Climate Change Fuels Rise of Mosquito-Borne Diseases in Temperate Regions

With summer heat comes pool parties, beach days, backyard cookouts, and, of course, swarms of bloodthirsty mosquitoes.

But while insect bites have always been a side effect of time spent outdoors, the species doing the biting are changing in historically temperate regions like New England. As climate change makes these areas warmer and wetter, their ranges are expanding—and any diseases they carry come with them.

In Connecticut, for example, a statewide mosquito monitoring program has detected 54 different species, including invasives like the Asian tiger mosquito, which can transmit potentially serious diseases including dengue and Zika. The mosquito’s historical territory is in hot and humid climates farther south, but it has been moving north.

“There are a number of new species that are creeping into our area,” said Philip Armstrong, chief scientist at the Connecticut Agricultural Experiment Station, which coordinates the state’s mosquito trapping and testing program.

Programs like these are key for preventing mosquito-borne diseases, especially as climate change alters the risks. “You really do have to test the mosquitoes to know where the hot spots are for these viruses,” Armstrong said. “By the time we learn about human cases, it’s usually too late to do anything.”

There aren’t statewide monitoring programs in much of the country. Instead, a patchwork quilt of more than 1,000 mosquito control agencies tries to keep ahead of an evolving problem. Most are run at the local level, with a wide range of organizational structures and monitoring practices.

The US ought to have a national surveillance database collecting and sharing information from all monitoring programs, said Dan Markowski of the American Mosquito Control Association, a nonprofit that works to reduce mosquitoes and vector-transmitted diseases. But, he added, “it all obviously comes back to money.”

Last week, Connecticut announced that mosquitoes in the state have already tested positive for West Nile virus this season. The virus, which appears every summer, has become the leading cause of mosquito-borne disease in the Northeast. While most infections are asymptomatic, it can cause flu-like symptoms and has resulted in more than 3,300 deaths since it first appeared in the US in 1999.

Connecticut established its monitoring program two years before that to monitor for a different virus: Eastern equine encephalitis, a rare but serious mosquito-borne disease that can cause neurological issues and has a roughly 30 percent fatality rate. While it is still uncommon, outbreaks are becoming more frequent in New England.

“There’s these cycles of increased virus activity, and we didn’t see that before, historically,” Armstrong said. “It has the hallmarks of something that’s being affected by climate change.”

In temperate regions like the Northeast, global warming can alter mosquito-borne disease risks not only by expanding the range of virus-carrying insects but also by lengthening the transmission season, reducing the number of mosquito predators and changing habitats, among other factors. Researchers predict that tropical mosquito-borne diseases like Zika, dengue, chikungunya, and malaria will likely become established in temperate areas because of climate change.

“As the temperatures rise, you can actually speed up mosquito development, so you can have multiple cycles of mosquitoes every year in new areas,” said Brian Leydet, who studies mosquito- and tick-borne diseases at the State University of New York College of Environmental Science and Forestry.

Beyond mosquitoes, Leydet said environmental changes can also affect viruses and their original hosts, such as birds and deer, creating complex ecological shifts that can be difficult to study.

Leydet helped establish a monitoring program in St. Lawrence County, New York, in 2024, after an Eastern equine encephalitis outbreak. Unlike Connecticut, New York does not have a statewide program, and many counties “lack the infrastructure and budget to conduct regular mosquito monitoring,” according to the project announcement.

Monitoring programs are typically labor-intensive and expensive, requiring teams to set and check traps, alongside specialists who can identify, sort, and test the samples. While some traps attract female mosquitoes with “stinky water” loaded with decaying organic material, others require dry ice to release carbon dioxide that mosquitoes sniff out when hunting for mammals. But in rural parts of New York, dry ice can be hard to come by—the team had to make their own.

Even beyond the logistical and resource challenges, communication, coordination, and data sharing for mosquito monitoring can be a challenge.

“One of the problems with states that are not comprehensively doing these surveillance programs is that surveillance is hit or miss,” Leydet said. “A lot of these surveillance programs are run by the counties, and they’re not really talking to each other.”

Leydet’s lab found that this patchwork system means some invasive mosquito species are flying under the radar.

Funding is another challenge to better surveillance. “If the county doesn’t have money or resources, these programs fade away,” Leydet said. “If we don’t have these surveillance programs, then all we’re doing is responding to a problem when it’s already a problem, and that’s never how prevention works.”

That could change with a bill introduced in the New York State Legislature this session, which would lay the groundwork for a comprehensive mosquito surveillance program—rather than what the bill calls the “sparse and disintegrated” current system—to help public health officials respond before an outbreak.

Proactive measures can include removing pools of standing water, applying targeted larvicides in breeding habitats, and warning the public to use insect repellent and cover bare skin when outdoors, though outbreaks may require more widespread pesticide spraying.

Leydet said greater centralization would be a start, but widespread coordination in large states like New York can be a challenge: “There is general interest in these programs, but when you start seeing what they cost, it’s like, ‘Maybe we’re not that interested.’”

Still, he said, “any help is better than nothing.”