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 byisolating 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 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.