A mosquito-borne virus called the Eastern equine encephalitis virus is circulating in Orange County, Florida, health officials warned this week. The virus was identified in a group of chickens researchers use to monitor levels of mosquito-borne diseases in the area.
Eastern equine encephalitis virus (EEEV) is dangerous: When it spreads to humans, the virus can infect the brain, and turns fatal in about a third of people, according to the Centers for Disease Control. But the disease is also extremely rare in humans—it only affects an average of seven people in the United States each year. However, like all viruses transmitted by mosquitoes, there’s a risk that it could have a bigger impact under a warmer climate. The number of annual cases may start to creep up. “I would not be surprised at all to see that happening,” says Thomas Unnasch, an expert in EEEV and distinguished professor in the College of Public Health at the University of Southern Florida.
Florida is the only place in the country where EEEV circulates year-round, and any virus present in other states originated in Florida, Unnasch says. The virus is carried by a type of mosquito called Culiseta melanura, and during its life cycle, it passes from birds to mosquitoes and back again. Culiseta melanura mosquitoes don’t bite humans directly—instead, they can pass on the EEEV virus to other types of mosquitoes that do interact with people.
The changing climate makes environments more accommodating for people-biting mosquitoes, says Sadie Ryan, a professor in the department of geography and Emerging Pathogens Institute at the University of Florida. Research shows that the range of mosquitoes, and the range of mosquito-borne diseases, is likely to expand under a warming climate: hotter, wetter temperatures are generally beneficial for mosquito life cycles. That might make it more likely for people to come in contact with mosquitoes carrying EEEV. “We could see more of that bridge happening,” she says.
Culiseta melanura mosquitoes that carry EEEV thrive in swampy, isolated areas, Unnasch says, and warming temperatures north of Florida could expand the region they transmit the virus in. “It could expand the range,” he says.
Some migratory birds also carry EEEV, which is why it can appear in swampy areas of states like New York and Massachusetts. It usually doesn’t linger for longer than a few years, though, Unnasch says. “The virus gets introduced, it bumps along for four or five years, and then a severe winter will kill mosquitoes and block the transmission of the virus.” However, if warming climates up north mean that winters are less severe, the virus might be able to carve out a more permanent foothold in those places, he says. “It may actually become much more strongly established.”
Hurricanes are made more severe by warming temperatures, but those changes might actually hurt EEEV-carrying mosquitoes, notes Unnasch. In the Florida panhandle, for example, Hurricane Michael knocked down trees in the swamps that served as a habitat for Culiseta melanura. However, uprooted trees leave behind water-logged hollows. “Those could be wonderful habitats, once they’re able to repopulate,” he says.
In addition, urbanization in places like Florida can intersect with climate change as it brings people’s homes and workplaces into closer contact with wild areas, including swamps where mosquitoes thrive. It may make spillover between animal reservoirs of a virus and people more common, Unnasch says.
While there’s evidence of general trends in mosquito activity in a changing climate, there are still a lot of unknowns about the ways EEEV transmission, specifically, might change. That’s likely in part because it’s so rare, Ryan says: It’s hard to find out about things that don’t happen very often.
However, because the disease is so dangerous, it’s good to be aware of the possibilities, she says. “Human cases are rare, but human cases are so bad. It’s useful to be alert to what could change.”