Science Provides Answer to West Nile’s Quiet Season | Western Colorado

Ebel is a professor of microbiology, immunology and pathology at Colorado State University. His specialty is vector-borne diseases—particularly those borne by mosquitoes. For years, he has studied West Nile virus and tracked its patterns using trap networks across the northern Front Range that seek out West Nile’s bloodsucking chauffeurs, the Culex mosquito.

The disease, which is spread by mosquitoes biting infected birds, is often asymptomatic or mild in humans. But it can be serious in some people, causing neurological damage that can take years to heal. Worse, it can be fatal in some cases.

Last year, traps across the state were brimming with Culex mosquitoes, sometimes more than 10,000 collected in just a few nights. The incredible mosquito boom contributed to Colorado’s worst West Nile season in decades — the worst in the country last year. When the season ended, Ebel and other Colorado mosquito experts worried that a giant horde of mosquitoes would survive the winter and emerge in the spring, ready to continue the sick party.

But that didn’t happen. Instead, Colorado’s West Nile season has been far below average. In data available last week, there were only 30 documented West Nile infections in the state, with 12 hospitalizations and two deaths. Compare that to last year’s totals: 634 infections, 386 hospitalizations and 51 deaths.

Ebel said he’s seen similarly low case numbers this year among the birds his lab tests as part of the Rocky Mountain Raptor Program. And his traps are also mostly free of Culex mosquitoes.

“I can’t give you a good answer as to why that is,” he said. “I don’t know.”

The weather may have something to do with it — this summer has been much warmer and drier along the Front Range than last year, when the season was unusually wet and cool. Ebel said it’s also possible that virus-carrying mosquitoes, gathered in such large numbers, caught the virus themselves.

In his research, he noted that the percentage of mosquitoes carrying West Nile has remained about the same. That means the virus is still out there and spreading. There just aren’t enough mosquitoes to spread the disease to as many people.

Rather than be frustrated by the mystery, Ebel sees it as a learning opportunity. In trying to better understand West Nile virus, even in a bad year, 2024 is a data point.

“I have to admit I was very surprised that there were so few this year,” he said. “So I think that tells us that the early season (mosquito) population is not related to the late season population from last year.”

FROM BOOM TO FALL

For Bob Hancock, a biology professor at Metropolitan State University in Denver, the observation may be the most puzzling discovery of the year — how a mosquito boom turned so quickly into a mosquito epidemic disaster.

Hancock’s specialty is studying blood-sucking insects, and his enthusiasm for the subject is so great that he became known as the Mosquito Man. Hancock hypothesized that the way a country efficiently manages its water supply creates a mosquito factory, even in drought years. There should always be plenty of water—whether in reservoirs or irrigation ditches or around crops—for mosquitoes to thrive.

But this year challenges that notion. He said 2024 strengthened the argument that more rain equals more mosquitoes.

“I think it’s because of the rain, especially the early rain,” he said.

(It should be noted that Hancock called The Sun from a nature preserve in Weld County, where he had taken a group of students to look for mosquito larvae in a marsh habitat. He was holding a long scoop in his hand as he spoke.)

Hancock said it’s possible that Culex mosquitoes are much more selective egg-laying than previously thought. Although there was still plenty of water in ponds and drains across the state this spring, perhaps the Culex mosquitoes’ preferred egg-laying sites were dry.

He said it’s also possible that all those mosquitoes from last year didn’t survive the winter—maybe a deep freeze or something else hit them. As he talked, he began formulating research ideas to better understand what happened.

Then he burst out laughing and said something he had said many times before, thinking about the customs associated with West Nile in Colorado:

“It’s so beautifully complicated.”