Colorado has had an unusually quiet West Nile season. Scientists want to know why.

In what is normally his busiest time of the year, scientist Greg Ebel is feeling something odd this year. He’s … relaxed?

“It’s been fairly uneventful for the first time in four years,” he said.

Ebel is a professor of microbiology, immunology and pathology at Colorado State University. His wheelhouse is what are known as vector-borne diseases — in particular diseases transmitted by mosquitoes. For years, he has studied West Nile virus and tracked its patterns through a network of traps in the northern Front Range that search for West Nile’s blood-sucking chauffeurs, mosquitoes from the Culex genus.

The disease, which mosquitoes pick up by biting infected birds, often passes with few to no symptoms in people. But it can be severe in some people, resulting in neurologic damage that can take years to heal from. Even worse, in some cases it is deadly.

Last year, traps across the state overflowed with Culex mosquitoes, sometimes more than 10,000 collected over just a couple nights. The incredible mosquito boom contributed to the worst West Nile season in decades in Colorado — the worst last year in the entire country. When the season ended, Ebel and other mosquito experts in Colorado worried the giant horde of mosquitoes would over-winter and pop out in spring ready to keep the diseased party going.

But that hasn’t happened. Instead, Colorado’s West Nile season has been well below average. In data available as of last week, there have been only 30 documented infections of West Nile reported in the state, with 12 hospitalizations and two deaths. Compare that to last year’s tallies: 634 infections, 386 hospitalizations and 51 deaths.

Ebel said he’s seeing similarly low case numbers this year among the birds that his lab tests for the Rocky Mountain Raptor Program. And his traps are also mostly bare of Culex mosquitoes.

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

Searching for clues

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

In his research, he’s noticed that the percentage of mosquitoes carrying West Nile has remained about the same. So that means the virus is still out there, spreading. It’s just there are not enough mosquitoes around to spread the disease to that many people.

Rather than be frustrated by this mystery, Ebel is taking it as a learning opportunity. In trying to better understand West Nile virus, even in a down year, 2024 is a datapoint.

“I have to admit that I was very surprised that it was as low as it is this year,” he said. “So I guess what that tells us is that the early-season (mosquito) population is not coupled to last year’s late-season population.”

From boom to bust

To Bob Hancock, a professor of biology at Metropolitan State University of Denver, that observation may be the most perplexing finding from this year — how a mosquito boom so quickly turned into a mosquito bust.

Hancock’s specialty is studying blood-sucking insects, and his enthusiasm for the topic is so great it has led to him being known as Mosquito Man. Hancock has hypothesized that the way  the state manages its water supply effectively creates a mosquito factory, even in drought years. There should always be plenty of water — whether in reservoirs or irrigation ditches or around farm fields — for mosquitoes to thrive.

But this year is challenging that notion. He said 2024 has reinforced the case for more rain equaling more mosquitoes.

“It looks like to me that it’s rain-driven, especially early rain-driven,” he said.

(It should be noted that Hancock called The Sun from a state wildlife area in Weld County, where he had taken a group of students to look for mosquito larvae in wetland habitat. He held a long dipper in his hand as he spoke.)

Hancock said it’s possible that Culex mosquitoes are much more selective egg-layers than previously known. While there was still plenty of water to be found in ponds and drainages across the state this spring, perhaps the preferred egg-laying sites for Culex mosquitoes 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 zapped them. As he spoke, he began to formulate research ideas to better understand what happened.