Notes from the Field: Increase in Eastern Equine Encephalitis Virus Activity — Vermont, 2023–2024

Eastern equine encephalitis (EEE) virus (EEEV) is a mosquitoborne alphavirus maintained in an enzootic cycle with mosquitoes and birds. EEEV can be transmitted to humans and susceptible animals by mosquitoes that bite both mammals and birds. 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. Approximately one third of human cases are fatal, and many survivors experience long-term neurologic sequelae (1). In the United States, a majority of EEE cases occur in states along the Atlantic Coast, Gulf Coast, and Great Lakes. EEEV was first detected in Vermont during a 2010 serosurvey of hunter-harvested deer and moose (2,3). After a 2011 outbreak of EEEV on a Vermont emu farm (4), statewide mosquito surveillance for EEEV was implemented in 2012. During 2012-2022, two human and four animal EEE cases were reported to the Vermont Department of Health (VDH). During 2023-2024, EEEV activity in mammals and mosquitoes increased, prompting targeted outreach in affected areas. This report describes EEEV activity in Vermont during 2023-2024 based on human and equine cases and mosquito surveillance data.