Eastern Equine Encephalitis Basics

Eastern equine encephalitis, or EEE, can be deadly in up to 90% of equine cases. Here’s what you should know.

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Eastern Equine Encephalitis Basics
Eastern equine encephalitis is a mosquito-borne disease that poses a threat to horses. | Photo: iStock
Eastern equine encephalitis (EEE) is an annual disease threat to horses in the Gulf and Atlantic Coast states and the Great Lakes region. Cases have been reported as far north as Canada and from some inland states such as Iowa, Arkansas, and Kentucky.

The causative agent, Eastern equine encephalitis virus (EEEV), is silently maintained by a songbird/mosquito transmission cycle. Infected ornithophilic (“birdloving”) Culiseta melanura mosquitoes living near freshwater hardwood marshes transmit EEEV to nestling songbirds; subsequently, new hatches of mosquitoes acquire the virus by biting the infected birds. The ecological range of C. melanura limits the westward extent of EEEV infections.


While songbirds do not become ill from the infection, transmission of EEEV to other birds, horses, or humans can cause disease. Transmission of EEEV to mammalian hosts involves mosquito vectors with broader feeding habits such as Aedes spp. In temperate regions, EEE cases in horses have a summer/fall seasonality with few cases occurring in cool months and complete cessation in the winter. In subtropical regions such as Florida, there is year-round risk of EEE transmission with a peak in the summer months. Horses and humans are not part of natural maintenance of EEEV as they do not produce sufficient viremia to allow transmission to mosquitoes; instead, they are considered dead-end hosts.

Clinical Signs

Once an equid is infected by a mosquito bite, clinical signs can appear in 5 to 15 days. EEEV infections in horses, mules, or donkeys are typically severe and up to 90% of ill horses do not survive. Clinical signs range from fever and/or dull mentation (thus the old name “sleeping sickness”) to blindness, loss of coordination (ataxia), head pressing, inability to rise, seizures, and death. Fevers in the acute clinical phase can be very high (105-106°F).

Clinical signs associated with EEE can overlap those observed with West Nile encephalitis or equine herpesvirus-1 myeloencephalopathy (EHM, the neurologic form of the virus), thus laboratory confirmation of EEE is critical. A combination of clinical signs, appropriate environment, and lack of appropriate vaccination history along with a positive EEE IgM Capture ELISA serologic (blood) test confers a strong presumptive diagnosis of EEE. For horses that die or are euthanized, PCR identification of EEEV nucleic acid in the brain, isolation of EEEV from brain tissue, or specific staining of fixed brain tissue confirms the diagnosis.

Vaccinating Against EEE

Despite widely available and effective USDA-licensed vaccines, each year horses in North America succumb to EEE. The American Association of Equine Practitioners considers EEE vaccination among the core vaccines and has specific risk-based guidelines established by age and residence of the horse. As the vaccines are killed virus products, adherence to a 2- or 3-dose primary series is critical to engender protective immunity in foals.

In 2014, a total of 136 equine cases of EEE were reported from 15 states. The USDA Animal and Plant Health Inspection Service (APHIS) Veterinary Services (VS) branch collaborates with the U.S. Centers for Disease Control and Prevention (CDC) and state veterinary and public health officials to facilitate communication about EEE disease cases in horses and confirm equine cases in each state. CDC collects EEE case information using its ArboNET reporting system, an electronic surveillance and reporting system used to track and report arboviral activity, including EEE, in humans and animals. During the transmission season, VS disseminates the equine arbovirus case information to state animal health officials for confirmation and posts the number of confirmed cases to the USDA equine infectious anemia disease information website.


  • Eileen N. Ostlund—eileen.n.ostlund@aphis.usda.gov—515/337-7551—USDA APHIS/VS/ Science, Technology, and Analysis Services (STAS)—National Veterinary Services Laboratories, Ames, Iowa
  • Rebecca Jones—rebecca.d.jones@aphis.usda.gov—970/494-7196—USDA/APHIS/VA/STAS—Surveillance Design and Analysis Center for Epidemiology and Animal Health—Fort Collins, Colorado

This is an excerpt from Equine Disease Quarterly, funded by underwriters at Lloyd’s, London.


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