile puzzle
The first thing to know about West Nile is that the virus would rather not infect us.
Humans are dead-end hosts to the virus. It can only be passed by mosquitoes, and the likelihood of a mosquito biting a West Nile-infected human and then spreading the virus on to where it can perpetuate is extremely low. The same is true for horses, the other main victim of West Nile virus in North America.
Instead, West Nile prefers birds, which is also what the Culex tarsalis mosquito, its primary vector in the state, most likes to feast on. (Birds typically survive West Nile infections, often with little problem.)
“West Nile makes its living by being transmitted between birds and mosquitoes,” said Greg Ebel, a professor of microbiology, immunology and pathology at Colorado State University and the director of the university’s Center for Vector-Borne Infectious Diseases. “People and horses get sick and die when they just sort of haphazardly stumble into this thing that is happening in nature all the time.”
So there’s the first challenge: Because humans are tangential to the real West Nile transmission patterns, it’s tough to know what we don’t know. What factors amplify the transmission cycle? What factors diminish it? It’s possible there are massive West Nile clusters happening somewhere across the West that we have no idea about because there aren’t humans present to be infected.
Ebel, whose lab tracks West Nile across Larimer County, said the virus shows consistently mysterious patterns in the region. The first place he detects West Nile in mosquito populations each spring is in the Fossil Creek area in the southeastern corner of Fort Collins. Then it pops up in the northeastern corner. It isn’t until much later that it shows up on the western side of Fort Collins. He has no idea why.
“I think there are a lot of things going on,” he said.
Some of the environmental and human-caused factors that would, on the surface, seem to make for ideal West Nile breeding grounds may not be what they appear.
Take water, for instance. Conventional wisdom would hold that wetter years are buggier, which would lead to more West Nile. After all, mosquitoes lay their eggs in small pools of water. But Ebel said the connection doesn’t necessarily hold true.
For starters, Culex tarsalis are not the mosquitoes most likely to be biting people in their backyards — though another mosquito that is more likely, Culex pipiens, can also carry West Nile.
“How many mosquitoes are out there isn’t that critical,” Ebel said.
Wetter years may also work against the transmission cycle because it gives water-thirsty birds and egg-laying mosquitoes a chance to spread out, Ebel said. Years with drier springs, which concentrate water in only a few places, bringing vector and host closer together, may be better for jump-starting the cycle.
Hancock, the MSU Denver professor, said too much water at the wrong time might kill a West Nile cycle before it can get going. An especially fast but full spring runoff could create a “lethal flush” that washes away mosquito larvae in wetlands.
Lastly, severe drought can hold mosquito populations in check. But Hancock said even that’s not a guarantee for a mellow West Nile season. In Colorado, when Mother Nature doesn’t provide, humans will still find ways to create precipitation.
“If we’re irrigating a lot, we’re creating other breeding sites,” he said. “So it’s really a delightfully complicated problem.”
Making a human connection
Of course, to have a bad West Nile year in terms of infections, you also need people near where these mosquito-and-bird transmission cycles are happening — and that’s where all that irrigation comes in.
Look at the top counties for West Nile cases in Colorado and a pattern soon emerges. The highest number of cases this year occurred in Boulder and Montrose counties, followed closely by Weld. Last year, Larimer County — which is the state’s all-time leader in West Nile infections — topped the list, and Delta County frequently appears near the top, as well.
All of those counties have at least relatively sizeable populations and significant amounts of irrigated farmland. Add in Colorado’s status as a major thoroughfare along the Central Flyway for migratory birds and the equation starts to come into focus. People + Water + Birds + Culex mosquitoes = human West Nile infections.
“I don’t think there’s anything different in the base-level ecology of Colorado,” Ebel said in trying to explain why Colorado stands out among other states. “It’s the people around and what we are doing with the environment.”
Colorado’s three worst years for West Nile infections
| Year | Neuroinvasive Infections | Deaths |
|---|---|---|
| 2003 | 622 | 66 |
| 2022 | 132 | 19 |
| 2021 | 92 | 11 |
When controlling the data for population size, Hancock said Delta County — home to acres of fruit orchards and sweet corn fields — has the highest rate of West Nile in the state. People in the county are 15 to 16 times more likely to get West Nile than someone living in Denver, he said.
But it’s not just proximity to agricultural operations that can drive up the chances of getting West Nile. How communities handle storm drainage can also have an impact.
Hancock said the favorite habitat of Culex tarsalis is a cattail marsh. The marshes can exist in naturally occurring wetlands. But they are also common near human-made reservoirs, along creekside bike paths, within golf courses and in stormwater containment basins adjacent to new construction.
“You can see human developments right next to massive cattail marshes,” Hancock said.
Predicting the effects of climate change
The fact that people have created so many of the environments that give rise to the mosquitoes infecting people with West Nile makes it even more complicated to predict how climate change might alter the dynamics.
Hancock is skeptical that it will. No matter how hot and dry it gets in Colorado, human intervention will likely preserve mosquito breeding grounds.
“We have made a relatively controllable mosquito factory,” he said. “So I predict climate change is going to affect everything, but I predict it will impact Colorado less than other places for mosquito epidemiology.”
Instead, the bigger impacts from a warming climate appear to come down to the added time it creates for mosquitoes.
House, the state public health veterinarian, said it takes a hard freeze to kill off mosquito populations for the winter. But in years where that hard freeze is delayed, West Nile season can stretch on and on. As climate change makes fall the fastest-warming season in Colorado, that means West Nile season is poised to lengthen.
“We’re definitely seeing a late season, and the longer the season goes, the more at risk people are,” House said.
Warmer temperatures also create a hack for mosquito biology. Heat speeds up the replication of the virus inside a mosquito. That means the hotter it is, the faster a mosquito is ready to spread the virus to something else — what’s called the “extrinsic incubation period.”
“That’s really important because life is quite dangerous for mosquitoes,” Ebel, the CSU professor, said. “They get killed all the time, they get eaten. Even if it’s just one day, that’s a day when mortality isn’t hacking away at mosquitoes.”
All of these factors can make Colorado’s West Nile problem seem dizzying and unsolvable. But Ebel, Hancock and other researchers are optimistic. They talk of producing mathematical models that predict West Nile spread. Doing so would allow officials to better target prevention efforts, like spraying.
Ebel hopes to conduct more genetic sequencing of West Nile virus samples, to get a better sense of how it moves across the map.
But first, they will need to answer some surprisingly elusive questions about the mosquito that’s to blame.
Building a home for Culex tarsalis
The enclosures Hancock has built to hold his delicate Sabethes mosquitoes are small, scarcely bigger than a microwave.
For his prospective Culex tarsalis enclosure, he had to go bigger. The box is roughly the size of a large refrigerator, and he might need to build it bigger still. This is because tarsalis likes to breed in airborne swarms. You need a lot of mosquitoes and a lot of space to make it work.
Hancock knows of one lab colony in existence in California. But he’s hoping to establish a colony that behaves a little more like the mosquitoes in Colorado do — and that means collecting them in the wild.
“It’s so deep and rich,” he said. “Every group of mosquitoes has their own ways.”
At times, Hancock’s ways can seem a little unique, as well. Feeding his lab colonies — he also has a growing colony of bedbugs for a different research project —with his own blood is done mostly out of convenience, he said. Besides, after years of research on blood-sucking bugs, he no longer develops welts or reacts to their bites, though getting to that point involved quite a bit of itchiness and pain, he said.
If he’s able to establish a tarsalis colony, though, he won’t let them feast on him. Because the mosquitoes will be collected from the wild, they could potentially be carrying West Nile or other diseases when they come into the lab. To feed them, he will use blood from a scientific supplier and a machine that warms the blood to human temperature. No one likes a cold meal, after all.
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If all goes well, the Mosquito Man could soon be decoding one of the great, basic mysteries that makes West Nile such a difficult disease to track and predict. And that could lead to an answer that helps turn the tide on rising West Nile cases in Colorado.
“We don’t even know one of the fundamental questions: When and in what conditions does a bird-loving mosquito eat humans?” Hancock says.
He pauses to think of how to sum up the absurdity of the situation — how a tiny mosquito that doesn’t really want to bite us can transmit a virus that doesn’t really want to infect us and how it all can so profoundly affect human health.
“That’s like us eating dog food.”