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Vermont documents rise in Eastern equine encephalitis cases in people, horses in 2023-24
Mary Van Beusekom, MS
Today at 1:53 p.m.
Eastern Equine Encephalitis In 2023 and 2024, Vermont reported an increase in humans, horses, and mosquito pools that were positive for Eastern equine encephalitis (EEE), a mosquito-borne disease with a 33% case-fatality rate, researchers from the Centers for Disease Control and Prevention (CDC) and Vermont health departments report.
For the study, published last week in Morbidity and Mortality Weekly Report, CDC researchers reviewed an investigation conducted by the Vermont Department of Health (VDH) and the Vermont Agency of Agriculture, Food, and Markets (VAAFM).
Each June through October, VAAFM traps, identifies, and pools mosquitoes from about 100 sites in the state. VDH and CDC labs conduct weekly testing of mosquito pools and investigate EEE virus (EEEV) detections and suspected human and animal infections. Most US EEE cases occur along the Atlantic Coast, Gulf Coast, and Great Lakes.
“EEEV causes severe neuroinvasive disease in humans; although an EEE vaccine is available for horses, no human vaccine is currently licensed, and treatment is supportive,” the researchers wrote. “Approximately one third of human cases are fatal, and many survivors experience long-term neurologic sequelae.”
Increasing detections in mosquito pools
From 2012 to 2015, EEEV was detected in 42 mosquito pools, but no virus was found in pools from 2016 to 2022. In 2023, EEEV was identified in 14 mosquito pools in three towns, and in 2024, in 86 pools in 16 towns in northern and western Vermont.
In the 100 total mosquito pools, the most common mosquitoes to test positive for EEEV were Culiseta melanura (the primary enzootic vector), Coquillettidia perturbans (primary bridge vector that feeds on people, horses, and birds), and Culex pipiens-restuans (bridge vector).
After the state reported the first two EEE infections in people in 2012, no cases were diagnosed again until 2024, when two men from the same northwestern county contracted neuroinvasive EEE. One man was identified before mosquito detections in July, and he survived, but the other patient died in early September.
From 2012 to 2013, VDH investigated four EEE cases in horses, all but one dying of their infections. None had traveled or received EEE vaccination. No equine cases were reported from 2014 to 2022, but in 2023 and 2024, three fatal equine EEE cases were reported in northern Vermont, none of which had recently traveled or been vaccinated against EEE.
Role of climate change, human behavior, evolving diagnostics
In response to the increased EEEV activity, VDH and VAAFM continued surveillance, planned potential mosquito-control activities, and shared more EEE risk-and-prevention communications. This included readying permits, contracts, and outreach plans in case aerial mosquito spraying were needed; conducting a municipal health briefing; and publishing health advisories, press releases, social media posts, weekly town health-officer notifications, and multilingual flyers.
Area-specific viral activity and risk levels should be communicated alongside mosquito bite prevention messaging to reduce the risk for infection.
The agencies recommended wearing protective clothing and Environmental Protection Agency–approved repellents and limiting outside time at dawn and dusk. Equine EEE vaccination was also recommended, but because horses don’t produce enough virus to transmit it back to mosquitoes, vaccination doesn’t disrupt the transmission cycle or reduce the risk to people, the authors said.
“Since the 1930s, EEE outbreaks in the northeastern United States have become more frequent and have expanded northward, with a record-breaking surge in 2019,” they wrote. “The reasons for this increase are unknown but are likely tied to climate and landscape changes, human behavior, increases in mosquito and bird populations, and evolving diagnostic and surveillance practices.”
The CDC recommends that health departments use a One Health approach that includes EEEV surveillance in mosquitoes, susceptible domestic animals, and people. “Area-specific viral activity and risk levels should be communicated alongside mosquito bite prevention messaging to reduce the risk for infection,” they concluded.
Vermont documents rise in Eastern equine encephalitis cases in people, horses in 2023-24
Mary Van Beusekom, MS
Today at 1:53 p.m.
Eastern Equine Encephalitis In 2023 and 2024, Vermont reported an increase in humans, horses, and mosquito pools that were positive for Eastern equine encephalitis (EEE), a mosquito-borne disease with a 33% case-fatality rate, researchers from the Centers for Disease Control and Prevention (CDC) and Vermont health departments report.
For the study, published last week in Morbidity and Mortality Weekly Report, CDC researchers reviewed an investigation conducted by the Vermont Department of Health (VDH) and the Vermont Agency of Agriculture, Food, and Markets (VAAFM).
Each June through October, VAAFM traps, identifies, and pools mosquitoes from about 100 sites in the state. VDH and CDC labs conduct weekly testing of mosquito pools and investigate EEE virus (EEEV) detections and suspected human and animal infections. Most US EEE cases occur along the Atlantic Coast, Gulf Coast, and Great Lakes.
“EEEV causes severe neuroinvasive disease in humans; although an EEE vaccine is available for horses, no human vaccine is currently licensed, and treatment is supportive,” the researchers wrote. “Approximately one third of human cases are fatal, and many survivors experience long-term neurologic sequelae.”
Increasing detections in mosquito pools
From 2012 to 2015, EEEV was detected in 42 mosquito pools, but no virus was found in pools from 2016 to 2022. In 2023, EEEV was identified in 14 mosquito pools in three towns, and in 2024, in 86 pools in 16 towns in northern and western Vermont.
In the 100 total mosquito pools, the most common mosquitoes to test positive for EEEV were Culiseta melanura (the primary enzootic vector), Coquillettidia perturbans (primary bridge vector that feeds on people, horses, and birds), and Culex pipiens-restuans (bridge vector).
After the state reported the first two EEE infections in people in 2012, no cases were diagnosed again until 2024, when two men from the same northwestern county contracted neuroinvasive EEE. One man was identified before mosquito detections in July, and he survived, but the other patient died in early September.
From 2012 to 2013, VDH investigated four EEE cases in horses, all but one dying of their infections. None had traveled or received EEE vaccination. No equine cases were reported from 2014 to 2022, but in 2023 and 2024, three fatal equine EEE cases were reported in northern Vermont, none of which had recently traveled or been vaccinated against EEE.
Role of climate change, human behavior, evolving diagnostics
In response to the increased EEEV activity, VDH and VAAFM continued surveillance, planned potential mosquito-control activities, and shared more EEE risk-and-prevention communications. This included readying permits, contracts, and outreach plans in case aerial mosquito spraying were needed; conducting a municipal health briefing; and publishing health advisories, press releases, social media posts, weekly town health-officer notifications, and multilingual flyers.
Area-specific viral activity and risk levels should be communicated alongside mosquito bite prevention messaging to reduce the risk for infection.
The agencies recommended wearing protective clothing and Environmental Protection Agency–approved repellents and limiting outside time at dawn and dusk. Equine EEE vaccination was also recommended, but because horses don’t produce enough virus to transmit it back to mosquitoes, vaccination doesn’t disrupt the transmission cycle or reduce the risk to people, the authors said.
“Since the 1930s, EEE outbreaks in the northeastern United States have become more frequent and have expanded northward, with a record-breaking surge in 2019,” they wrote. “The reasons for this increase are unknown but are likely tied to climate and landscape changes, human behavior, increases in mosquito and bird populations, and evolving diagnostic and surveillance practices.”
The CDC recommends that health departments use a One Health approach that includes EEEV surveillance in mosquitoes, susceptible domestic animals, and people. “Area-specific viral activity and risk levels should be communicated alongside mosquito bite prevention messaging to reduce the risk for infection,” they concluded.