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Venezuelan Equine Encephalomyelitis

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. Humans can also contract this disease. 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. In humans, the virus usually causes flu-like symptoms such as fever and headaches.

Do we have this disease in Canada?

No. Venezuelan Equine Encephalomyelitis has never occurred in Canada. It was first recognized in Venezuela in 1936, when it caused a major outbreak of encephalomyelitis in horses. The disease has occurred only in the western hemisphere, in particular South and Central American countries where sporadic outbreaks occur.

Is there a human health risk associated with this disease?

Humans become infected mainly through bites from infected mosquitoes. Human disease generally 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.

How is the disease transmitted and spread?

The VEE virus is transmitted primarily by mosquitoes, which bite an infected animal and then bite and feed on another animal or human. The speed with which the disease spreads depends on the subtype of the VEE 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 suitable vectors (mosquitoes) in the environment, a factor that is largely determined by climatic conditions.

What are the clinical signs of VEE?

Horses infected with VEE may show one or more of the following signs:

• fever;
• depression;
• loss of appetite;
• weakness;
• central nervous system disorders such as lack of coordination, circling, head pressing, abnormal stance, paddling movement of the limbs, and convulsions.

Some horses experience subclinical or unapparent infections, where others may experience a mild to severe clinical course of the disease that may be fatal. Central nervous system disorders are usually not observed until four to five days into the illness. In some cases, horses infected with VEE show no clinical signs before dying.

How is this disease diagnosed?

A presumptive diagnosis may be made on the basis of clinical signs, geographic location of the affected horse, season of the year, and presence of mosquitoes in the environment. The clinical signs of VEE, however, can be confused with those of other diseases that affect the nervous system such as eastern equine encephalitis, western equine encephalitis, African horse sickness, West Nile virus, rabies, tetanus, and meningitis. Laboratory and blood tests are necessary for a definitive diagnosis.

Are there any treatments?

There is no treatment for this viral disease.

What is being done to prevent this disease from entering Canada?

Venezuelan equine encephalomyelitis is a reportable disease under the Health of Animals Act. This means that all suspected cases must be reported to the Canadian Food Inspection Agency (CFIA). All reported suspect cases are immediately investigated by inspectors from the Agency. The 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 CFIA port-of-entry inspections.

Canada’s emergency response strategy in the event of an outbreak of a foreign animal disease is to eradicate the disease and re-establish the country’s disease-free status as quickly as possible. In an effort to eradicate VEE, the CFIA would employ its "stamping out" policy, which would include:

• the humane destruction of all infected and exposed animals;
• surveillance and tracing of potentially infected or exposed animals;
• strict quarantine and animal movement controls;
• strict decontamination of infected premises; and
• zoning to define infected and disease-free areas.
• instituting insect control measures during mosquito and biting fly season in the infected zone.

In the case that the disease is not detected before the virus has become widespread in an insect vector population, the eradication strategy may be modified to include vaccination of susceptible animals.

How to get more information?

Contact your Canadian Food Inspection Agency Area Office:

You can find your local Canadian Food Inspection Agency District Office on the CFIA Web site or by consulting the blue pages of your local phone directory.

September 2003
P0323E-03