Eastern equine encephalitis (EEE) is a rare but serious disease caused by EEE virus (EEEV), which circulates between mosquitoes and birds. Mosquitoes that bite both birds and mammals can pass it to people and to animals such as horses.
| In people | severe neuroinvasive disease; about one in three cases is fatal, and many survivors have lasting neurologic problems |
| Vaccine | available for horses; none licensed for people |
| Treatment | supportive only |
| Where | most U.S. cases occur in states along the Atlantic Coast, the Gulf Coast and the Great Lakes |
Vermont's history with the virus
| Year | |
|---|---|
| 2010 | EEEV first detected in Vermont, in a survey of deer and moose taken by hunters |
| 2011 | an outbreak on an emu farm |
| 2012 | statewide mosquito surveillance begins; the state's first two human cases |
| 2012–2013 | four horses with EEE, three of which died — none had traveled or had a veterinarian-given EEE vaccine |
| 2012–2015 | EEEV found in 42 mosquito pools (a median of nine a year) |
| 2016–2022 | no positive pools; no horse cases after 2013 |
Each June to October, the Vermont Agency of Agriculture, Food, and Markets traps mosquitoes at about 100 sites, sorts them into pools (1 to 50 mosquitoes of one species, from one trap on one date), and has them tested weekly for EEEV at state and CDC laboratories.
2023–2024: the virus comes back
| 2023 | 2024 | |
|---|---|---|
| Positive mosquito pools | 14, from 3 towns | 86, from 16 towns in northern and western Vermont |
Of those 100 positive pools, the commonest species were:
- Culiseta melanura (64) — the main species keeping the virus going among birds;
- Coquillettidia perturbans (9) — the main bridge vector, which feeds on people, horses and birds;
- Culex pipiens-restuans (6) — another bridge vector.
Horses: three cases in northern Vermont in 2023 and 2024. None had traveled recently or had records of the recommended EEE vaccination, and none survived.
People: in 2024, two men from the same northwestern county developed neuroinvasive EEE — the first human cases since 2012. One, identified in July, before any positive mosquitoes had been found, survived; the other died in early September.
EEE cases in humans and horses, and EEE virus detections in mosquito pools, Vermont, 2012–2024. CDC
How Vermont responded
The Vermont Department of Health and the Agency of Agriculture kept coordinating surveillance, prepared for possible mosquito control, and stepped up warnings:
- permits, contracts and outreach plans drawn up in case aerial spraying against adult mosquitoes was needed;
- a municipal health briefing, health advisories, press releases and social media;
- weekly notices to town health officers and multilingual flyers where infected mosquitoes were found.
Advice to residents: wear protective clothing and use Environmental Protection Agency–approved repellent; limit outdoor activities at dawn and dusk, when mosquitoes bite. Horse owners were urged to vaccinate — though horses are dead-end hosts (they don't carry enough virus to reinfect mosquitoes), so vaccinating them protects the horses but does not lower the risk to people.
The bigger picture
Since the 1930s, EEE outbreaks in the northeastern United States have become more frequent and spread northward, with a record surge in 2019. The reasons are unknown but likely include changes in climate and landscape, human behavior, more mosquitoes and birds, and changing diagnosis and surveillance.
CDC recommends:
- a One Health approach — surveillance in mosquitoes, susceptible domestic animals and people — with integrated vector management and clear communication of local risk alongside bite-prevention advice;
- that clinicians and veterinarians consider EEE in people and horses with sudden fever or neurologic illness, especially in summer and fall;
- yearly vaccination of horses before mosquito season where the virus is established.
Sources
Based on Strelau KM, Pareles E, Kelso P, et al., "Notes from the Field: Increase in Eastern Equine Encephalitis Virus Activity — Vermont, 2023–2024," MMWR volume 75, number 16, Centers for Disease Control and Prevention; a work of the United States government in the public domain.
Licence: CC0 1.0 (public domain) · Adapted from www.cdc.gov
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