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West Nile Virus was once a nominal concern for Utahns but since August 2003, when it was first detected in the Beehive State, infections in Utah residents have steadily risen.

"West Nile Virus or WNV is mainly spread to humans through the bite of an infected mosquito," says Utah State University ecologist Norah Saarman. "Unfortunately, populations of mosquitoes capable of spreading the virus are increasing in our state and throughout the United States."

Saarman and her students, Katelyn Graybeal, Tyler Seeley, Emily Calhoun and Eric Jenkins; USU Department of Watershed Sciences faculty member Andre De Lima Moraes, along with colleagues at VecTech, Inc. and the Connecticut Agricultural Experiment Research Station, recently published about the expanding range of the Southern House Mosquito, Culex quinquefasciatus, also known as "Quinx," along with hybrids resulting from interbreeding among Quinx and the Northern House Mosquito, Culex pipiens, across mid-latitudes of North America, including Utah, in the journal One Health.

"Both of these species spread West Nile Virus, but Quinx, which is newer to Utah, is better at it," says Saarman, assistant professor in USU's Department of Biology and the USU Ecology Center.

With AI-fueled identification tools, coupled with DNA technology, Saarman and her team are improving the ability to identify mosquito species and monitor their range. They report Quinx is not just spreading into new areas; an advanced guard of its hybrids, also capable of spreading diseases, are moving ahead of Quinx in hybrid zone boundaries.

"Essentially, disease-carrying genes from Quinx, within hybrid mosquitoes, can spread ahead of the actual species like outriders or scouts, especially if they provide a selective advantage to the mosquitoes carrying them," she says.

Climate change and land use change are the main drivers behind mosquitoes' forward migration.

"Mosquitoes need at least two things for survival and those are blood from vertebrate hosts and water," Saarman says. "As Utah's population grows and our state becomes increasingly urbanized, mosquitoes flock to welcoming habitats, including watered lawns, storm drains and households with pets, unintentionally created by human development."

The team's study provides critical data for tracking range shifts, improving vector surveillance and improving understanding of West Nile Virus and other mosquito-borne disease risks.

In Utah, WNV is the most prevalent human illness spread by mosquitoes. According to the , more than 47 people in the state have become seriously ill from the virus in 2025, including three Utah residents who died from the viral infection.

"It's well known that West Nile Virus is highly underreported, with only one in more than 112 infections detected in case counts," Saarman says. "This means the 2025 case estimate in Utah could be well over 5,000."

As Cache Valley and the rest of Utah present dazzling autumn hues and temperatures cool, the risk of unwelcome mosquito-borne diseases wanes. But the cold-blooded fliers remain active until after the first hard freeze. The risk of illness from the virus remains worrisome, Saarman says, as WNV is a serious neuroinvasive disease that can progress to dangerous and lingering complications, including meningitis, encephalitis, Acute Flaccid Paralysis and death.

Cases of WNV usually spike in early September in Utah, she says, which is what happened this year.

"Despite our state's dry summers, we think mosquitoes are finding the water they need from human sources," Saarman says. "We think the timing of the spike is because it takes from around May to August for the virus to amplify in the bird-to-mosquito and mosquito-to-bird transmission cycle to a high enough rate to cause spillover to humans and human companion mammals, including dogs, cats and horses."

She says the evidence-based information her team is gathering will advance integrated approaches to public health by linking ecology and evolution to emerging disease risk in both human and wildlife populations.

"Our targeted approach will also help mosquito abatement managers minimize off-target impacts of insecticide and other interventions," Saarman says. "Being able to monitor the level of insecticide resistance, in which species it's occurring and where it's taken place is a management challenge. Our data will help to counter this challenge."