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

West Nile virus was once a nominal problem for Utahns, but since August 2003, when it was first discovered in the Beehive State, the number of cases among Utahns has been steadily increasing.

“West Nile virus, or WNV, is primarily transmitted to humans through the bite of an infected mosquito,” says Utah State University ecologist Nora Saarman. “Unfortunately, the population of mosquitoes capable of spreading the virus is increasing in our state and throughout the United States.”

Saarman and her students Caitlin Graybeal, Tyler Seely, Emily Calhoun and Eric Jenkins; USU Department of Watershed Sciences lecturer Andre De Lima Moraes, together with colleagues from VecTech, Inc. and the Connecticut Agricultural Experiment Station recently published conclusions on the range expansion of the southern house mosquito Culex quinquefasciatus, also known as “Quinx”, and hybrids resulting from crosses between the Quinx and the northern house mosquito Culex pipiens, in mid-latitude North America, including Utah, in the journal One health.

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

Using artificial intelligence-based identification tools combined with DNA technology, Saarman and her team are improving the ability to identify mosquito species and monitor their range. They report that Quinx isn't just expanding into new areas; the vanguard of his hybrids, also capable of spreading disease, advances ahead of Quinx within the borders of the hybrid zone.

“Essentially, disease vector genes from Quinx within hybrid mosquitoes may spread ahead of actual species such as outriders or scouts, especially if they provide a selective advantage to the mosquito carriers,” she says.

Climate change and land use change are the main drivers of further mosquito migration.

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

The team's research 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 mosquito-borne disease in humans. According to Centers for Disease ControlIn 2025, more than 47 people in the state became seriously ill from the virus, including three Utahns who died from the virus infection.

“It is well known that West Nile virus is greatly underreported, with case counts showing only one in more than 112 cases,” says Saarman. “This means the number of cases in Utah could exceed 5,000 in 2025.”

As the Cache Valley and the rest of Utah enjoy a dazzling fall and cooler temperatures, the risk of unwanted mosquito-borne illnesses decreases. But cold-blooded flyers remain active until the first severe frost. The risk of illness from the virus remains alarming, Saarman said, as WNV is a serious neuroinvasive disease that can progress to dangerous and long-lasting complications, including meningitis, encephalitis, acute flaccid paralysis and death.

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

“Despite the dry summer in our state, we think mosquitoes are finding the water they need from human sources,” says Saarman. “We believe the timing of the surge is due to the fact that it takes the virus from about May to August to accelerate in the bird-to-mosquito and mosquito-to-bird transmission cycles to a high enough rate to cause spread to humans and companion mammals, including dogs, cats and horses.”

She says the science-based information her team collects will inform the development of comprehensive public health approaches by linking ecology and evolution to the risk of new diseases in both humans and wildlife populations.

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