Crisper fall weather is descending, signaling the coming end of another mosquito season that this year saw modest outbreaks of West Nile virus and eastern equine encephalitis.

The good news has been that the disease-carrying mosquitoes would rather bite birds than humans, a factor in keeping the maladies relatively rare. The bad news is that a warming world is expected to add months to mosquito season and, worse, that species with a stronger taste for humans are headed north.

The Centers for Disease Control and Prevention says this year there have been just 880 U.S. cases of West Nile, the most common mosquito-borne disease in the continental U.S. EEE is rarer still, with just 13 cases in seven states this year.

That rarity is a good thing because both can be deadly.

Though most cases are mild or asymptomatic, one in 150 cases of West Nile can be severe (as was the recent case of Anthony Fauci, former director of the National Institute of Allergy and Infectious Diseases) and one in 10 severe cases result in death, according to the CDC. The numbers are more sobering for EEE, with most of the cases reported each year being severe and 30 percent, on average, resulting in death. Seven of the known cases this year have been fatal.

The cooler temperatures that come with autumn are beginning to ease the EEE outbreak. In early October, Massachusetts public health officials lowered EEE risk warnings in the worst-hit parts of the state from critical to moderate. The risks for West Nile remained unchanged — high or moderate over large portions of the state — and experts warn that cooler weather alone doesn’t stop transmission. Mosquitoes remain active until killed by frost, which has been happening later in recent years.

In fact, recent studies have projected that by 2050 longer autumns and earlier springs will extend the U.S. mosquito season by as much as two months. Those months are expected to be warmer and wetter, providing more standing water where mosquitoes can breed. The extra time also means more gestational cycles so more biting by females, who must have a blood meal before laying eggs.

Professor Flaminia Catteruccia with mosquito cages at Harvard Chan School.

Photo by Dylan Goodman

“You have more bites, more areas where they’re able to live, more months when they’re active, and more places for them to breed. That means larger populations,” said Matthew Phillips, a research fellow in infectious diseases at Harvard Medical School and Massachusetts General Hospital. “All of this is expected based just on changes in climate that affect mosquitoes.”

Evidence of the trends has already been seen, Phillips said. In 2021, during one of the hottest Decembers on record, the CDC recorded 30 cases of West Nile virus. Even at MGH in chilly Boston, the trend has been evident, albeit with diseases spread by hardier insect vectors.

“We were seeing cases of anaplasmosis and babesiosis, diseases that are spread by ticks and can be potentially pretty serious,” Phillips said. “Typically, you’d see them in summertime, but we were seeing those in the middle of winter.”

Experts are also keeping an eye on two invasive species that have already established themselves in the nation’s Southeast and are beginning to spread north. The mosquitoes, Aedes aegypti and Aedes albopictus, can carry several viral diseases, including West Nile and EEE. But unlike the mosquitoes currently spreading those diseases, both prefer humans. Estimates by Canadian researchers in 2020 showed the species spreading to the West Coast and the Canadian border by 2080.

Epidemiologists note the two species pose some additional threats. Besides West Nile and EEE, these mosquitoes can carry Zika, which caused 3,500 cases of microcephaly among infants during Brazil’s 2015-16 outbreak. They can also spread the tropical diseases Dengue fever — called “break-bone fever” because of the intensity of its pain — and Chikungunya, a tropical fever with no known treatment or cure. Public health officials in Florida and California reported cases of Dengue fever this year.

Aedes aegypti is more efficient at spreading diseases like Zika and Dengue, but when discussing near-term threats, both Phillips and Flaminia Catteruccia, a mosquito expert at the Harvard T.H. Chan School of Public Health, point to albopictus as the most concerning. Not only is it hardier, it’s already starting to appear.    

“It’s only recently been seen in Massachusetts and is very good at transmitting viruses,” said Catteruccia, professor of immunology and infectious diseases and a Howard Hughes Medical Institute investigator. “That’s my bit of worry: If it becomes really prevalent, we might see more transmission. But it remains to be established whether the environmental conditions, especially the long winters here, will be hospitable enough for these mosquitoes to survive.”

Alongside the new threats is the possibility of an older one that may make a reappearance: malaria. In 2023, there were malaria outbreaks in Florida, Texas, and Maryland that could not be traced to someone arriving from a malaria-endemic country. The apparently local acquisition of the disease is concerning because malaria was responsible for more than 600,000 deaths in 85 countries in 2022. It’s also not a newcomer to the U.S. Malaria circulated widely here from Colonial times until it was eradicated in 1951.

Catteruccia said that malaria has an advantage in favor of its U.S. spread: the Anopheles mosquitoes that host the malaria parasite are already widespread here. Counterbalancing that is the fact that those mosquitoes prefer animals to humans. Cold winters also provide a shield.

“Malaria used to be here in the states, so the mosquitoes are around and are potential vectors of malaria,” Catteruccia said. “But malaria has a very complex lifecycle, so especially here in the north, I don’t see this becoming an issue for the time being.”

With shifting disease patterns already happening, Phillips said our understanding of the epidemiology of those diseaseshas to change as well. Physicians who diagnose patients during winter shouldn’t automatically rule out ailments traditionally seen in summer. And those diagnosing in summer shouldn’t rule out ailments from warmer regions.

“One thing that climate change does is it changes the traditional epidemiology of mosquito-borne diseases,” Phillips said. “We’re used to these diseases in the summertime, and they’re showing up in winter. We’re used to them being in the tropics and they’re showing up in temperate climates. These traditional epidemiological associations are breaking down and, as they break down, we need better disease monitoring to know where they’re going and what they’re doing.”