Venezuelan Equine Encephalomyelitis (VEE) - Donkey

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This is an infectious, zoonotic, mosquito-borne virus of equidae. VEE virus usually exists as a non-pathogenic strain in rodents and birds, and has Culex mosquitoes as the vector.

This virus is enzootic in many parts of the Americas. Epidemics of VEE occur when strains of this endemic form mutate and give rise to an epidemic strain of virus capable of causing disease in equidae and humans. The mechanism of development of these virus strains is still not clear, nor is the mechanism of their release into the greater population, capable of causing epidemics (Sahu, 2003). VEE is found in high levels in the blood of equidae which can act as an amplifier in the epidemiological cycle. This differs from other causal agents of equine encephalomyelitis where, due to insufficient virus levels in the blood for them to be infectious, equines are the dead end host.

Outbreaks of VEE are mainly confined to South America and sporadic outbreaks date back to the 1930s or earlier. However, Central America and Mexico have experienced more recent outbreaks, lately in 1993 and 1996 (Gonzalez-Salazar, 2003).

Transmission cycle of VEE (Image courtesy of The Donkey Sanctuary)

Clinical signs

There is a range in the severity of clinical signs seen:

  • Subclinical: no overt signs
  • Moderate: characterised primarily by anorexia, fever and depression
  • Severe non-fatal: anorexia, high fever, stupor, weakness, staggering, blindness and permanent neurological sequelae
  • Severe fatal: anorexia, high fever, stupor, weakness, staggering, blindness and neurological signs. Not all fatal cases show neurological signs (Anon, 1998)

There are few documented cases in donkeys. In a suspected case in Jamaica the main sign was lethargy. This contrasted with a fatal infection in a recently imported horse (A. Trawford, personal communication).

Different strains of VEE have variable virulence. The so-called Trinidad donkey strain, isolated during the epizootic in 1943, has been shown to be lethal for 50% of donkeys and horses infected (Eddy, 1973). Donkeys infected with one particular serotype of VEE have been shown to have similar pathological lesions to the horse (Sahu et al, 2003).


  • Virus isolation or antigen detection in clinical cases
  • Serological tests


  • Supportive and symptomatic treatment of mild cases


  • A vaccine is available and, as equids can amplify the virus, an important component of control is vaccination of all the susceptible equines in a population
  • Vector control

Literature Search

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Use these links to find recent scientific publications via CAB Abstracts (log in required unless accessing from a subscribing organisation).

Venezuelan Equine Encephalomyelitis in donkeys publications


  • Anzuino, J. (2008) Exotic infections In Svendsen, E.D., Duncan, J. and Hadrill, D. (2008) The Professional Handbook of the Donkey, 4th edition, Whittet Books, Chapter 14

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