A rare but deadly virus spread by mosquitoes has been detected in different parts of the country, and scientists say a combination of rainy weather and warmer winters is to blame.
The virus is called eastern equine encephalitis, or EEE. It primarily infects horses and birds, but in instances when it makes its way to humans, the disease is particularly virulent, often leading to severe neurological problems.
In Massachusetts, one death from EEE has been reported, and three other cases have been diagnosed.
In Michigan, health officials have confirmed one case in a teenage girl, and are investigating other potential cases.
The virus has also been found in three horses in Massachusetts, six horses and two deer in Michigan, two horses in Connecticut and four chickens in Delaware this month. Mosquito samples also show the virus was present in New Hampshire.
The Centers for Disease Control and Prevention says it usually receives reports of about seven cases a year, which means the number of the reported cases so far this year are in line with what's expected.
"We don’t have any indication that this is an outbreak year," said Carolyn Gould, a medical epidemiologist with CDC’s Division of Vector-Borne Diseases.
She noted that in some years, case counts have reached as high as 15.
According to the CDC, the majority of people who are infected with EEE never know it. "We believe fewer than 5 percent actually develop encephalitis, or inflammation of the brain," Gould said.
But for those who do fall ill, the illness is swift and devastating.
Symptoms start four to 10 days after a person is bitten and include headache, high fever, chills and vomiting. As the disease progresses, the patient can experience disorientation, seizures and coma.
Inflammation of the brain leads to death in about one-third of cases, and people who do survive are often left with brain damage.
The eastern equine encephalitis environment
Most cases of EEE occur in states that border the Atlantic, Gulf and Great Lakes. The main carrier of the virus is a species of mosquito called Culiseta melanura, which feeds on birds.
Another mosquito species, Coquillettidia perturbans, can also carry the virus. It feeds on birds as well as horses and people, meaning it can spread the virus from an infected bird to a human.
The mosquitoes are found primarily in swampy areas — specifically, red maple or white cedar swamps and wetlands.
The trees are an important component, said Phil Armstrong, a research scientist with the Connecticut Agricultural Experiment Station. "Wetlands with mature trees, with mature root systems ... create little water pockets in the swamp environment. The mosquito larvae develop around the roots of these trees that grow in swamps."
Armstrong runs Connecticut's statewide surveillance program for mosquitoes and mosquito-borne diseases.
"This year has been particularly bad for us," he told NBC News. He blames unusually high levels of rain. When water tables are high in the swamps, it creates the perfect habitat for the mosquitoes that carry EEE.
Armstrong explored the link between EEE cases and climate change in a paper he wrote in the New England Journal of Medicine in 2013. In addition to rainfall, milder winters may allow mosquitoes to survive year-round and migrate north. Typically, EEE season runs July through September.
"If you look back at the last 10 or 15 years, the trend is we’re seeing greater number of cases over time," said Armstrong.
Although EEE is the most severe mosquito-borne virus for humans, it's nowhere near as common as other mosquito-borne illnesses, like the West Nile virus.
And infectious disease specialists have their eyes on mosquitoes that are transmitting diseases in other parts of the world, too, such as yellow fever and the Mayaro virus in South America, dengue in Asia and Rift Valley fever in Africa.
"We have to continuously monitor what's going on outside of our country because these things can be brought into the U.S. so quickly," said Elitza Theel, an associate professor of lab medicine and pathology and director of the infectious disease serology laboratory at the Mayo Clinic in Rochester, Minnesota.
"We always have to be on our toes to help detect infections as quickly as we can," Theel told NBC News.
The CDC says the best way to avoid EEE is to avoid mosquito bites. That means using an EPA-approved mosquito repellent, wearing long sleeves and pants in wooded, swampy areas, and getting rid of standing water in flower pots, bird baths and anything else in your yard that holds water.
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