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Equine Viral
Arteritis
Equine Typhoid,
Epizootic Cellulitis–Pinkeye,
Epizootic Lymphangitis Pinkeye,
Rotlaufseuche
Last Updated: August 2009
Importance
Equine viral arteritis (EVA) is an economically important viral disease of equids.
Stallions can become long term carriers of the virus, and transmit it during breeding.
Although carrier stallions can be bred if precautions are taken, the need to mate them
with seropositive or vaccinated mares decreases their desirability as breeders. Acute
illness also occurs in some horses. Although deaths are very rare in healthy adults,
pregnant mares that become infected may abort, and very young foals may die of
fulminating pneumonia and enteritis. Equine viral arteritis has recently increased in
prevalence, possibly due to increased transportation of horses and semen.
Etiology
Equine viral arteritis is caused by equine arteritis virus (EAV), an RNA virus in
the genus Arterivirus, family Arteriviridae and order Nidovirales. Isolates vary in
their virulence and potential to induce abortions. Only one serotype has been
recognized. Limited genetic analysis suggests that EAV strains found among donkeys
in South Africa may differ significantly from isolates in North America and Europe.
Species Affected
Equine arteritis virus is found in the Equidae. Antibodies to this virus have been
reported in horses, ponies, donkeys and zebras. Illness occurs mainly among horses
and ponies, but clinical signs have also been reported in experimentally infected
donkeys. EAV might also be able to cause disease in South American camelids:
polymerase chain reaction assay (PCR) detected viral nucleic acids in an alpaca that
had aborted.
Geographic Distribution
Antibodies to EAV have been found in most countries where testing has been
done. Seropositive horses have been reported in North and South America, Europe,
Asia, Africa and Australia. Infections are common among horses in continental
Europe, but rare in the United Kingdom. This virus appears to be absent from Iceland
and Japan.
Transmission
Equine arteritis virus can be transmitted by the respiratory and the venereal
routes. Acutely affected horses excrete the virus in respiratory secretions; aerosol
transmission is common when horses are gathered at racetracks, sales, shows and
other events. This virus has also been found in urine and feces during the acute stage.
It occurs in the reproductive tract of acutely infected mares, and both acutely and
chronically infected stallions. In mares, EAV can be found in vaginal and uterine
secretions, as well as in the ovary and oviduct, for a short period after infection.
Mares infected late in pregnancy may give birth to infected foals. Stallions shed EAV
in semen, and can carry the virus for years. Transmission from stallions can occur by
natural service or artificial insemination. Some carriers may eventually clear the
infection. True carrier states have not been reported in mares, geldings or sexually
immature colts; however, EAV can occasionally be found for up to six months in the
reproductive tract of older prepubertal colts.
Equine arteritis virus can be transmitted on fomites including equipment, and
may be spread mechanically by humans or animals. This virus is inactivated in 20–30
minutes at 56-58ºC (133-136ºF), but can remain viable for 2 to 3 days at 37-38ºC (99-
100ºF) and for up to 75 days at 4-8ºC (39-46ºF). Semen remains infectious after
freezing.
Incubation Period
The incubation period varies from 2 days to 2 weeks. Infections transmitted
venereally tend to become apparent in approximately one week.
Clinical Signs
Most EAV infections, especially those that occur in mares bred to long-term
carriers, are asymptomatic. The clinical signs are generally more severe in old or very
Equine Viral Arteritis
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young animals, and in horses that are immunocompromised
or in poor condition. Fulminant infections with severe
interstitial pneumonia and/ or enteritis can be seen in foals
up to a few months of age. Systemic illness also occurs in
some adults. In adult horses, the clinical signs may include
fever, depression, anorexia, limb edema (particularly in the
hindlimbs), and dependent edema of the prepuce, scrotum,
mammary gland and/or ventral body wall. Conjunctivitis,
photophobia, periorbital or supraorbital edema and rhinitis
can also be seen. Some horses develop urticaria; the hives
may be localized to the head or neck, but are sometimes
generalized. Abortions or stillbirths can occur in mares that
are pregnant when they are exposed. Abortions are not
necessarily preceded by systemic signs. Temporary
decreases in fertility, including reduced quality sperm and
decreased libido, may be seen in stallions during the acute
stage of the disease. The decrease in sperm quality has been
attributed to increased scrotal temperature and edema, and
can persist for up to 4 months. The quality of the semen is
not decreased in carrier stallions. Except in cases of severe
disease in foals, deaths are rare.
Outbreaks have not been reported among donkeys or
mules, and although antibodies have been found, there is
little information on the clinical signs in these species.
Fever, sometimes accompanied by mild depression, mild
conjunctivitis, a slight oculonasal discharge and/or mild
dependent edema were reported in donkeys inoculated with
an EAV strain that is moderately virulent for horses. The
clinical signs were mild, and could readily have been
missed if the animals were not closely monitored. Donkeys
inoculated with a South African donkey strain developed
mild to more severe clinical signs including fever,
depression, serous ocular and nasal discharge, and
conjunctivitis. Pregnant donkey mares did not abort, and
their foals were clinically normal when born. Horses
inoculated with this donkey strain had very mild clinical
signs.
Post Mortem Lesions
In acute cases, the lesions are characterized by edema,
congestion and hemorrhages of the subcutaneous tissues,
visceral organs and lymph nodes. These changes are often
found in the subcutaneous tissues of the limbs and
abdomen, the thoracic and abdominal lymph nodes, and the
small and large intestines (especially the colon and cecum),
but may occur throughout the body. Accumulations of
clear, yellowish fluid may be found in the peritoneal cavity,
pleura and pericardium. Foals may also have pulmonary
edema, interstitial pneumonia, emphysema, splenic infarcts
and enteritis. In mares that abort, the endometrium may be
swollen and congested, and can contain hemorrhages.
Aborted fetuses are often partially autolyzed, but may be
well preserved. In some fetuses, the only gross lesions may
be excess fluid in the body cavities and signs of interlobular
interstitial pneumonia.
Morbidity and Mortality
The prevalence of equine arteritis virus varies
significantly among horse breeds. In the U.S., this infection
is particularly common among Standardbreds. In a survey
done in 1998, 24% of unvaccinated Standardbreds, 4.5% of
Thoroughbreds, 0.6% of Quarter horses, and 3.6% of
Warmblood horses had antibodies to this virus. In Europe,
the seroprevalence is particularly high among Warmbloods.
Breed-related differences in seroprevalence might be due to
genetic differences, but they are more likely to be caused by
different management practices. In experimentally infected
horses, the breed has no apparent effect on susceptibility to
infection or the establishment of carriers. Overall,
approximately 10-70% of infected stallions become carriers
of EAV.
Outbreaks are much less common than infections, and
tend to occur when horses have been moved or semen
shipped. The clinical signs are generally more severe in old
or very young animals, and in horses that are
immunocompromised or in poor condition. They also vary
with the route of exposure, and the strain and dose of the
virus. Asymptomatic infections are common, and deaths are
very rare in healthy adults; young foals are more likely to
die. The abortion rate varies from less than 10% to as high
as 50-60%.
Diagnosis
Clinical
Equine viral arteritis should be considered when the
clinical
signs
include
fever,
depression,
edema,
conjunctivitis, nasal discharges and abortions. This disease
is difficult to differentiate from other systemic and
respiratory illnesses of horses.
Differential diagnosis
The differential diagnosis includes equine influenza,
equine infectious anemia and African horse sickness, as
well as infections with Getah virus, Hendra virus, equine
rhinitis A and B viruses, equine adenoviruses, and equine
herpesviruses 1 and 4. Equine viral arteritis also resembles
purpura hemorrhagica and other streptococcal infections, as
well as poisoning from the toxic plant Berteroa incana
(hoary alyssum).
Laboratory tests
Equine viral arteritis can be diagnosed by virus
isolation, the detection of viral antigens or nucleic acids,
and serology.
In recently infected animals, equine arteritis virus may
be recovered from nasal secretions, blood and semen as
well as from a number of tissues and fluids at necropsy.
Carrier stallions can be identified by isolating the virus
from semen; EAV is not found in the respiratory secretions,
blood or urine of carriers. This virus can be isolated in
rabbit, equine and monkey kidney cells or cell lines. RK–13
(rabbit kidney) cells are the system of choice. The identity
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of the virus can be confirmed using serum neutralization,
reverse-transcription
PCR
(RT-PCR)
assays,
immunofluorescence or immuno-histochemistry.
RT-PCR can also detect viral nucleic acids in clinical
samples from live horses or tissues taken at necropsy.
Immunohistochemistry can identify viral antigens in
necropsy samples, as well as in skin biopsies from acutely
infected horses; the latter technique is not completely
reliable. Histopathology is helpful in clinical cases.
Serological
tests
include
virus
neutralization,
complement fixation, agar gel immunodiffusion, indirect
fluorescent antibody, fluorescent microsphere immunoassay
(MIA) and enzyme–linked immunosorbent assays (ELISA).
A complement-enhanced micro-neutralization test is often
used to detect antibodies to EAV. In acute cases, a fourfold
increase in titer should be seen in serological tests.
Carriers can also be detected by breeding the stallion to
two seronegative
mares,
which are checked for
seroconversion four weeks after breeding.
Samples to collect
In live horses with acute infections, EAV can be
isolated from nasopharyngeal swabs or washings,
conjunctival swabs and unclotted blood samples. Because
heparin may inhibit the growth of RK-13 cells, blood
samples should be anticoagulated with acid citrate dextrose
or EDTA. Clinical samples can also be tested by RT-PCR.
Samples should be collected as soon as possible after the
onset of the clinical signs. Samples for virus isolation
should be refrigerated and shipped cold. Carriers can be
identified by detecting EAV in semen using virus isolation
or RT-PCR. Semen samples should contain the sperm-rich
fraction of the ejaculate; virus is not found in the pre-sperm
fraction.
At necropsy, tissues should be collected for
histopathology; the cecum, colon, spleen and associated
lymph nodes, as well as the adrenal cortex, are especially
helpful. In aborted fetuses or stillborn foals, samples for
virus isolation should be taken from the placenta, fetal
fluids and fetal tissues, especially the lymphoid tissues,
lung and spleen. In young foals with fatal disease, necropsy
samples should include a variety of organs and lymph
nodes associated with the respiratory and gastrointestinal
tracts; they should include the lungs, liver and spleen.
Tissue samples can also be collected for RT-PCR and
immunohistochemistry. EAV antigens can be found in
organs with or without lesions. Immuno-peroxidase staining
can detect viral antigens in formalin-fixed or frozen tissues.
Paired acute and convalescent serum samples should be
collected to diagnose acute disease. Serology is also helpful
in surveillance and to confirm the absence of infection
before vaccination.
Recommended actions if
equine viral arteritis is suspected
Notification of authorities
Equine viral arteritis is a reportable disease in many
states. Each state should be checked for specific
regulations.
Federal: Area Veterinarians in Charge (AVIC):
http://www.aphis.usda.gov/animal_health/area_offices/
State Veterinarians:
http://www.usaha.org/Portals/6/StateAnimalHealthOfficials
.pdf
Control
Acutely infected horses should be isolated to prevent
transmission in secretions and excretions. Precautions
should also be taken to avoid spreading the virus on
fomites. EAV is readily inactivated by detergents, common
disinfectants and lipid solvents. No specific treatment is
available; however, most healthy horses other than young
foals recover on their own. Good nursing and symptomatic
treatment should be used in severe cases. Vaccination can
also help contain outbreaks.
Venereal transmission can be controlled by good
management and vaccination. To protect pregnant mares
from abortion, they should be separated from other horses
and maintained in small groups according to their predicted
foaling dates. Newly acquired horses should be isolated for
3 to 4 weeks. Vaccination appears to prevent uninfected
stallions from becoming long term carriers. Stallions that
are not carriers should be vaccinated before the start of the
breeding season. Prepubertal colts are given the vaccine
when they are 6-12 months old. Carrier stallions are
identified and bred only to well vaccinated or naturally
seropositive mares. Similarly, semen that contains EAV
should be used only in these mares. Because first-time
vaccinates may shed field viruses for a short time after
exposure, these mares should be isolated from seronegative
horses, particularly pregnant mares, for three weeks after
breeding. Naturally infected mares and those that are not
first-time vaccinates are isolated for 24-48 hours, to protect
other horses from the viruses present in semen.
Carrier stallions should be housed where they can be
physically separated from uninfected horses; in one case,
stallions apparently became infected by indirect exposure to
semen. However, this appears to be rare. EAV is sensitive
to sunlight and low humidity, and uninfected stallions have
been kept near carriers for years without infection.
Excellent hygiene and decontamination of fomites should
be practiced when breeding infected horses or collecting
semen. Although some experimental methods may be
promising, there is no proven method to eliminate carriage
in stallions.
Mandatory
notification,
annual
testing,
the
identification of carrier stallions and control of transmission
from these animals, and selective vaccination are being
used in New Zealand to eradicate equine viral arteritis.
Equine Viral Arteritis
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Public Health
There is no indication that equine arteritis virus can
infect humans.
Internet Resources
Food and Agriculture Organization of the United Nations
(FAO). Manual for the Recognition of Exotic Diseases
of Livestock
The Merck Veterinary Manual
http://www.merckvetmanual.com/mvm/index.jsp
USDA APHIS. Equine viral arteritis
http://www.aphis.usda.gov/animal_health/animal_disea
ses/eva/
World Organization for Animal Health (OIE)
OIE Manual of Diagnostic Tests and Vaccines for
Terrestrial Animals
http://www.oie.int/international-standard-
setting/terrestrial-manual/access-online/
OIE Terrestrial Animal Health Code
http://www.oie.int/international-standard-
setting/terrestrial-code/access-online/
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*Link defunct as of 2009