A blood-sucking creature that comes at dusk is frightening Vermonters this summer.
No, it’s not a vampire. It’s Culiseta melanura — one mosquito species responsible for spreading the eastern equine encephalitis virus, a rare and sometimes fatal disease.
A man in New Hampshire died of the disease in August, and a person in Chittenden County has recently tested positive for the virus. That case and an uptick in mosquitoes turning up with the virus led the Vermont Department of Health to issue an advisory last week recommending that Vermonters in several northwestern Vermont communities stay inside from 6 p.m. to 6 a.m.
Those high-risk areas include Alburgh, Colchester, Swanton and Burlington, the state’s largest city. Several Burlington events, including Oktoberfest and the Intervale Center’s Summervale, have been canceled over EEE concerns.
Other events, like Burlington’s Twilight series, are scrambling to find indoor locations, according to mayor’s office spokesperson Joe Magee. Schools in high-risk areas are moving around sports practices and back-to-school events to protect against the virus, Vermont Public reported.
VTDigger spoke with Patsy Kelso, the state epidemiologist, to learn more about EEE and its spread in Vermont.
What is EEE?
Eastern equine encephalitis is a severe disease caused by a virus found in several species of animals and spread through two different species of mosquitoes. Mosquitoes pick up the virus from biting and feeding on birds, then pass along the virus to humans and horses, Kelso said.
EEE is not new to the region. It is more common in eastern Massachusetts and southern New Hampshire, according to the U.S. Centers for Disease Control and Prevention, but Vermont has had waves of outbreaks in humans and animals, Kelso said. The last fatalities were in 2012, when two people died in Rutland County.
Vermont did not detect the virus in mosquitoes or humans from 2016 to 2022. “It stays around for a few years, and then it disappears for a while,” Kelso said.
Then in 2023, 14 mosquito pools tested positive and one horse died of the virus. “We kind of anticipated that we’d see it again this year,” she said.
One important distinction when discussing EEE is that not everyone who gets the virus shows symptoms, and not everyone who shows symptoms gets encephalitis, the most severe complication of the disease, Kelso said.
In fact, most people with the virus do not develop symptoms. If they do, the incubation period is typically around four to 10 days, when they typically start to see indicators like a fever, chills and joint pain. Kelso recommended that people with these symptoms contact their doctor to rule out other causes and keep an eye on the course of the illness.
Only 5% of symptomatic patients get encephalitis, which includes a more severe fever, stiff neck, a really bad headache, seizures and vomiting, Kelso said. People who develop these symptoms should seek medical care immediately. Of the patients who develop encephalitis, about one-third die. There is no current cure for EEE.
There is limited data on who EEE affects most because the sample size each year is so small, but the evidence suggests people under the age of 15 or over the age of 50 are most susceptible to the most severe complications of the disease, she said.
What do we know about the disease this year?
Besides the human case, most data on EEE comes from the state Agency of Agriculture, Food and Markets and the state health department testing thousands of mosquito pools each year.
The agency sets up traps throughout the state that are designed to attract and collect specific species of mosquitoes, including the ones that spread EEE and West Nile Virus. You can see whether your community has one of those traps in this health department map.
Agency employees painstakingly look at each mosquito to identify its species and put them in groups or “pools,” Kelso said. They then send the mosquitoes out to the health department or CDC labs for testing each week.
Last month, the percent of pools testing positive for EEE reached a peak of around 8%, far higher than the seasonal average of 1% to 2%, the health department reported on its website. At least one pool has tested positive in 15 towns in Chittenden, Franklin, Grand Isle and Addison counties.
The positive pools triggered the health department’s existing disease plan to issue the advisory about staying indoors, Kelso said. Although no pools have tested positive in Burlington, the recent Chittenden County case in a human prompted the department to extend the advisory to the city.
Kelso said it is good for Vermonters on the border with New Hampshire to keep tabs on the spread of the disease there. But for the most part, mosquitoes are not big travelers. “Mosquitoes typically don’t travel more than about a mile from where they hatch,” she said.
What can be done to prevent EEE?
Asked exactly how dangerous it was to spend time outdoors in high-risk areas, Kelso emphasized that only 66 of the more than 2,200 pools tested — about 3% — actually came back positive for the virus.
Still, “it’s an individual decision that people need to make based on how concerned they are,” she said.
“Being one of those unfortunate five percenters,” or people who get encephalitis, “a third of whom will die, it really comes down to that,” she said. “So the health department’s message is we strongly recommend that people avoid being outside as much as possible in those evening and early morning hours.”
The health department has other recommendations for protecting yourself against mosquito bites on its website. Among its recommendations are to wear long tops and bottoms, use effective insect repellent, cover baby carriages and playpens with mosquito netting, fix holes in your home’s window screens, and get rid of standing water outside your home.
Kelso said people should consider using insect repellent even if wearing long clothing, since mosquitoes can bite through thin layers of fabric.
The health department’s surveillance plan calls for the use of pesticides or larvicides when necessary to limit mosquito-based diseases. At this point, no specific spraying projects are planned, Kelso said, although some regions of the state have mosquito control programs simply to deal with the nuisance.
“We have discussed it, and we don’t feel that (for) any of the towns that are currently high-risk, that we have data that warrants taking that action,” she said.
The department plans to keep tracking the progress of mosquito testing week to week, she said. If it were to go about mosquito control, it would need to go through several steps — particularly an environmental review of the area and a public meeting — before spraying could go forward, she said.
“We would never be able to decide on a Friday, ‘OK, the conditions are such that it warrants aerial application of pesticide,’ and then have that happen on Saturday because too many things need to happen,” she said.
What’s the outlook of EEE?
EEE falls off when mosquitoes do, which is around the first hard frost, typically early-to-mid- October in the Champlain Valley. Kelso said the prevalence of mosquitoes and the virus they spread starts to lower when the temperature dips below 50 degrees, even before the frost kills them off.
She said based on the multiyear pattern of the virus, it seems possible the outbreak could continue into next summer.
“I’m guessing we’ll still see it in the next year or two or three, and then it will probably go away again for a little while,” she said.
Unfortunately, climate change and its warming effect could extend the mosquito season and the challenges that go with it, Kelso said.
“We might start trapping and testing mosquitoes in May in coming years, if May is warm enough and we have mosquito activity, whereas now it’s really like late June or July,” she said.
Read the story on VTDigger here: EEE has come to Vermont. Here’s what you need to know about this mosquito-based virus..