These guidelines were prepared by the technical divisions of the Pan American Health Organization.
For more information please contact us


Equine encephalitides are viral zoonoses that occur episodically and cause outbreaks in equines and, less frequently, in humans. The ethiological agents are viruses belonging to the genus Alphavirus in the Togaviridae family. Infection is maintained in natural foci between wild reservoirs anD mosquitoes.  Three viruses are important: Western equine (WEE), eastern equine (EEE) and Venezuelan equine encephalomyelitis (VEE).  A fourth member, Higlands J virus, occurs in the eastern United States, primarily in Florida.

In Central America, a growing activity of syndromes compatible with equine encephalitis has been observed in horses during the last three years, particularly in Belize, Guatemala,Honduras and Panama.  Laboratory isolations of the virus and/or serological typing have shown EEE and VEE viral infection. These two agents are known to cause high morbidity and mortality among horses, donkeys and mules that, in turn, become amplifiers of the infection that eventually spreads to human by mosquito bites.

The VEE virus has caused major epidemics in the Americas. In 1969, an  outbreak started in Ecuador and spread through Peru, Colombia,  Venezuela, Central America and Mexico reaching finally Texas in 1971.  Close to 50,000 equines died. In Ecuador, alone, 31,000 human cases were  reported and 310 deaths.

In 1995, an outbreak occurred in Venezuela and Colombia that affected  seven States in Venezuela and the Department of La Guajira in  Colombia. This outbreak registered more than 40,000 human cases and 46  deaths. The outbreak was associated with one period of unusually intense rain with a subsequent increase of mosquito populations, vectors of VEE  virus and the lack of vaccination among the equine population.
The current situation in Central America is similar:
** The existence of natural foci where viral infection persists in wild reservoirs. The reservoirs include rodents: wild birds and bats.
** An increase in the population of mosquito vectors.  Stagnant waters in flooded lands favors an increase in mosquito breeding sites, and consequently, the growth of their population and a greater exposure of the domestic animals and humans to mosquito bites and infections carried by them.
** A susceptible equine population.  With the exception of a few areas in Guatemala and Belize, registries of horse vaccinations against EEV and/or EEE in the last four years are non-existent in the countries affected by Hurricane Mitch.  This can translate into a population that is entirely susceptible to the infection can be implied. The displacement of humans and this congregation in vulnerable areas, such as overcrowded refugee camps, surrounded by infected domestic animals, stagnant waters, and poor hygienic conditions, will be part of the scenario of a possible epidemic.
We are alerting the health authorities in these areas to improve surveillance of neurological syndromes compatible with equine encephalomyelitis in equines and to strengthen coordination among sectors for this purpose. Special consideration should be given to high-risk areas where there are larger numbers of horses, mules and donkeys (late predictors of equine encephalitides epidemics) and no history of vaccination against EEE and/or VEE exists in order to take timely and adequate preventive and control measures such as:

1. Immediate vaccination of horses mules, and donkeys.

2. Detection and treatment of mosquito breeding sites, especially those close to permanent and temporary human settlements (refugees).

3. Equine case monitoring and reporting, characterized by:
--  Fever, anorexia and depression accompanied by nervous signs such as: deep depression, difficulties with equilibrium, resting the head on objects, insomnia. Several animals might show hypersensitivity to touch and sound, excitation, walk in circles, blindness and bumping into obstacles. They often fall and are unable to get up.

4. Monitoring of probable human cases, characterized by:
    --  Fever headache, nausea, vomiting, muscular and pains in the joints, chills, photophobia and prostration, convulsions or alteration of the state of conscience, somnolence, hyperacusia and coma.

-- In people under three years of age: Fever, alterations of the state of conscience or convulsions.

For more information, you can consult the Manual "Bases for  Epidemiological Surveillance of Venezuelan equine encephalitis in the  Region of the Americas", which includes definition of concepts,  guidelines for the establishment and development of programs for  epidemiological surveillance and examples of formats for the  management of the information system during normal conditions and in  case of outbreaks or epidemics.

Albino J. Belotto
Regional Advisor
Veterinary Public Health Program
División of Disease Prevention and Control
PAHO/WHO, Washington, D.C.
Tel: 202-974-3191