What is EVA?   EVA is caused by the equine arteritis virus (EAV), which is present in most horse breeds.  This virus is not new to the horse industry, gaining notoriety in 1953 after an outbreak on a Standardbred farm in Ohio.  In 1984, an epidemic occured on approximately 41 Thoroughbred breeding farms in Kentucky.  In 2006, EVA appeared in the state of New Mexico, crippling a Quarter Horse breeding farm, and in Normandy, France, where eight stallions were affected, causing restricted movement of horses within the area and the cancellation of shows. 
   EVA is not a disease to take lightly, but can be managed quite sucessfully with the use of appropriate vaccinations and education.  It has sometimes been referred to as a political disease, in that countries which have EVA present still put restrictions on any semen entering into the country - some feel to reduce competition from imported semen.  EVA-positive horses can also be restricted internationally if showing in other countries.

Mode of Transfer  The most common mode of transfer for this agent is primarily through the respiratory system via particles in the nasal discharge of infected horses; however, what makes this virus so dangerous is that it can harbour itself in the accessory glands of the reproductive tract of an unsuspecting stallion.  The organism can then be shed in the semen of the asymptomatic carrier.  Why would this be a problem?  Although the semen appears unaffected by the virus, the fertility intact, mares that are inseminated with contaminated semen will develop the disease and spread it via the respiratory route to other horses in close proximity.  If a pregnant mare contracts EVA, the foal will usually die and be aborted.  If the foal is born, it will only live for a short period of time.  The abortion rate varies from ten to 75 percent of mares infected with EAV.
   EQV is testosterone dependent, which means only stallions can be long term carriers, as once gelded, the virus will die out.  Mares, geldings, and foals that are infected with EAV will only shed the virus for a few weeks, with all disease transmission terminating after three weeks.
   In the acute stage, the virus can be shed via nasal transmission for a period of one to two weeks.  Even vaccinated horses can exhibit the symptoms for this period and need to be isolated from other horses during the incubation time frame.
   Stallions that are infected with EVA can be blood-only positive, which means they are carriers, and shed the virus through respiratory means but do not shed the virus in their semen; or blood- and semen-positive, which means they are carriers and shed the virus through their semen and possibly when exhibiting symptoms.  A blood-positive stallion can become a semen-positive stallion.

Clinical Signs of EVA   Most horses do not exhibit signs of EVA when infected by EAV, but may show a combination of symptoms such as nasal discharge, fever, swelling, loss of appetite, depression, and muscle soreness.  The symptoms may also include swelling of the genitals in stallions or mammary glands in mares.  Because the symptoms mimic other respiratory diseases, it is difficult to determine that EAV is the causative agent without laboratory testing.

Treatment   Since EVA is caused by a virus, antibiotics will not help any infected animal.  This disease is quite manageable with the proper vaccination program in place.  Vaccination of mares with ARVAC, a modifed live vaccine available from Fort Dodge Animal Health, a division of Wyeth, 21 days prior to breeding allows for sufficient development of EVA antibodies, and possible prevention of abortion if the mare is then bred to a known shedder stallion.
   Seronegative stallions, those that have not been exposed to the disease and have not developed antibodies, can be vaccinated with this vaccine to prevent contamination and becoming shedders of the virus.
   The vaccine is a modified, live virus and will cause the vaccinated horse to possibly exhibit signs of the disease.  Isolation of vaccinated horses for a period of at least 21 days is a must to prevent other horses from becoming contaminated.  Vaccination will result in a seropositive horse and it is impossible to distinguish between a horse infected via active route or via vaccination.  It is imperative that when vaccinating, proper documentation is put in place to ensure the horse is identified as a seropositive through vaccination and not through infection.

Recent Updates   The state of Montana has recently put restrictions on the import of semen into the state.  Any semen entering this state will require a stallion owner to obtain a permit accompanied by a USDA-Aphis approved lab test result stating the stallion tested negative for Equine Infectious Anemia within the last 12 months, and EVA within the last six months.  Proof of vaccination must be shown for those stallions that have been vaccinated.  If the stallion is seropoitive (antibody-positive), the semen of this stallion must undergo virus isolation testing of the semen to ensure non-shedder status.  Any veterinarian that inseminates semen without the requisite permit will be penalized.  While this is a proactive means for control of the spread of EVA, the restrictions have some issues which need to be addressed, in particular, the fact that only the breeding stallion has been isolated in this permit, no other horse - be it gelding, mare, or stallion.
   Other areas may have other types of restrictions in place to to the recent outbreaks.  It is advisable for the stallion owners to include a clause in their contracts regarding this matter to identify potential permit "problems" prior to shipment of semen. 

Guidelines   In the United States, guidelines were proposed by a working group of Veterinarians and representatives from several breed organizations. 
   The following is what is recommended for stallions and mares in a breeding operation.

For Stallions    It is recommended that at least 60 days prior to the breeding season, the stallion should be blood tested for the presence of antibodies to EAV.
   If the stallion is antibody negative in the blood, the stallion owner should consider vaccination with the licensed modified live vaccine and isolated for a period of 30 days.  annual boosters are recommended.
   If the stallion is found to be antibody positive in the blood, and has not been vaccinated, then he must have his semen tested to determine if he is shedding the virus in his semen by collection of two separate ejaculates on week apart and submitting the samples to an approved laboratory.
   If the stallion is proven to be a non-shedding stallion while being antibody positive in the blood, the owners may wish to retest the semen as the stallion can move to a shedding state.
   Those stallions that are found to be blood and semen positive can still be used in a commercial breeding operation if managed accordingly.  Stallion owners and managers are advised to disclose the shedding status to the mare owners and advise them to vaccinate the breeding mare prior to receiving shipped semen.

For Mares Breeding to an EVA Shedding Stallion   The mare should have blood tested at least 30 days prior to breeding for presence of EAV.  If the mare is founnd to be blood negative, she should be vaccinated with the modified live vaccine and isolated for 21 days to allow for sufficient immunity to develop.  The mare should not be bred until after the 21 day period.  After being bred, the mare will need to be isolated again for a period of 21 days.  The current licensed vaccine available for horses is not approved for use in pregnant mares.  If the mare is to be bred again to a shedding stallion, an annual booster vaccination is required 21 days prior to being used for breeding purposes.
   Any mares that are blood positive for EAV can be bred to a shedder stallion without any prior vaccination.  If breeding a positive mare to a known shedding stallion via live cover, it is advisable to isolate the mare after breeding and any vehicle used to transport said mare be disinfected and cleaned thoroughly.