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Climate Change May Spread North America's Deadliest Mosquito-Borne Pathogen

By Henry Gass, E&E reporter
9/30/2013 10:49:54 AM

A number of infectious diseases are poised to move up the Eastern Seaboard as the climate warms.

Scientists have been finding that the environmental changes associated with climate change -- particularly warmer winters and above-average rainfall -- are fueling the spread of diseases from West Nile virus to dengue fever that used to be found only in warm equatorial regions (ClimateWire, June 4, 2012).

Researchers also worry about the return of an old, and deadly, disease.

Eastern equine encephalitis (EEE), a mosquito-borne disease endemic in horses and humans in the eastern United States, is rarely found in humans compared with other vector-borne diseases like dengue and West Nile.

But with an estimated case-fatality rate of 35 to 75 percent, EEE is the deadliest mosquito-borne pathogen in North America, and there is no specific treatment for humans. Half of EEE survivors also suffer permanent neurological damage and require long-term care costing as much as $3 million per patient over a lifetime.

Since EEE's first outbreak in humans in southern Massachusetts in 1938, involving 34 cases in humans and 25 deaths, EEE has had a sporadic presence in the Northeast. Some years would feature a couple of human cases in the region; many years would feature none at all.

But since a disease outbreak in New Hampshire in 2005, scientists have witnessed a sustained resurgence of EEE virus activity in the Northeast and a northward expansion of the virus into regions where it was historically rare or unknown, reaching as far as southern Canada.

Last summer, Vermont documented its first ever cases of EEE in humans.

Photo by Flickr user dr_relling

A need to find the right habitat

Heidi Brown, who teaches epidemiology and biostatistics at the University of Arizona, is researching the EEE resurgence in the northeastern U.S. She said that climate change could be a factor in the virus's resurgence and expansion, but that there "could be a lot of factors."

"We see years with no cases," she said. "Then all of a sudden we're getting a situation where we're having them every year."

Temperature changes could play a big role, Brown said. Mosquitoes are highly temperature- and precipitation-dependent. Warmer winters enable some species to survive to the next summer, and longer, warmer summers extend the amount of time they have to breed, increasing the number of infected.

Cases of EEE are also expanding north, with instances reported as far north as the Maritimes and southern Ontario.

Ted Andreadis, who heads the Department of Environmental Sciences at Connecticut's Agricultural Experiment Station, said these new temperature conditions could have caused a buildup of infected Culiseta melanura mosquitoes, the main carrier of EEE, which then overflowed into the human population. Andreadis said South America is experiencing a similar situation with a dengue resurgence in Aedes aegypti mosquitoes.

Extreme weather could play a role

Extreme weather could also play an important role in EEE endurance. Not only does higher average rainfall help mosquitoes and their love of damp, humid environments, but the trunks of uprooted trees also happen to be the ideal habitat for C. melanura.

"This mosquito develops in these freshwater swamps and develops in subterranean crypts, so their abundance is typically related to rainwater and groundwater levels," Andreadis said.

"So if you have more extreme weather events with lots of rain, these mosquitoes are going to do very well," he added.

Brown said she's still looking for data to support the uprooted tree trunk theory. Knowing for sure whether the mosquito has such a specific habitat could help control EEE, but Brown said even then the virus would be hard to contain.

"They're living creatures that want to maintain their population," she said. Even if toppled trees were actively removed from forests in the northeast U.S., "my guess is [the mosquitoes] would find another place to be."

Tracking 'a new phenomenon'

Little is certain about what's happening with EEE or how to deal with it. Brown thinks climate change could be a factor in the virus's resurgence and spread. Andreadis has "a suspicion" that climate is playing a role. But there are plenty of other factors that could be involved.

Brown and Andreadis suspect increasing human encroachment into forests could be one cause, bringing humans into closer proximity to the virus than in previous decades. The resurgence and increased range might also be a result of climate disrupting the movement of birds in the region, which initially carry the virus before being bitten by C. melanura and transmitting the virus to them.

"This is a new phenomenon," Andreadis said. "I don't think this is trivial, and I don't think this is coincidental. I think this is real, and we need to figure out exactly what's going on."

Brown and Andreadis said they'll both be tracking the virus over the winter. Brown said she'd like to do more research on the impact of extreme weather events on the virus. Andreadis said more research needs to be done on whether the virus has been mutating and how it overwinters, "if it does at all."

Even for a pathogen that's been reported only about 270 times in the United States since 1964, according to the Centers for Disease Control and Prevention, the stakes are high.

"Case fatality rates, the numbers are as high as they get," Brown said. "It's a scary disease."

Reprinted from ClimateWire with permission from Environment & Energy Publishing, LLC. 202-628-6500.

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