Venezuelan Equine

Encephalomyelitis

Venezuelan equine encephalomyelitis (VEE) is a

mosquito-borne viral disease of all equine species,

such as horses, asses, and zebras. After infection,

equines may suddenly die or show progressive 

central nervous system disorders. Humans also can

contract this disease. Healthy adults who become

infected by the virus may experience flulike 

symptoms, such as high fevers and headaches.

People with weakened immune systems and the

young and the elderly can become severely ill or die

from this disease.

The highly pathogenic form of VEE has not

occurred in the United States since 1971. However, in

1993 an outbreak of VEE in the State of Chiapas,

Mexico, prompted the U.S. Department of

Agriculture’s (USDA) Animal and Plant Health

Inspection Service (APHIS) to temporarily increase

its surveillance activities and tighten its quarantine

requirements for equine species entering the United

States from Mexico.

History

In 1936, VEE was first recognized as a disease

of concern in Venezuela following a major outbreak of

equine encephalomyelitis. From 1936 to 1968,

equines in several South American countries suffered

devastating outbreaks. In 1969, the disease moved

north throughout Central America, finally reaching

Mexico and Texas in 1971. Federal and State animal

health officials, the U.S. military, and the affected

communities took swift action to control this outbreak.

During the 1993 outbreak in southern Mexico,

Mexican animal health officials effectively contained

the disease by quarantining the affected area,

surveying a widespread area for the disease, and

promoting a vaccination campaign. As a precautionary measure, APHIS restricted the importation of

Mexican equines and encouraged vaccination of all

equines in the U.S. States bordering Mexico.

A more recent outbreak that began in

northwestern Venezuela in April 1995 spread westward to the State of Guajira in Colombia. This

outbreak is the largest in the region since 1971. A

prolonged and heavy rainy season and the resulting

increase in mosquito populations are believed to be

largely responsible for this 1995 outbreak. Control

measures instituted by Colombia include vaccination

of equines in the affected areas, movement

restriction of equines both between and within the

affected areas, insecticide application, public

education, community mobilization campaigns to

eradicate mosquito breeding sites, and surveillance

of humans and equines.

Clinical Signs

Equines infected with VEE may show one or

more of the following signs:

• Fever 

• Depression 

• Loss of appetite 

• Weakness

• Central nervous system disorders (lack of 

coordination, chewing movements, head 

pressing, “sawhorse” stance, circling, paddling 

motion of the limbs, and convulsions).

In some cases, horses infected with VEE may 

show no clinical signs before dying.

Confusion With Other Diseases

The clinical signs of VEE can be confused with

those of other diseases that affect the central

nervous system. These include eastern equine 

encephalitis, western equine encephalitis, African

horse sickness, rabies, tetanus, and bacterial 

meningitis. VEE might also be mistaken for toxic 

poisoning. Definitive diagnosis can be made by

isolating the virus in a laboratory or by testing blood

for the presence of antibodies to the virus.

How It Spreads

The virus that causes VEE is transmitted 

primarily by mosquitoes that 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

mosquito populations.

Enzootic subtypes of VEE are diseases endemic

to certain areas. Generally these serotypes do not

spread to other localities. Enzootic subtypes are

associated with the rodent-mosquito transmission

cycle. These forms of the virus can cause human

illness but generally do not affect equine health.

Epizootic subtypes, on the other hand, can

spread rapidly through large populations. These

forms of the virus are highly pathogenic to equines

and can also affect human health. Equines, rather

than rodents, are the primary animal species that

carry and spread the disease. Infected equines 

Factsheet

 

 
 

APHIS
develop an enormous quantity of virus in their 

circulatory system. When a blood-feeding insect feeds

on such animals, it picks up this virus and transmits it

to other animals or humans. Although other animals,

such as cattle, swine, and dogs, can become infected,

they generally do not show signs of the disease or

contribute to its spread.

Control Measures

During outbreaks, the most effective way to

prevent further spread of disease is to quarantine

infected equines. Controlling mosquito populations

through pesticide treatments and eliminating

insect-breeding sites will also enhance disease

control. These measures should be accompanied by a

large-scale equine immunization program. Equines in

the United States should be vaccinated for VEE only

when there is a serious threat that the disease could

spread to this country.

Report Suspicious Signs

Owners of equines have an essential role in

preventing VEE from spreading. They should

immediately report to a veterinarian or to State or

Federal animal health officials any animals showing

signs suspicious of VEE.

Additional Information

For more information, contact

USDA, APHIS, Veterinary Services

Emergency Programs

4700 River Road, Unit 41

Riverdale, MD 20737-1231

Telephone (301) 734-8073

Fax (301) 734-7817

or visit our Web site at www.aphis.usda.gov/vs.

United States Department of Agriculture • Animal and Plant Health Inspection Service • Safeguarding American Agriculture

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