Why does Larimer County always have higher levels of West Nile virus? Here's what we found

When it comes to West Nile virus infections in humans, Larimer County's case count exceeds all other counties in Colorado. And year after year, that's typical.

In fact, data shows Larimer, Weld and Boulder counties have reported far more cases since the virus was first detected in 2002 and began being tracked in 2003. Case numbers from 2003 to 2022:

• Larimer: 1,012

• Weld: 860

• Boulder: 837

Denver metro area counties have reported half as many cases but have higher populations than Larimer, Weld and Boulder:

• Adams: 431 cases

• Arapahoe: 234

• Denver: 317

• Jefferson: 286

So why does West Nile virus appear to be more prevalent in Northern Colorado?

We asked Colorado public health officials and an insect-borne disease researcher to see if we could answer this question or provide insights. Here's what we learned:

Rivers and agriculture use create more standing water

When West Nile virus first came into the state, Larimer County and areas toward the plains along the South Platte River basin quickly became the dominant spot for West Nile, said Brian Foy, a Colorado State University professor who studies insect-borne diseases in his Foy Lab in CSU's Center for Vector Borne Infectious Diseases.

That's because that’s where the water is, where irrigation is prevalent and where it gets used for agricultural purposes, he said.

The mosquitoes don’t live in the river but in the canals and flooded fields: anywhere water pools.

As you go farther away from the river basins or into the mountain elevations, the number of cases drops, Foy said.

In recent years, he said, West Nile has become more prevalent on the Colorado River basin into Grand Junction, where you also see more irrigation.

"In our state, it’s a disease on river basins, including the Arkansas River," Foy said.

While Denver metro counties may have a higher population, they don't see as much agricultural use of the land. Larimer has both a population center and a lot of ag use.

Outside of Northern Colorado and the Denver metro area, Grand Junction's Mesa County has a higher rate of cases, with 290 since 2002.

Do some counties just not report West Nile virus activity?

Not all counties monitor West Nile virus in mosquitoes, but all health care providers and labs in every county are required to report human cases to public health departments, according to AnneMarie Harper, communications director for the Colorado Division of Disease Control and Public Health Response.

In each case, patients are interviewed to find out where they may have been exposed, she said.

Currently, 11 Colorado counties monitor mosquito activity, which includes trapping them and testing them for West Nile, Harper said: Adams, Arapahoe, Boulder, Delta, Denver, Jefferson, La Plata, Larimer, Mesa, Pueblo and Weld counties.

This information is used to to guide decisions regarding mosquito control and prevention. So while not all counties monitor West Nile activity in mosquitoes, all cases in humans must be reported.

Why don't we have better treatments or a vaccine for West Nile?

CSU has been studying mosquito-borne diseases since the 1980s, well before West Nile virus ever showed up in Colorado, Foy said.

But in 2023, there is no specific treatment for neuroinvasive West Nile, only supportive treatment, according to health officials.

Why is that?

There is a vaccine for livestock, like horses, Foy said. But “to develop those for humans is a different ballgame.”

Science can easily develop a vaccine, and six or seven have been made and brought to phase 2 clinical trials, he said. But the reality is that a West Nile vaccine isn't commercially viable because it’s less likely to make money for a pharmaceutical company.

If you’re only vaccinating people in small river basin areas of Colorado, for example, the companies would be investing billions of dollars without getting the return they want.

"It's going to take time and effort and money, but that’s going to be a hard barrier,” Foy said.

His lab, however, is working on vaccines for wildlife, like the birds that play a part in West Nile transmission, Foy said.

How do mosquito control methods affect cases?

That's still unclear.

Harper said it can be more difficult to treat agricultural areas like ponds and pastures with mosquito control measures, citing this as a possible reason why cases are higher in those areas.

But control methods across cities, counties and states are not consistent, Foy said, so researchers are trying to understand what different entities are doing now.

For example, Fort Collins and Larimer County have triggers for insecticide spraying, like when the vector index reaches a certain level. Towns to the east, like Timnath, spray throughout the summer, even when the vector index falls below Larimer County's. 0.5 threshold. Other towns don't spray at all.

But this is a question that could be answered with help from CSU as part of a new partnership funded by the Centers for Disease Control and Prevention.

The Rockies and High Plains Vector-borne Diseases Center has received $8.75 million to study zoonotic diseases in Colorado, New Mexico, Utah, Wyoming and the Texas panhandle in collaboration with the Colorado Department of Public Health and Environment, Texas Tech University and the Salt Lake City Mosquito Abatement District.

The new center will study West Nile, but also tick-borne Colorado tick fever and flea-borne plague.

The team will collaborate not just with academia but also with public health departments and vector control companies, all while training people for the workforce, Foy said.

Larimer County reports first West Nile death of 2023

Larimer County reported its first fatality due to West Nile virus on Tuesday.

The 66-year-old resident had been hospitalized since the end of July with neuroinvasive West Nile virus, according to a news release from the Larimer County Department of Health and Environment.

“We are saddened to report the passing of one of our residents," Tom Gonzales, Larimer County public health director, said in the news release. "Unfortunately, we will likely continue to see cases of West Nile Virus for the next month or two. We continue to see positive pools of mosquitoes from the weekly trapping and testing.”

In Larimer County, there have been 29 confirmed cases this year, including eight hospitalizations, the health department reported.

In the state of Colorado, there have been three deaths due to West Nile virus this year, according to the Colorado Department of Public Health and Environment. The first death of the season was reported in Weld County on Aug. 4.

There are 72 cases of reported West Nile virus in Colorado, according to state data. Forty-one people have required hospitalization.

"That’s a sharp increase from the beginning of last week when there were 36 human cases, including one death, and 19 people hospitalized," Harper said in an email to the Coloradoan. "This is also more cases than we would typically see at this time of year."

Data from the past five years shows an average of two cases having been reported by this week of the summer.

This article originally appeared on Fort Collins Coloradoan: Why West Nile virus is more prevalent in Larimer County, Northern Colorado