Ecologists report spread of disease-carrying mosquitoes and their hybrids in North America

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

“West Nile virus, or WNV, is primarily transmitted to humans through the bite of an infected mosquito,” says Norah Saarman, an ecologist at Utah State University. “Unfortunately, populations of mosquitoes capable of spreading the virus are increasing in our state and across 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 from VecTech, Inc. and the Connecticut Agricultural Experiment Research Station, recently published findings on the growing range of the southern house mosquito, Culex quinquefasciatus, also known as “Quinx,” as well as hybrids resulting from crossings between Quinx and the northern house. Mosquito, Culex pipiens, in mid-latitudes of North America, including Utah, in log One health.

“Both of these species spread West Nile virus, but Quinx, which is newer to Utah, is better at it,” says Saarman, an assistant professor in the USU Department of Biology and USU Ecology Center.

Using AI-based identification tools and DNA technology, Saarman and his team are improving the ability to identify mosquito species and monitor their range. They report that Quinx is not only spreading into new areas; an advanced guard of its hybrids, also capable of spreading diseases, outpaces Quinx within the limits of the hybrid zone.

“Essentially, Quinx disease-carrying genes, in hybrid mosquitoes, can spread before actual species like scouts or pathfinders, especially if they confer a selective advantage to the mosquitoes carrying them,” she says.

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

“Mosquitoes need at least two things to survive: blood from vertebrate hosts and water,” says Saarman. “As Utah’s population grows and our state becomes increasingly urbanized, mosquitoes flock to hospitable habitats, including watered lawns, storm drains and households with pets, created unintentionally by human development.”

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

In Utah, WNV is the most common human disease transmitted by mosquitoes. According to the Centers for Disease Control, more than 47 people in the state became seriously ill from the virus in 2025, including three Utah residents who died from viral infection.

“It is well known that West Nile virus is highly underreported, with only one in more than 112 infections detected in the case count,” says Saarman. “This means the 2025 case estimate in Utah could be well over 5,000.”

As Cache Valley and the rest of Utah experience dazzling fall hues and cooler temperatures, the risk of unwanted mosquito-borne diseases decreases. But cold-blooded birds remain active until after the first severe frost. The risk of illness from the virus remains concerning, Saarman says, because WNV is a serious neuroinvasive disease that can progress to dangerous and persistent complications, including meningitis, encephalitis, acute flaccid paralysis and death.

WNV cases typically spike in early September in Utah, she said, which has happened this year.

“Despite our state’s dry summers, we believe mosquitoes find the water they need from human sources,” says Saarman. “We believe the timing of this peak is because it takes approximately May to August for the virus to amplify in the bird-mosquito and mosquito-bird transmission cycle to a rate high enough to cause spread to humans and pet mammals, including dogs, cats and horses.”

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

“Our targeted approach will also help mosquito control managers minimize off-target impacts of insecticides and other interventions,” says Saarman. “Being able to monitor the level of insecticide resistance, in which species it occurs and where it occurs poses a management challenge. Our data will help address this challenge.”