How Climate Change Spread This Deadly Mosquito-Borne Illness to the US Northeast

A 41-year-old man in New Hampshire died last week after contracting a rare mosquito-borne illness called eastern equine encephalitis virus, also known as EEE or “triple E.” It was New Hampshire’s first human case of the disease in a decade. Four other human EEE infections have been reported this year in Wisconsin, New Jersey, Massachusetts, and Vermont.

Though this outbreak is small and triple E does not pose a risk to most people living in the United States, public health officials and researchers alike are concerned about the threat the deadly virus poses to the public, both this year and in future summers. There is no known cure for the disease, which can cause severe flu-like symptoms and seizures in humans 4 to 10 days after exposure and kills between 30 and 40 percent of the people it infects. Half of the people who survive a triple E infection are left with permanent neurological damage. Because of EEE’s high mortality rate, state officials have begun spraying insecticide in Massachusetts, where 10 communities have been designated “critical” or “high risk” for triple E. Towns in the state shuttered their parks from dusk to dawn and warned people to stay inside after 6 p.m., when mosquitoes are most active.

Like West Nile virus, another mosquito-borne illness that poses a risk to people in the U.S. every summer, triple E is constrained by environmental factors that are changing rapidly as the planet warms. That’s because mosquitoes thrive in the hotter, wetter conditions that climate change is producing.

“We have seen a resurgence of activity with eastern equine encephalitis virus over the course of the past 10 or so years,” said Theodore G. Andreadis, a researcher who studied mosquito-borne diseases at the Connecticut Agricultural Experiment Station, a state government research and public outreach outfit, for 35 years. “And we’ve seen an advancement into more northern regions where it had previously not been detected.” Researchers don’t know what causes the virus to surge and abate, but Andreadis said it’s clear that climate change is one of the factors spurring its spread, particularly into new regions.

The first triple E outbreak on record occurred in Massachusetts in the 1830s in horses — the reason one of the three Es stands for “equine.” It wasn’t until a full century later, in 1934, that mosquitoes were incriminated as potential vectors for the disease. The first recorded human cases of the disease also occurred in Massachusetts four years later, in 1938. There were 38 human cases in the state that year; 25 of them were fatal. Since then, human cases have mostly been registered in Gulf Coast states and, increasingly, the Northeast. From 1964 to 2002, in the Northeast, there was less than one case of the disease per year. From 2003 to 2019, the average in the region increased to between four and five cases per year.

The disease is spread by two types of mosquito. The first is a species called Culiseta melanura, or the black-tailed mosquito. This mosquito tends to live in hardwood bogs and feeds on birds like robins, herons, and wrens, spreading the virus among them. But the melanura mosquito doesn’t often bite mammals. A different mosquito species, Coquillettidia perturbans, is primarily responsible for most of the human cases of the disease reported in the U.S. The perturbans mosquito picks up the EEE virus when it feeds on birds and then infects the humans and horses that it bites. Toward the end of the summer, when mosquitoes have reached their peak numbers and start jostling for any available blood meal, human cases start cropping up.

Andreadis, who published a historical retrospective on the progression of triple E in the northeastern U.S. in 2021, said climate change has emerged as a major driver of the disease.

“We’ve got milder winters, we’ve got warmer summers, and we’ve got extremes in both precipitation and drought,” he said. “The impact that this has on mosquito populations is probably quite profound.”

Warmer global average temperatures generally produce more mosquitoes, no matter the species.

Studies have shown that warmer air temperatures up to a certain threshold, around 90 degrees Fahrenheit, shorten the amount of time it takes for C. melanura eggs to hatch. Higher temperatures in the spring and fall extend the number of days mosquitoes have to breed and feed. And they’ll feed more times in a summer season if it’s warmer — mosquitoes are ectothermic, meaning their metabolism speeds up in higher temperatures.

Rainfall, too, plays a role in mosquito breeding and activity, since mosquito eggs need water to hatch. A warmer atmosphere holds more moisture, which means that even small rainfall events dump more water today than they would have last century. The more standing water there is in roadside ditches, abandoned car tires, ponds, bogs, and potholes, the more opportunities mosquitoes have to breed. And warmer water decreases the incubation period for C. melanura eggs, leading one study to conclude that warmer-than-average water temperatures “increase the probability for amplification of EEE.”

Climate change isn’t the only factor encouraging the spread of disease vectors like mosquitoes. The slow reforestation of areas that were clear-cut for industry and agriculture many decades ago is creating new habitat for insects. At the same time, developers are building new homes in wooded or half-wooded zones in ever larger numbers, putting humans in closer proximity to the natural world and the bugs that live in it.

On an individual level, the best way to stay safe from EEE and other mosquito-borne diseases is to prevent bites: Wear long sleeves and pants at dusk and dawn, when mosquitoes are most prone to biting, and regularly apply an effective mosquito spray. But there are also steps that local health departments can take to safeguard public health, like testing pools of water for mosquito larvae and conducting public awareness and insecticide spraying campaigns when triple E is detected. Massachusetts is an example of a state that has been proactive about testing mosquitoes for triple E in recent summers.

The most effective way to protect people from this disease would be to develop a vaccine against it. A vaccine already exists for horses, but there is little incentive for vaccine manufacturers to develop a preventative for triple E in humans because the illness is so rare.

“Although EEE is not yet a global health emergency, the recent uptick in cases has highlighted our lack of preparedness for unexpected infectious disease outbreaks,” a group of biologists wrote last year in the open-access scientific journal Frontiers. “It would be wise to follow proactive active control measures and increase vigilance in the face of these threats.”

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