Kentucky's mosquito season is getting longer. See how many more days in your area

Cities in Kentucky and neighboring states are seeing more warm, humid days suitable for mosquitoes each year, bringing a heightened risk of disease to the region.

Louisville’s mosquito season increased by an average of 16 days from 1979-2022, according to data and analysis from Climate Central and gridMET . Other cities across the Ohio River Valley saw larger increases.

The analysis defined “mosquito days” as having both an average relative humidity of 42% or higher, and a daily minimum and maximum temperatures between 50-95 degrees Fahrenheit.

As Kentucky’s climate becomes warmer and wetter, mosquitoes in the state have more time to populate — and more opportunity to potentially spread diseases to humans, according to Shannon LaDeau, a disease ecologist at the Cary Institute of Ecosystem Studies in New York.

“That length of time really does have an effect on how abundant the population of mosquitoes can get,” she said, “but also how likely they are to be able to transmit a pathogen ... like West Nile virus.”

While mosquito-suitable days in the Louisville area have varied in recent decades, they’ve trended upward on average.

In recent years, the city has averaged about 170 mosquito days each year, or about 47% of the year.

Every other Kentucky area examined in the Climate Central analysis also saw increases.

Hazard, for example, saw 27 additional days suitable for mosquitoes each year compared to 1979 — with 164 mosquito days in 2022.

Nationally, the analysis found an overall trend toward longer mosquito seasons.

“From 1979 to 2022, 173 locations (71% of the 242 analyzed) experienced an increase in annual mosquito days,” according to Climate Central.

And along with the Northeast, the Ohio River Valley region — defined in the data to include Kentucky and most of its neighbors — “saw the largest region-wide increase in mosquito days since 1979.”

How mosquitoes benefit from warm climates

Kentucky is home to at least 50 different recorded species of mosquito, LaDeau said, and most don’t typically bite humans.

“It's really a handful of species that are a nuisance or a risk to humans,” she said. “Unfortunately, some of those are the most likely to benefit from longer mosquito seasons.”

Mosquito populations benefit from a longer habitable season each year, but how warm it is during those days can also make a difference.

Heat accelerates the physiological processes of mosquitoes — including the insect’s growth from eggs to adulthood.

Even a difference of a few degrees can shave days off of a mosquito’s growth to adulthood, LaDeau said, allowing populations to grow faster.

“A couple of days doesn't seem like that much,” she said, “but when you're building a population over generations, within a given six months, it makes a big difference.”

But warmer days can also help another internal process: the replication and buildup of a virus a mosquito may be carrying.

“It's both the length of days, but also how warm it is during those days, that will influence changes in vector-borne disease,” LaDeau said.

The Centers for Disease Control and Prevention reported more than 1 million cases of vector-borne diseases in U.S. from 2003-23.

Kentucky is vulnerable. The Louisville Metro Department of Public Health and Wellness has detected local mosquitoes carrying West Nile virus in recent years.

“Rising temperatures and changing rainfall patterns can boost mosquito and tick populations and make it easier for them to spread harmful pathogens to people,” according to the CDC . “As a result, illnesses like Lyme and West Nile virus diseases are increasing.”

However, some communities are seeing fewer mosquito days compared to 1979, according to Climate Central.

Most of these locations are in the South, “where summer temperatures may frequently exceed the upper range” suitable for mosquitoes.

Amplified risk in disadvantaged neighborhoods

The public health threat brought on by larger mosquito populations disproportionately affects less affluent communities.

Low-income households have less access to mosquito controls, like air conditioning, and health care in the event of disease outbreaks.

But the physical environment of disadvantaged neighborhoods can also support mosquito populations.

“Areas with histories of disinvestment in housing and infrastructure,” research shows , “are potential footholds for (Asian tiger mosquito) populations and vector-borne disease transmission.”

In neighborhoods with less wealth, abandoned property, illegal dumping and maintenance problems may follow.

Standing water in abandoned tires and trash cans offer a favorable breeding ground for mosquitoes.

“That really profoundly affects mosquito ecology in those places,” LaDeau said.

Connor Giffin is an environmental reporter for The Courier Journal. Reach him directly at cgiffin@gannett.com or on X @byconnorgiffin .

This article originally appeared on Louisville Courier Journal: Kentucky's mosquito season is getting longer. See how many more days in your area