Hot Topic—EVA
Recently there has been increased interest in Equine Viral
Arteritis (EVA) by veterinarians and horse owners as it has
been identified in both New Mexico and Utah in 2006. In
light of this interest, the following article serves to
briefly address some of the more common concerns in regard
to EVA: transmission risks, impact of EVA, control and
prevention including vaccination and recommended
bio-security measures.
Equine
Viral Arteritis is a disease of both horses and donkeys and
the virus, equine arteritis virus (EAV) is widely
distributed among equid populations throughout the world. It
is called arteritis because of the distinctive lesions
present in the acute phase of infection; a panvasculitis
most distinct in the muscle wall of small arteries. Horses
infected with EAV display a range of presentations from
subclinical infection to an influenza-like illness in adult
horses, abortion in pregnant mares, and interstitial
pneumonia in neonatal foals. When illness does occur—usually
within 3-7 days of exposure—EVA can be difficult to diagnose
because it is clinically similar to several other equine
diseases such as equine rhinopneumonitis, influenza, equine
infectious anemia (EIA), hoary allysum intoxication, and
purpura hemorrhagica. Most notable is that approximately
30-60% of exposed stallions become persistently infected
carriers following natural EAV infection, whereas neither
mares nor geldings become persistently infected with the
virus. It is a manageable disease if the control program
aims at minimizing or eliminating direct or indirect contact
of susceptible horses with the secretions/excretions of
infected animals. Most critical to success of a control
program is the aim to restrict dissemination of EAV in
breeding populations and the prevention of the establishment
of the carrier state in stallions and post-pubertal colts.
Transmission Risk: EAV infection can be transmitted among
horses in five different ways:
• Respiratory—primary route of acute transmission, common at
racetracks, shows, and sales.
• Venereal—virus shed in the semen of a carrier stallion
(cooled or frozen semen can be infectious).
• Body Secretions—urine, feces, etc.
• In Utero—virus passes across the placenta from an acutely
infected mare to her unborn foal (uncommon).
• Indirect Contamination—tack and/or equipment shared among
horses.
There is a very real risk of EVA being transferred
indirectly via personnel and fomites. Special care should be
taken when handling semen in laboratories prior to
insemination or preparation for shipping.
The Impact of EVA: EVA can have a significant economic
consequences for both the breeding and performance sectors
of the horse industry. Direct financial losses resulting
from outbreaks of the disease on breeding farms can be
summarized as follows: losses due to abortion and/or disease
and death in very young foals, decreased commercial value of
stallions that become persistently infected with the virus,
reduced demand to breed to carrier stallions because of the
added expense, and inconvenience involved in vaccinating and
isolating mares before and after breeding and denied export
markets for carrier stallions and infected semen. An
outbreak of EVA at a racetrack, equestrian event or horse
show can have considerable impact: potential for widespread
transmission (returning home after exposure). Direct
financial losses through abortion, infected stallions,
disruption of training schedules, reduced competition
entries, and event cancellations. At the international
level, affected trade in horses and semen with denied export
opportunities for carrier stallions, EAV infective semen
and, with some countries, trade in all categories of horses
that hve “undocumented” antibodies to the virus.
Control & Prevention: EVA is a manageable disease.
• It has been possible to design effective strategies for
control and prevention.
• A major factor in determining success of any control
program is minimizing or eliminating direct or indirect
contact of susceptible horses with the secretions/excretions
of infected animals.
• Effective strategies for control and prevention must be
horse industry driven; it is the industry’s disease to
control or spread.
• There is no zoonotic concern.
• The aim is to restrict dissemination of EAV in breeding
populations, prevent outbreaks of virus-related abortion or
illness in young foals and prevent the establishment of the
carrier state in stallions and post-pubertal colts.
There is a safe and effective EVA vaccine (Arvac®, Fort
Dodge Animal Health).
• It has been shown to be both safe and effective for use in
stallions, non-pregnant mares, geldings, fillies and colts.
• There is no evidence that a vaccinated stallion will shed
virus in their semen or develop the carrier state.
• It is not labeled for use in the last trimester pregnant
mare, or in foals less than 6 weeks of age and should be
avoided in these situations except in times of high risk of
natural infection and only if recommended by a veterinarian.
Vaccine: Arvac®, Fort Dodge Animal Health
• Modified live vaccine
• Only licensed vaccine
Indications and Dosage:
• For the vaccination of healthy non-stressed horses.
• If all horses are not being vaccinated at the same time,
isolate those being vaccinated from those you wish to remain
sero-negative. There is a minimal potential for vaccine
virus to be shed and spread to other horses.
• Administer one 1mL dose intramuscularly.
• Vaccinate males and young animals at any time (not labeled
for use in foals less than 6 weeks of age).
• Stallions should be vaccinated not less than 3 weeks prior
to breeding.
• Vaccinate mares preferably as maidens or when open.
• According to the label, mares in foal should not be
vaccinated until after foaling and then not less than 3
weeks prior to breeding.
• In the face of high risk of exposure to wild-type virus,
pregnant mares can be vaccinated.
• Maiden and barren mares may be vaccinated anytime but
should be vaccinated not less than 3 weeks prior to
breeding.
• Repeat with annual booster dose.
Vaccination: Primary vaccination affords protection against
clinical disease for several years. If initial vaccinates
are exposed to field virus for the first time via venereal
or aerosol transmission, they will probably have a limited
re-infection cycle and be short-term shedding of the field
strain virus (approx. 1 week) Revaccination normally results
in a pronounced increase in antibody titers and protection
against the disease.
Biosecurity Measures:
• Isolate all new arrivals for 3 to 4 weeks.
• If at all possible segregate pregnant mares from other
horses. Maintain pregnant mares in small groups according to
predicted foaling dates.
• Prior to each breeding season, blood-test all new breeding
stallions for EAV titers. Culture semen from all sero-positive,
non-vaccinated stallions for infectious virus.
• Annually vaccinate all non-carrier breeding stallions at
least 4 weeks prior to start of each breeding season.
• Physically isolate any EAV-carrier stallions.
• Observe strict precautions when breeding or collecting
semen. Risk of advertently transferring infection via
indirect contact.
• Limit breeding carrier stallions to vaccinated mares or
mares with natural titers.
• Vaccinate sero-negative mares at least 3 weeks prior to
breeding to a known carrier stallion or artificial
insemination (AI) with infectious semen.
• Isolate initial vaccinate mares for 3 weeks post breeding
from all but known EAV sero-positive horses.
• It is especially important that these mares do not have
contact with pregnant mares by any route, aerosol,
respiratory and/or indirect contact.
• In breeds and/or areas with high prevalence of EAV
infection, vaccinate all immature colts before 270 days of
age (seven months). If implemented—over a number of years
this would greatly reduce the number of carrier stallions
and effectively eliminate the primary reservoir.
• Determine the infectivity status of all semen for AI.
The information in this article has been provided with
permission by OSU
