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In this Aug. 26, 2019, file photo, Salt Lake City Mosquito Abatement District biologist Nadja Reissen examines a mosquito in Salt Lake City. (AP Photo/Rick Bowmer)

Colorado has just endured its worst West Nile season since the virus became endemic in the state in the early-2000s.

As of Wednesday, the state had seen 613 cases of West Nile, including 373 hospitalizations and 309 neuroinvasive cases — the most severe form of the disease. There have been 45 deaths reported.

Those numbers beat every other year in Colorado’s history with West Nile, save for the epidemic year of 2003, when the virus really took hold in the state. The 45 deaths are more than double the number last year, which previously held the mark as the deadliest West Nile year in the state since 2003.

Only California, Nebraska and Illinois have also reported more than 100 cases of West Nile so far this year, but none has even half as many cases as Colorado does. (With the first hard freezes of fall, Colorado’s West Nile season is officially over, but it may last longer in other states.)

So West Nile is emerging as a significant public health threat in Colorado, though certainly not on the level of other viral illnesses like COVID, flu or RSV. But the tools we have to deal with it are lacking.

Spraying and trying to kill mosquito larvae are pretty much it. Health authorities otherwise encourage people to wear bug spray and long pants and sleeves.

There is no West Nile vaccine for humans. So are we just stuck with this situation?

Two professors at Colorado State University hope not. And they are in the early phases of trying a novel approach to controlling West Nile: If we can’t vaccinate humans, can we vaccinate birds?

But first, let’s back up.

Why isn’t there a West Nile vaccine for humans?

The idea of vaccinating against West Nile isn’t far-fetched. There already is a vaccine for horses, who can also get sick and die from West Nile. And it works quite well.

“I think most people are now including it in their core vaccines every year, so it’s dramatically decreased the number of cases,” said Kathryn Wotman, an assistant professor in CSU’s College of Veterinary Medicine.

There have also been several promising clinical trials on potential human vaccines, including some that have completed Phase 2 trials in the typical three-phase system. But there, they hit a brick wall.


First there’s the challenge of conducting large Phase 3 trials, during which a vaccine is tested for safety and effectiveness in the field. In Colorado this year, the county with the highest number of West Nile cases was Denver, with 93. But that amounted to only 0.13 cases per every 1,000 people in the city.

Based on those numbers, a vaccine clinical trial would need to enroll tens of thousands of people in Denver to be able to test whether it could effectively prevent cases of West Nile.

“If case counts are low in areas chosen for clinical trials, enrollment might take years to complete,” a team of scientists from the Centers for Disease Control and Prevention’s Division of Vector-Borne Diseases, based in Fort Collins, wrote in a recent article for The New England Journal of Medicine.

There’s also a market consideration: Once a vaccine is approved, would enough people want to get it to be able to recoup the costs invested in its research and development?

So that brings us back to the birds.

How a bird vaccine for West Nile might work

West Nile is a virus primarily passed back and forth between Culex mosquitoes and passerine birds. Humans are a dead-end host for the virus; only birds build up enough virus in their systems that mosquitoes can extract the virus in a bite and continue the transmission chain.

This means that if you control West Nile in birds, you might be able to lower its spread to humans.

CSU professor Brian Foy, who researches mosquito-borne diseases, has worked this angle a bit before with a project to study whether it’s possible to dose birds with the antiparasitic drug ivermectin. When mosquitoes get a blood meal full of ivermectin, they die.

Colorado State University professor Brian Foy works in an outdoor “mesocosm” used to study the biting behavior of mosquitoes on chickens. The research is part of Foy’s effort to understand whether feeding ivermectin to birds can help kill mosquitoes and reduce the spread of West Nile virus. (Provided by Brian Foy)

That project is set for field trials next summer, Foy said. But he is now also talking with fellow CSU professor Gregg Dean, a vaccine researcher, to study whether they can create a bird vaccine for West Nile using bacteria found in yogurt.

Dean has developed this vaccine “platform” to fight other viruses, first rotavirus and, later, the SARS-CoV-2 coronavirus. It’s essentially a genetically modified version of the beneficial gut bacterium Lactobacillus acidophilus that Dean can tweak so that it resembles whatever virus he wants to target. Its superpower, though, is that it produces a vaccine that can be taken orally and stored at room temperature and that has a long shelf life.

So, problem, meet potential solution: Vaccine-infused bird seed that gives birds immunity against West Nile virus.

Will it work?

Foy is optimistic but also experienced enough to know that optimism isn’t always enough.

“There’s a lot of challenges,” Foy said. “This is really, really kind of early days in figuring things out.”

There needs to be testing for safety and efficacy. The scientists will need to know how many birds in a given area feed at feeders and how many feeders need to have the vaccine to provide adequate coverage. They’ll need to know how long immunity lasts, which means they’ll need to better understand the avian immune system.

It’s a lot to overcome, but you never know until you try.

“I don’t have any clue about how long immunity would last,” Foy said. “Our first challenge is to prove it could work.”

John Ingold is a co-founder of The Colorado Sun and a reporter currently specializing in health care coverage. Born and raised in Colorado Springs, John spent 18 years working at The Denver Post. Prior to that, he held internships at...