Eastern/Western Equine Encephalomyelitis (EEE/WEE)

Disease Overview

Eastern Equine Encephalomyelitis (EEE) and Western Equine Encephalomyelitis (WEE), sometimes called “sleeping sickness,” are caused by viruses that attack the brain and spinal cord. Birds and rodents are the primary reservoirs, and the diseases are spread by mosquitos. Unvaccinated horses are particularly susceptible to EEE and WEE and often serve as sentinels of the diseases, which also affect humans. EEE is more common than WEE and is found widely in several regions of the United States, especially in areas with high mosquito populations.

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Transmission Parasite Life Cycle

EEE and WEE are transmitted only by the bite of infected mosquitoes; direct transmission between horses and people does not occur.

Clinical Signs

Infection affects the central nervous system.

  • Moderate to high fever (>101.5)
  • Severe depression
  • Lack of appetite
  • Behavior changes
  • Impaired vision
  • Circling or head pressing
  • Muscle twitches
  • Inability to swallow
  • Weakness, incoordination
  • Paralysis of one or more limbs
  • Inability to stand
  • Convulsions, death

Risk Factors

  • High mosquito population
  • Standing water
  • Vaccination is recommended as a core protection strategy


“Eastern & Western Equine Encephalomyelitis,” American Association of Equine Practitioners, copyright 2020, aaep.org/guidelines/vaccination-guidelines/core-vaccination-guidelines/easternwestern-equine-encephalomyelitis.

“Equine Encephalitis (EEE/WEE/VEE),” USDA Animal and Plant Health Inspection Service, Dec. 29, 2020, www.aphis.usda.gov/aphis/ourfocus/animalhealth/animal-disease-information/equine/eee-wee-vee/equine-encephalitis.

“Eastern Equine Encephalitis (EEE),” Equine Disease Communication Center: Disease Fact Sheet, EDCC and AAEP, 2017, aaep.org/sites/default/files/Documents/Outside%20Linked%20Documents/DiseaseFactsheet_EEE%20Cobranded.pdf.

“Western Equine Encephalitis (WEE),” Equine Disease Communication Center: Disease Fact Sheet, EDCC and AAEP, 2017, https://aaep.org/sites/default/files/Documents/Outside%20Linked%20Documents/DiseaseFactsheet_WEE_FINAL%20Cobranded.pdf