Equine Encephalomyelitis

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Also Known As

Equine encephalitis


Equine encephalomeylitis (EE), also referred to as equine encephalitis,is caused by viruses. Three major strains include Eastern, Western and Venezuelan. The virus is spread by mosquitoes and is transferable to humans. Each strain is unique to an area so that EEE is seen in Eastern North America, WEE in Western areas and Venezuelan is widespread in South America.

The equine encephalomeylitis viruses are carried by mosquitoes and once a disease-carrying mosquito bites a horse, the virus travels to the lymph nodes and begins reproducing copies of itself.

Initially, horses are quiet and depressed with clinical neurologic signs generally occurring 5 days after infection. EE viruses attack the central nervous system and unvaccinated horses are very susceptible to infection. The disease appears within five days after a mosquito transmits the virus to the horse. Most deaths occur within 2-3 days after onset of clinical signs.

An initial diagnosis may be made on the basis of clinical signs, the location of the affected horse, and season of the year. A specific diagnosis can be made only by virus isolation and identification or by detecting a specific increase in antibodies in the horse's blood.


Equine encephalitis diseases are caused by viruses. These viruses are usually spread by mosquitoes and are transferable to humans making them zoonotic diseases.

While the core proteins of all strains of EE are similar, structural differences among the strains from different biological sources and different geographical sources have lead to the multiplicity of groupings such that EEE virus strains have been grouped over the years by various investigators into 3, then 4, then 2, then 7 groups (Morris, 1994). There are biological differences among epidemic/epizootic and endemic/enzootic strains. There also are differences in the virulence of strains from different geographical regions.


  • Fever
  • Listlessness
  • Loss of appetite
  • Irritation and nervousness
  • Paralysis
  • Inability to stand and move
  • Death


No specific therapy for viral encephalitis has been shown to be effective. Supportive care includes fluids if the horse is unable to drink, judicious use of anti-inflammatory agents, and anticonvulsants if necessary. Good nursing care is essential.

Veterinary care in the early stages is crucial. Treatment of the symptoms with drug and vitamin therapies may help a horse survive the effects of the virus.


Vaccination is the best preventative for EEE, WEE, and VEE. Mares should be vaccinated 3-4 weeks before foaling.

Horses that contract the disease should be quarantined and steps taken to reduce the population of biting insects.

Horse owners who live in an area where any form of EE is prevalent should make sure they follow the recommended vaccination schedule. In the event a horse shows symptoms of any form of EE a veterinarian should be called immediately since early care is crucial for survival of the horse with the fewest after effects.

Mosquito control through elimination of breeding sites and control of mature mosquitoes, as well as protection from mosquito bites, increase the success of preventive measures. Horses may be partially protected from mosquitoes by using repellents and by stabling with fans and screens to limit insect access.

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EquiMed Staff

EquiMed staff writers team up to provide articles that require periodic updates based on evolving methods of equine healthcare. Compendia articles, core healthcare topics and more are written and updated as a group effort. Our review process includes an important veterinarian review, helping to assure the content is consistent with the latest understanding from a medical professional.