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Nature of the disease
Eastern, Western and Venezuelan equine viral encephalomyelitis (EEE, WEE and VEE respectively) are arthropod-borne diseases of horses that occur in the Western Hemisphere. 
They are caused by alphaviruses from the Togaviridae family and are serious zoonoses. EEE and WEE have caused outbreaks of disease in poultry and other farmed bird species.
OIE List B disease
Susceptible species
Clinically, horses and humans are the most important natural hosts. WEE virus occasionally causes encephalitis in pigs. WEE and EEE viruses have caused mortalities in poultry and farmed pheasants.
Each of the viruses also infects, generally sub-clinically, a wide range of mammalian, bird and other hosts.
The three viruses are found in North, Central and South America, with WEE being the most widely distributed — from south-west Canada to most of South America east of the Andes.
Clinical signs 
In horses, EEE and WEE are indistinguishable clinically. Sub-clinical infections can occur with both viruses. There is biphasic fever with clinical signs appearing during the second peak. Affected horses show:
  • Hypersensitivity to sound and touch
  • Involuntary muscle movements (especially of the shoulder and facial muscles)
  • Walking aimlessly or in to objects
  • Occasionally periods of excitement or intense pruritus

The acute phase is followed by depression with increasing incoordination, paralysis, coma and death. Case fatality rates are much higher with EEE (up to 90%) compared to WEE (20–30%).

Clinical signs of VEE encephalitis are similar to those above, although mortality rate is lower. There is a second clinical syndrome in which signs are more attributable to generalised infection:

  • Fever
  • Weakness
  • Depression
  • Anorexia
  • Colic
  • Diarrhoea
Post-mortem findings 
There are no characteristic gross lesions. Histologically, changes are seen in the central nervous system. With VEE, necrotic foci may be seen in internal organs such as pancreas, liver and heart.
Differential diagnosis 
Specimens required for diagnosis 
Blood samples (50 ml) should be collected from febrile horses in the acute stage of the disease for virus isolation.
Brain tissue — specimens of cerebral cortex, medulla, pons and cerebellum — is best collected from horses killed in the acute stage of the disease, or from horses within 24 hours of death. These should be put in sterile containers with duplicate specimens collected into neutral buffered formalin for histopathology.

Blood samples (20 ml) from horses in the acute and convalescent stages of the disease for serology, an ELISA test is available.

Mosquitoes (Cluiseta melanura, Aedes vexans, Culiseta tarsalis, Cluuicoides spp, Culex spp.)are responsible for transmitting all three viruses.
The natural cycle for EEE and WEE viruses is between birds and/or small mammals and mosquitoes. Small outbreaks or sporadic cases occur in humans and horses, which are generally regarded as ‘dead end’ hosts. With VEE, the natural cycle is poorly understood, but is believed to involve culicine mosquitoes and forest rodents. 

Epidemics in horses and humans occur at irregular intervals. There is a high viraemia in horses and direct horse-mosquito cycles occur during epidemics. The migratory birds should be considered as a factor of spread of the disease. 

Risk of introduction   
The possible source of introduction of these diseases are:
  • Infected mosquitoes being carried on international flights,
  • Importation of infected horse, donkey or birds
  • Introduction by migratory birds

 Whether these diseases could establish would depend on the presence of suitable hosts and mosquito species in the recipient country.
In the case of VEE horses can get viraemias high enough to spread disease and theoretically the virus could be introduced with live horses imported from endemically affected countries.

Control / vaccines  
Inactivated and attenuated vaccines are available either monovalent or polyvalent for each of the viruses. In endemic areas, annual vaccination is recommended.
Vaccination programs should be supported by mosquito control programs where practicable.
  • Equine Enephalomyelitis, In Merck Veterinary Manual, National Publishing Inc. Eight ed, 1998, Philadelphia, p 931-934
  • GEERING WA, FORMAN AJ, NUNN MJ, Exotic Diseases of Animals, Aust Gov Publishing Service, Canberra, 1995, 440p
  • Office International des Epizooties, 2002