Newly Arrived Mosquito, Kissing Bugs Bring New Disease Threats to Colorado

West Nile virus is currently the best-known insect-borne illness in Colorado, and its effects can be absolutely devastating. Last year, Westword detailed the struggles of Rick Enstrom, a Denver-area resident who was essentially paralyzed by West Nile and is still battling back more than two years after having been stricken.

Colorado led the country in reported West Nile cases in 2023, but the virus could be getting some unwanted competition.

According to Bob Hancock, a biology professor at Metropolitan State University of Denver, a mosquito capable of carrying dengue, yellow fever and Zika has now become a fulltime resident of one Colorado community and could spread across the state under the right circumstances. Meanwhile, insects that can already be found in the Denver area known as kissing bugs — blood-suckers whose name refers to their tendency to bite people on the face — are raising new concerns because they can carry the parasite that causes Chagas disease, a horrific ailment that a growing number of scientists want declared endemic in the United States.

The mosquito in question, known by scientists as Aedes aegypti, is “an insanely effective invader” that thrives in tropical or subtropical climates. Colorado’s high-altitude and arid environment were thought to be uninhabitable, but the mosquito “is somehow defying its own biology by radical, aggressive northward movement,” Hancock notes. As for Chagas disease, he says, “This stuff is nothing short of a nightmare.”

Hancock, who’s been studying mosquitos since the 1990s, is an internationally known expert in the field. He’s earned the nickname “The Mosquito Man” and even has his own theme music courtesy of The Diane Jobe Band. (Jobe is married to Hancock, who plays trumpet in her group; they were recently filmed playing the ditty about mosquitos for a Canadian Broadcasting Corporation documentary, which is expected to debut in 2026.)

Aedes aegypti are characterized by what Hancock calls “their dark color and striking, silvery-white markings on their legs and their bodies.”

“We don’t have anything else in the state that looks like them,” he says. Thus far, “they’re only established in one place so far: the Grand Junction area,” on Colorado’s Western Slope. Personnel in Grand Junction “caught them in their traps a couple of years ago as part of their normal mosquito surveillance,” Hancock continues, “and they demonstrated that they showed up the next year by changing some of their tactics and setting special traps to collect both the adults and, with something called an ovitrap, the eggs the mosquitos lay. Then, this past season, they were able to demonstrate that they’re still there. So apparently, they made it through the winter.”

This scenario is particularly troubling given how resistant Aedes aegypti are to the type of mosquito-mitigation methods that have proven effective locally in the past.

“The behavior of our typical mosquitos is that most of them become active at dusk,” Hancock explains. “That’s the peak of their activity, but they also show some activity into the night, and often have a dawn peak of activity, too. But they’re not buzzing around during the middle of the day unless you’re walking in a shady area. That’s why we aggressively deploy aerial spraying at dusk, and that gives us the ability to wipe out huge numbers of mosquitos in order to knock the populations down.”

In contrast, Aedes aegypti are active during the day and “very ready to bite us,” Hancock points out.

“Most mosquitos breed in mating swarms, where a group of males are flying in a particular place at dusk, and when females go flying through that swarm, they get aggressively attacked in the air by these males. But Aedes aegypti males don’t swarm. They actually come to the host, which is us, and fly around in a little figure-eight holding pattern usually low on our bodies, around our legs. Then, when the females come to bite us, they beeline toward the female and try to copulate,” he explains. “They’re truly human-oriented, which is one of the reasons why they have caused so much historical and contemporary disease in humans.”

That doesn’t mean the Aedes aegypti mosquitos in Colorado are actively spreading sickness. However, the prospects for something like that happening are very real. Take dengue, which can be fatal, but more often results in aches, pains and/or nausea and vomiting.

“The way it would work in a place like this would be for a mosquito feeds on somebody who is viremic with dengue — likely someone who’s traveled in South America or Central America, or even in the southern U.S.,” Hancock says. “If the mosquito that took that infected blood meal of dengue is able to survive for a week or a little longer and then goes on to bite somebody else, then you could truly have a locally acquired case of dengue.”

Yellow fever and Zika are both passed on to humans under the same circumstances. A viral disease that can lead to organ failure, fever, aches and jaundice, causing yellow-hued skin and eyes, yellow fever can be fatal in some cases. Zika’s symptoms are often more mild, such as a headache, joint pain or rash, but in some cases Zika can trigger severe neurological disorder and brain defects when passed from a pregnant mother to an unborn child.

Can Aedes Aegypti Mosquitos Migrate to Denver?

Could Aedes aegypti successfully cross the Continental Divide and take root in greater Denver? Hancock doesn’t dismiss this possibility, especially in light of climate change. In his words, “we’ve seen too many examples of mosquitos setting up camp in places we’ve never seen them before. So relevant questions for the biology of this mosquito are: Does a mean increase in temperature that we measure over time as our world warms help them survive?…Or would a brutal, lethal freezing event keep these guys from getting established?”

Hancock says that kissing bugs have shown a tendency to carry Chagas disease, a disease he calls “somewhat comparable to malaria.” Chagas disease is carries by single-celled parasites called trypanosomes, which infest the blood.

“These parasites are very complex and, ultimately, very difficult to kill. If you’re unfortunate enough to get Chagas disease, the treatment against these parasites in your body is better described as chemotherapy. Some of the chemistry they use to kill these parasites has arsenic in it, so it’s a very serious disease that can ultimately lead to the failure of certain muscles,” he explains. “The worst two places the pathology can manifest itself would be the digestive-tract musculature and the cardiac muscle. What happens is that people who grew up in different parts of the world or extensively traveled in another part of the world go in for cardiac problems, and they’re finding out they’re caused by Chagas disease.”

At present, there have only been a handful of Chagas disease cases reported in the U.S., and they’ve been limited to eight southern states. Additionally, Colorado’s kissing bugs are less likely to pass along the disease than those living elsewhere. “The vector competency factor is certainly in our favor,” Hancock notes. “Our species isn’t as good a vector as some of the ones in Texas.”

Then again, insects are capable of travel. For example, mosquitos might catch a ride on a truck driving from other parts of the country.

“The number of mosquitos coming our way and the number of potential infesting events are only on the increase,” Hancock stresses. “So they’re coming, for sure. We can’t deny that. The big question is, if we catch them in our traps in the summer, are they going to be there the next year?”