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Medical Index
First Posted: July 15, 1008
Oct 5, 2013

Equine Encephalomyelitis

by Debora Johnson
Equine Encephalomelitis is an infectious viral disease that affects the brain of the horse. The mosquito vectors the disease from hosts: birds, reptiles and rodents. The Eastern strain is the most frequently found in the United States and has the highest mortality rate. There are two other known strains: Western and Venezuelan. This disease is also called sleeping sickness. The most prevalent time for infection is midsummer to frost. Horses can transmit the disease to one another through common food, water, and contact.

Note: "To obtain further data on the potential for snakes to harbor EEEV, Unnasch and colleagues from the Department of Biological Sciences at Auburn University in Auburn, Ala., wrangled snakes at the Tuskegee National Forest (an EEEV endemic area in east-central Alabama). The researchers collected blood samples from the snakes and tested them for antibodies (infection-fighting proteins) against EEEV and genetic material specific to EEEV by a laboratory process called quantitative reverse transcription polymerase chain reaction (qRT-PCR).

Key findings of the study were:

  • Cottonmouths were the most frequently caught snake
  • Sixty-six cottonmouths did not have antibodies to EEEV, indicating they were neither exposed nor infected with the virus
  • Fifty-four cottonmouths did have antibodies to EEEV, and 22.22% of those snakes also tested positive for EEEV via qRT-PCR
  • The highest levels of EEEV genetic material measured by qRT-PCR occured in the spring months
  • EEEV could not be cultured (grown in the laboratory) from any blood samples

'This is the first study to clearly demonstrate that snakes are competent hosts for the EEEV as the snakes in this study had EEEV circulating in their bloodstreams,' explained Unnasch. 'Previous studies have only found that snakes have antibodies to EEEV.'..." Snakes Linked to Spread of Equine Encephalitis Virus


Generally, symptoms will start to appear after five days of infectious exposure.

  • Fever
  • Depression
  • Diarrhea
  • Listless
  • Self-mutilation
  • Hyperexcitability
  • Irritability
  • Will not eat
  • Will not drink
  • Incoordination
  • Head pressing
  • Circling
  • Paralysis
  • Convulsions
  • Coma
  • Death

Make the horse comfortable. The outcome is not usually good.


  • Mosquito control
  • Annual Vaccination

For More Information:

Encephalitis in Horses
Annual Horse Vaccines
New Equine Vaccine Information

Medical Index