West Nile virus is back. Here's how Colorado is responding.
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SEVERANCE, Colo. — Ellie McEwen fiddled with a mosquito trap at the tailgate of her pickup.
The trap looks like a science fair project assembled the night before it was due. It consists of a mosquito net, a battery powered fan and a Coleman jug filled with dry ice.
McEwen secured the trap around a tree trunk at a housing development north of Highway 14. As the dry ice warms, it releases carbon dioxide that attracts mosquitoes.
“We just plug it in, and the fan sucks them in,” said McEwen, 25, who began trapping ‘skeeters professionally for Vector Disease Control International in 2020.
Mosquito trapping is one of the primary ways scientists monitor mosquito numbers and the prevalence of mosquito-borne illnesses, like West Nile virus.
In late June, Larimer County confirmed the presence of West Nile virus in mosquitos it tested. Since then, mosquitos in Denver, Boulder, Weld and Broomfield counties have also tested positive for the virus.
The Colorado Department of Public Health and Environment and Boulder County Public Health confirmed the state’s first human death of West Nile virus this year on July 25. The case involved an adult in Lafayette.
The virus reached the United States in 1999, at which point it devastated bird populations in New York and led to a wave of mysterious neurological conditions in people. Colorado recorded its first cases in 2002.
Although 80% of people who are infected with West Nile virus show no symptoms, West Nile virus can cause debilitating, sometimes fatal, neurological symptoms in a small number of patients (roughly 1 in 150 cases).
“Serious illness can occur in people of any age, however people over the age of 50 and some immunocompromised persons are at the highest risk for getting severely ill,” according to the World Health Organization.
From 1999 to 2024, Colorado recorded the second-highest number of West Nile virus cases with 6,774, trailing only California. In 2023 — a particularly bad year — Colorado recorded 51 deaths from the illness.
Mosquito management varies widely by city and county.
Severance, Colorado, for example, employs mosquito fogging operations to spray permethrin insecticide any time mosquito traps contain more than 100 mosquitoes.
Cities like Fort Collins test mosquitoes for West Nile virus and calculate the risk of infection before implementing fogging operations.
According to the Centers for Disease Control, when truck spraying operations are performed correctly using EPA approved insecticides, they don’t pose a risk to humans or the environment.
Although Colorado is home to many types of mosquitos, Culex tarsalis mosquitos, which are particularly adept at spreading West Nile virus, thrive in the state, said Greg Ebel, director of the Colorado State University Center for Vector Borne Infectious Diseases and a professor in the Department of Microbiology, Immunology and Pathology.
Ebel’s lab tests mosquitoes caught in traps, like the ones McEwen sets.
Testing for West Nile virus involves grinding the bugs into a “mosquito sludge,” using a centrifuge to extract a solution that contains the virus, then analyzing the solution's genetic information.
In order for a mosquito to become a disease vector, it must first become infected with the disease itself. For example, a mosquito might feed on the blood of a bird that is infected with high levels of West Nile virus.
“Compared to any other mosquito that I've ever studied, the likelihood of a Culex tarsalis mosquito getting infected if it is exposed to this virus is really, really high,” said Ebel.
The landscape of Northern Colorado could also play a role. Irrigated agriculture, with its network of reservoirs, ditches and furrows, provides numerous opportunities for mosquitoes to reproduce.
“The way that we use the land is quite favorable to Culex tarsalis,” said Ebel.
Climate change is expected to exacerbate human cases of West Nile virus, according to the Environmental Protection Agency.
“At warmer temperatures, mosquitoes transmit viruses faster,” said Ebel.
Coloradans can reduce their risk of West Nile Virus by following the “Four Ds:”
DEETWear insect repellant with at least 10 percent DEETDressDress in long pants and sleevesDusk and DawnAvoid exercising during dawn and dusk when mosquitoes are most activeDrainDrain standing water. (One small can of water can hold as many as 10,000 mosquito larvae.)
Although horses can be vaccinated against West Nile Virus, no such vaccine exists for humans. Treatment consists of supportive care.
Ebel said the future of West Nile Virus prevention will likely involve better tools for managing mosquito populations. He gave the example of research that fellow Colorado State University professor, Brian Foy, is leading, to develop bird seed that would make a bird’s blood toxic to mosquitos, as well as a vaccine against West Nile Virus for wildlife.
Mosquitos are most active in Colorado from June until September.
Type of story: News
Based on facts, either observed and verified firsthand by the reporter, or reported and verified from knowledgeable sources.
Based on facts, either observed and verified firsthand by the reporter, or reported and verified from knowledgeable sources.
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