A 41-year-old man in New Hampshire died last week after contracting a rare mosquito-borne disease 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 Wisconsin, New Jersey, Massachusetts and Vermont.
Although 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 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. Due to EEE’s high death rate, government officials began spraying insecticide in Massachusettswhere 10 communities have been designated as “critical” or “high risk” for triple E. Towns in the state closed their parks from dusk to dawn and warned people to stay inside after 6pmwhen mosquitoes are most active.
As West Nile virusanother mosquito-borne disease that poses a risk to people in the US every summer, triple E is limited by environmental factors that are changing rapidly as the planet warms. This is because mosquitoes thrive in the warmer, wetter conditions that climate change is causing.
“We’ve seen a resurgence of activity with eastern equine encephalitis virus in the last 10 years,” said Theodore G. Andreadis, a researcher who studies mosquito-borne diseases at the Connecticut Agricultural Experiment Station, a state government research facility. and public outreach outfit, for 35 years. “And we’ve seen an advance to more northern regions where it hadn’t been detected before.” Researchers don’t know what makes the virus rise and fall, but Andreadis said it’s clear that climate change is one of the factors driving its spread, especially to new regions.
The first triple E outbreak on record appeared in horses in Massachusetts in the 1830s—the reason one of the three E’s stands for “horses.” It was not 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 four years later, in 1938, in Massachusetts. There was 38 human cases in the state that year; 25 of them were fatal. Since then there have been human cases are mostly registered in the 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 rose to between 4 and 5 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 swamps and feeds on birds such as robins, herons and wrens, spreading the virus among them. But the melanura mosquito does not often bite mammals. Another mosquito species, Coquillettidia perturbans, is primarily responsible for most human cases of the disease reported in the US. The perturbans mosquito picks up the EEE virus when it feeds on birds and then infects the humans and horses it bites. By late summer, when mosquitoes have reached their peak numbers and begin jostling for any available blood meal, human cases begin to emerge.
Andreadis, what published a historical review on the progress of triple E in the northeastern US in 2021, said climate change has emerged as a major driver of the disease.
“We have milder winters, we have warmer summers, and we have extremes in both precipitation and drought,” he said. “The impact it has on mosquito populations is probably quite large.”
Warmer global average temperatures generally produce more mosquitoes, regardless of species.
Studies have shown that warmer air temperatures up to a certain threshold, around 90 degrees Fahrenheit, shorten the time it takes for C. melanura eggs to hatch. Higher temperatures in spring and fall extend the number of days mosquitoes have to breed and feed. And they will feed more often in a summer season when it is warmer – mosquitoes are ectothermic, meaning their metabolism speeds up in higher temperatures.
Rainfall also plays a role in mosquito breeding and activity, as mosquito eggs need water to hatch. A warmer atmosphere hold more moisturemeaning 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, swamps and potholes, the more opportunities mosquitoes have to breed. And warmer water reduces the incubation period for C. melanura eggs, which leads one study to conclude that warmer than average water temperatures “increase the likelihood for amplification of EEE.”
Climate change is not the only factor encouraging the spread of disease vectors such as mosquitoes. The slow reforestation of areas that were clear for industry and agriculture many decades ago is creating new habitat for insects. At the same time, Developers are building new homes in forested or semi-forested zones in ever-increasing numberswhich puts people closer 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 likely to bite, and regularly apply an effective mosquito spray. But there are also steps local health departments can take to protect public health, such as testing pools of water for mosquito larvae and conducting public awareness and insecticide spray campaigns when triple E is detected. Massachusetts is an example of a state that has been proactive in testing mosquitoes for triple E in recent summers.
The most effective way to protect people from this disease is to develop a vaccine against it. A vaccine already exists for horses, but there is little incentive for vaccine makers to develop a preventative for triple E in humans because the disease is so rare.
“Although EEE is not yet a global health emergency, the recent increase in cases has highlighted our lack of preparedness for unexpected infectious disease outbreaks,” a group of biologists wrote in the open-access scientific journal Frontiers last year. “It would be wise to pursue proactive active control measures and increase vigilance in light of these threats.”
