Venezuelan Equine Encephalomyelitis - Fact Sheet

What is Venezuelan equine encephalomyelitis?

Venezuelan equine encephalomyelitis (VEE) is a mosquito-borne viral disease, affecting all equine related species, such as horses, mules, donkeys and zebras.

Is VEE a risk to human health?

In humans, the virus usually causes flu-like symptoms such as fever and headaches.

Humans generally become infected through bites from infected mosquitoes. Human disease usually occurs at the same time as outbreaks of the disease in horses. Healthy adults may develop flu-like symptoms such as a high fever and headaches. People with weakened immune systems, young children and the elderly can become severely ill from this disease.

What are the clinical signs of VEE?

In horses, the disease typically starts as a mild fever that progresses into severe and often fatal encephalomyelitis (inflammation of the brain and spinal cord). In some cases, it can produce an acute disease causing sudden death.

Clinical signs may appear two to four days after infection, early signs of the infection include:

  • fever;
  • loss of appetite; and
  • mild excitability.

Frequently, a rapid progression of the disease ensues with depression, weakness and ataxia (lack of muscle coordination). These symptoms are then followed by signs of encephalitis such as muscle spasms, chewing movements, lack of coordination and convulsions. Some animals may stand quietly in their surroundings, whereas others may wander aimlessly or press their heads against solid objects. A characteristic paddling motion of the limbs may be observed when the horse is lying down.

Some animals experience sub-clinical or inapparent infections, where others may experience a mild to severe clinical source of the disease that may be fatal.

In some cases, horses infected with VEE show no clinical signs before dying.

Where is VEE found?

VEE has never been detected in Canada. The disease has occurred only in the western hemisphere, in particular South and Central American countries. In 1972, an outbreak of VEE extended into Texas.

How is VEE transmitted and spread?

The VEE virus is transmitted primarily by mosquitoes. The speed with which the disease spreads depends on the sub-type of the virus and the density of the mosquito population.

Infected horses carry a large amount of the virus in their blood and are, therefore, considered amplifying hosts that aid in the spread of the disease.

Highly pathogenic types of VEE virus spread rapidly through susceptible populations. In order for the disease to become established in an area, there must be a suitable mosquito population in the environment, a factor that is largely determined by climatic conditions.

How is VEE diagnosed?

Diagnosis may be made on the basis of clinical signs, geographic location of the affected animal, season of the year and presence of mosquitoes in the environment.

Laboratory and blood tests are necessary for a definitive diagnosis.

How is VEE treated?

There is no treatment for the disease. A vaccine has been used for control purposes in countries that have experienced VEE.

What is done to protect Canadian livestock from VEE?

The Canadian Food Inspection Agency (CFIA) imposes strict regulations on the import of animals and animal products from countries where VEE is known to occur. These regulations are enforced through port-of-entry inspections conducted by both the CFIA and Canada Border Services Agency.

VEE is a “reportable disease” under the Health of Animals Act. This means that all suspected cases must be reported to the CFIA for immediate investigation by inspectors.

How would the CFIA respond to an outbreak of VEE in Canada?

Canada’s emergency response strategy in the event of an outbreak of VEE would be to:

  • eradicate the disease; and
  • re-establish Canada’s disease-free status as quickly as possible.

In an effort to eradicate VEE, the CFIA may employ some or all of the following disease control methods:

  • the humane destruction of all infected and exposed animals;
  • surveillance and tracing of potentially infected or exposed animals;
  • strict quarantine and animal movement controls to prevent spread;
  • strict decontamination of infected premises; and
  • zoning to define infected and disease-free areas.

In the event that the disease is not detected before the virus becomes widespread in the insect population that is carrying it, the disease control strategy may be modified to include vaccinating animals at risk of contracting the disease.

Owners whose animals are ordered destroyed may be eligible for compensation.

Additional information

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