MONTPELIER -- Vermont public health officials don't know why the state is seeing its first human cases of a rare viral illness that has killed an elderly man and sickened another adult, but they are committed to stopping its spread by killing mosquitoes that transmit the disease, eastern equine encephalitis.
The Health Department began aerial spraying last week in two areas of Rutland and Addison counties, low-lying, swampy areas long known for mosquitoes. Officials had already begun ground spraying as part of an effort to kill the mosquitoes known to carry EEE.
But it remains a mystery why the disease was detected in humans in Vermont for the first time this summer, even though more cases have been reported over the last half century in Massachusetts than any other state and it has been reported in animals in the surrounding states and the province of Quebec.
"It is not always easy to predict when the factors are going to be right and it spills over and causes human, or even domestic animal, illness. This is a virus that hangs out in birds and mosquitoes," said Erica Berl, infectious disease epidemiologist with the Vermont Health Department.
People infected with EEE can develop two types of illness. One comes on suddenly and is characterized by chills, fever, malaise, joint and muscle pain, and lasts about one to two weeks. A more severe illness affects the central nervous system and causes fever, headache, irritability,
About 35 percent of those infected with the more severe form die. And EEE is unlike West Nile virus, another mosquito-borne illness, in which the most severe cases are reported in the elderly and those with weakened immune systems.
"This one can take young, healthy people," Berl said.
Those who survive the more severe form of EEE often have severe neurologic complications that can last a lifetime. "If a child gets it and survives and has neurologic problems, that's a long life of neurologic problems," Berl said.
EEE has a history in horses that goes back into the 1800s. It was first reported in people in the 1930s. The virus is carried in a species of mosquito that favors acidic, hardwood swamps, such as those in Addison and Rutland counties.
The area where the EEE was found is a low-lying swampy area from around Middlebury south to Brandon and has long been infamous for its mosquitoes. The area has Vermont's three municipal organizations charged with controlling mosquitoes.
Vermont has been testing for EEE and West Nile viruses for years, but it hadn't had a positive EEE test in humans and the first animal cases, in emus, came last year. There were two human cases of West Nile last year, the state's first human cases since 2003.
Now that both EEE and West Nile have been detected in humans, the Health Department decided to spray pesticides in two areas between Middlebury and Brandon. The threat won't disappear completely until after the first hard freeze.
To avoid getting bitten by mosquitoes, officials suggest wearing long sleeves and pants, avoid being outdoors at dusk and dawn, get rid of standing water and using repellents. Also, officials say, horses, emus, llamas and alpacas should be vaccinated. There is no vaccine for humans.
"The public education is a challenge to strike the right note, to get people to take precautions, but not totally panic," Berl said.