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Equine Viral Arteritis
Veterinary Services
May 2002
Equine viral arteritis (EVA) is an infectious viral disease of horses
that causes a variety of clinical symptoms, most significantly abortions.
The disease is transmitted through both the respiratory and reproductive
systems. Many horses are either asymptomatic or exhibit flu-like symptoms
for a short period of time. An abortion in pregnant mares is often the
first, and in some cases, the only sign of the disease. EVA has been
confirmed in a variety of horse breeds, with the highest infection rate
found in adult Standardbreds.
Breeders, racehorse owners, and show horse owners all have strong economic
reasons to prevent and control this disease. While it does not kill
mature horses, EVA can eliminate an entire breeding season by causing
numerous mares to abort. In addition, U.S. horses that test positive
for EVA antibodies and horse semen from EVA-infected horses can be barred
from entering foreign countries. As the horse industry becomes increasingly
internationalized, nearly all major horse-breeding countries are including
in their import policies measures to reduce the risk of EVA. The U.S.
Department of Agriculture's (USDA) Animal and Plant Health Inspection
Service's (APHIS) Veterinary Services (VS) program provides the equine
industry with EVA diagnostic and surveillance support.
History
More than a century ago, a disease fitting the clinical description
of what we now call EVA was reported in European veterinary literature.
However, the virus was not isolated from horses in this country until
1953 during an epidemic of abortions and respiratory disease.
The most recent EVA epidemic occurred in 1984 when this disease affected
41 thoroughbred breeding farms in Kentucky. This outbreak brought to
light two very important findings about EVA: the efficiency with which
an acutely infected stallion could venereally transmit the virus and
the high carrier rate that immediately occurred in stallions following
natural infection with the virus.
Transmission
EVA is primarily a respiratory disease. Particles from acutely infected
horses' nasal discharges are inhaled, often during the movement of horses
at sales, shows, and racetracks. Horses are herd animals that tend to
commingle, and this close contact facilitates the spread of the virus.
However, unlike other respiratory diseases, EVA can also be transmitted
venereally during breeding, either naturally or by artificial insemination.
When a mare, gelding, or sexually immature colt contracts the disease,
the animal will naturally eliminate the virus and develop a strong immunity
to reinfection. On the contrary, infected stallions are very likely
to become virus carriers for a long time. Once stallions are in the
carrier state, they transmit the virus to mares during breeding.
While the mare will shed the virus easily, a pregnant mare infected
with EVA may pass the virus to her unborn fetus. Depending on the stage
of pregnancy, the fetus can become infected, die, and be aborted. If
the infected foal is born, it will only live for a few days.
Symptoms
Many horses infected with EVA are asymptomatic. When symptoms do occur
in the acute stage of the disease, they can include any or all of the
following: fever, nasal discharge, loss of appetite, respiratory distress,
skin rash, muscle soreness, conjunctivitis, and depression. Other clinical
signs in infected animals are swelling around the eyes and ocular discharge,
swollen limbs, swollen genitals in stallions, and swollen mammary glands
in mares.
Abortion in pregnant mares is also a symptom of EVA. Abortion rates
in EVA-infected mares can be as low as 10 percent or as high as 70 percent.
Diagnosis
Horse owners should suspect EVA when respiratory symptoms accompany
an abortion in a mare. Since the clinical signs of EVA are similar to
those of other respiratory disease, and no characteristic lesions are
in EVA-aborted fetuses, only diagnostic tests can confirm the disease.
Virus isolation can be attempted from swabs of the nose, throat, or
eyes; semen, placentas, or fetal tissue; and blood samples. However,
the most common method of diagnosis is testing blood for the virus'
neutralizing antibodies that cause EVA. While the presence of these
antibodies alone does not indicate active infection, it does indicate
EVA exposure has occurred. Very high levels of antibodies on a single
sample or a rising antibody titer from paired blood samples collected
14 to 28 days apart indicate active infection.
Treatment
While there is no specific treatment for EVA, treatment should include
rest and in selected cases, antibiotics, which may decrease the risk
of secondary bacterial infection. Adult horses recover completely from
the clinical disease. However, the virus commonly persists in the accessory
glands of recovered stallions, so these carrier stallions continue to
shed the virus for years and remain a significant source of infection.
Prevention and Control
Fortunately, there is a way the industry can work to prevent and control
EVA. A safe, effective, and low-cost avirulent live virus is now available.
Combining this vaccine with isolation of the vaccinated animal from
noninfected horses can prevent the spread of EVA.
Since properly vaccinated EVA-negative stallions do not become carriers,
all EVA-negative colts less than 270 days old should be vaccinated.
The vaccine is not approved for use in pregnant mares.
Blood samples for EVA testing should be collected from all horses before
breeding, and virus isolation should be performed on imported semen
before use. Strict hygiene and disinfection of instruments and equipment
are essential to minimize spread of the virus. EVA-negative mares should
be bred only to EVA-negative, noncarrier stallions.
If blood test results are positive in a stallion, but there is no official
documentation of negative EVA status prior to vaccination, the stallion
must be tested for the presence of a carrier state. Virus isolation
can be attempted on the semen from two separate ejaculations, or by
mating two EVA-negative mares with the stallion. Twenty-eight days after
breeding, mares' blood should be tested for the development of the neutralizing
antibodies to the EVA virus.
Carrier stallions should be bred only to EVA-positive mares or mares
that are properly vaccinated. When breeding an EVA-positive or carrier
stallion to an EVA-negative, vaccinated mare, isolate both horses for
24 hours after breeding to prevent mechanical spread of EVA from voided
semen. If this is the first time the mare has been bred to a carrier
stallion, she should be isolated from other horses for an additional
21 days due to potential virus shedding.
All vaccinated horses should receive yearly boosters to protect against
infection and, for the stallions, to prevent the development of a carrier
state. In a generation or two, these practices could all but eliminate
the population of carrier stallions.
Additional Information
For more information about EVA, contact
USDA, APHIS, Veterinary Services
National Animal Health Programs
4700 River Road, Unit 43
Riverdale, MD 20737
Telephone (301) 734–6954
Fax (301) 734–7964
Current information on animal diseases is also available on the Internet
at www.aphis.usda.gov.
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or marital or family status. (Not all prohibited bases apply to all
programs.) Persons with disabilities who require alternative means for
communication of program information (Braille, large print, audiotape,
etc.) should contact USDA's TARGET Center at (202) 720–2600 (voice
and TDD).
To file a complaint of discrimination, write USDA, Director, Office
of Civil Rights, Room 326–W, Whitten Building, 1400 Independence
Avenue, SW, Washington, DC 20250–9410 or call (202) 720–5964
(voice and TDD). USDA is an equal opportunity provider and employer.
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