What’s behind Colorado’s West Nile virus surge?

Scientists express why they think infections rose sharply last year and what they’re doing to control this year’s mosquito population.

Jill McGranahan

April 13, 2022

What’s behind Colorado’s West Nile virus surge? — MSU Denver Biology Professor Bob Hancock, Ph.D., (left) and student Shawn Ward observe mosquitoes in a university laboratory. Photo by Alyson McClaran

As mosquito season hatches in Colorado, scientists want to learn more about what’s behind a significant jump in cases of West Nile virus last year.

Climate change, agricultural irrigation systems and suburban sprawl may all be contributing to the surge of the potentially deadly mosquito-borne virus.

In 2021, there were 175 confirmed cases of the West Nile virus in Colorado. Of those, 11 people died and 101 developed serious neuroinvasive cases such as meningitis and encephalitis. The number of cases represents a 120% increase from the previous five-year average and the largest number of cases in 18 years.

Professor Bob Hancock, Ph.D., (front left) and his students observe mosquitos in a flight tunnel as they fly toward stimuli in an olfactometer. Photo by Alyson McClaran

“The epidemiology of West Nile virus in Colorado is a beautifully complicated story,” said Robert G. Hancock, Ph.D., professor in the Department of Biology at Metropolitan State University of Denver. “Mosquitoes — unlike humans, who become immune once they have been infected — are carriers for life.”

Mosquitoes carry the virus after feeding on infected birds, then transmit it to humans via their bites. There is no vaccine for the virus. While the vast majority of cases are mild, with few if any symptoms, some people develop a serious brain infection that can result in permanent brain damage or death.

“For Colorado to have such an influx of cases,” Hancock said, “we have to believe there is a large number of young birds that have been infected. And we have this because Colorado is a major flyway for migratory species as well as a habitat for crows and jays, which are especially susceptible.”

Pinpointing the cause

Some scientists think the rise in West Nile virus cases may be a sign of things to come.

Studies show that warmer temperatures associated with climate change can accelerate mosquito development, biting rates and incubation of the disease within a mosquito, according to the Environmental Protection Agency.

RELATED: Summer bites

There isn’t yet enough data to determine what proportion of West Nile virus cases is directly driven by climate change, the EPA says, but Hancock thinks other factors may also be at play for last year’s increase, including heavily irrigated agricultural systems.

Mosquito in a lab — Students sort mosquito specimens under a dissection microscope at an MSU Denver Biology lab. Photo by Alyson McClaran

“Because we are a semi-arid, high-desert region using a lot of irrigation, the reservoirs seep and create cattail marshes, which are an ideal habitat for mosquitoes,” said Hancock, who’s known as the “Mosquito Man.” “If you look at water-use statistics from 2021, drier conditions and more irrigation led to much higher usage than in previous years.”

Hancock also believes the Covid pandemic contributed to the increased number of diagnosed cases, as more people were going to their doctors with symptoms such as severe headaches, which could indicate Covid or West Nile. Additionally, more people stayed close to their homes, which often have more standing water and poorer drainage compared with metropolitan areas, creating more opportunities for exposure.

MSU Denver Biology graduate Holly Medina, a supervising technician who works at Vector Disease Control International in Broomfield, believes suburban growth and its effect on water use are primary drivers.

“We made our own problem,” she said. “Colorado is growing. There is a lot of sprawl in the metro area, and these developments have poor drainage.”

MSU Denver Professor Bob Hancock, Ph.D. — MSU Denver Biology Professor Bob Hancock, Ph.D., examines a sample set of Colorado mosquitoes provided by Vector Disease Control International. Photo by Alyson McClaran

Reducing the mosquito population

Medina is one of several MSU Denver Biology graduates working to control and mitigate the mosquito population. She recruits current students to help with mosquito control from April through September.

“We bring in college students each April or May and train them on the different cycles of mosquito breeding,” she said. “We then divide them into two shifts: The daytime crew hunts for larvae, and the nighttime crew sprays pesticide to reduce the adult population.”

Students work in a biology lab — MSU Denver student Claire Sanford marks specimens with fluorescent powder with help from Shawn Ward, a senior in MSU Denver’s Biology program. Photo by Alyson McClaran

Hancock’s tips for mitigating mosquitoes:

Flip over water-collecting, slime-coated backyard tarps and buckets where mosquitoes like to breed, particularly in urban areas.
Be aware that in the Front Range, mosquito activity peaks at dawn and, especially, dusk.
Use insect repellent that contains DEET. There are a lot of myths about DEET, Hancock said, but it’s an effective insect repellent. “I’m a mosquito professional, and I’m 100% in favor of repellent containing DEET.”
Wear breathable, long-sleeve, light-colored shirts and long pants. Spray the DEET on your clothes instead of directly on your skin.
Take a head net along to the mountains.

The students are trained to identify the disease-carrying Culex mosquitoes as well as how to determine nesting areas.

Shawn Ward, a senior in Biology at MSU Denver, also works at Vector Disease Control International.

“I started working with VDCI in 2016 as a larvae technician, where I would drive around 10 hours a day looking for local water areas and searching for larvae, which I would treat with biological pesticide,” said Ward. “In subsequent years, I worked setting and collecting traps for species identification and WNV surveillance.”

Ward said that in his seven years with Vector, he has worked with and recruited multiple other MSU Denver students. “The great thing about these seasonal jobs is that it gives you that hands-on experience in the field,” he said.

Ward noted that his sister Heather, an MSU Denver alumna, worked with him for several years at VDCI and now works as a biological technician identifying mosquito species for the Anastasia Mosquito Control District in St. Augustine, Florida.

“We have a highly regarded Biology program,” said Hancock. “It’s an honor to see these students applying their degrees in such an important endeavor.”