EVA - EQUINE VIRAL ARTERITIS
Kirsten Voigt, DVM

Now that breeding season is upon us, it's time to start thinking about how to manage the breeding stock.  Equine Viral Arteritis (EVA) is a viral disease of horses and donkeys caused by the Equine Arteritis Virus (EAV). In recent years it has become more of a problem for breeders, and warrants some preventative action to keep it under control.  In the average adult horse, EVA may cause mild respiratory signs and is generally cleared from the body within 60 days.  Other clinical signs can include fever, nasal discharge, limb edema and sheath or mammary swelling.  In young foals it can cause severe pneumonia and death.  It becomes a problem for breeders when a mare that has never seen the virus becomes infected while pregnant.  Often her first and only clinical sign is abortion which can occur at any stage of the pregnancy after two months.  The fetal tissues can then be a source of infection for other pregnant mares. 

Other means of transmission are aerosolized droplets from nasal discharge, urine, and infected semen.  The virus can even survive in transported fresh, chilled or frozen semen.

Infected stallions are responsible for the maintenance of this disease in the horse population.  Once infected, 30-60 percent of stallions will become carriers.  This means that they will continue to shed the virus through their semen for weeks to months to years.  There is currently no way to clear a stallion of the virus.  

A carrier stallion has normal fertility and can still be bred with the proper management of all involved parties.  First off it is important to know the immune status of the stallion and the mare.  A blood test should be done to look for antibodies against the virus.  A horse may have antibodies from natural exposure or from vaccination.  The test cannot determine one from the other.  It is therefore very important to test stallions and document negative results before vaccinating.  If a stallion is negative and then vaccinated he is unlikely to become a carrier even if exposed.   

If a stallion has a positive blood test, his semen should be tested to see if he is a carrier.  If the semen tests positive, vaccinating will do nothing to change his status and is therefore unnecessary in this case. Carriers should only be bred to a mare that has antibodies either by past natural infection or from vaccination.   

If the stallion has a negative blood test he should be vaccinated immediately.  Breeding should not take place until 28 days after vaccination. Subsequently he should receive the vaccination annually at least 28 days before the start of each breeding season.  The same protocol should be used for a stallion with a positive blood test but that is not shedding the virus in the semen.  Having said this, if there are any plans for exporting the stallion, vaccination should be delayed until finding out the regulations of the intended destination.  Some countries do not allow in horses that test positive even if it from the vaccination. 

 A young colt between 6-9 months of age that is intended for breeding should be tested and if results are negative he should be vaccinated and have a  booster six months later.  If tested too soon he may show antibodies to EVA that have come from his mother and should be retested a month later to verify that results are true.   

Any mare that is to be bred to a carrier stallion should be vaccinated at least 21 days before the breeding. The vaccine is not labeled for use in pregnant mares so vaccinating before breeding will not only protect her but also give the best chance of passing on antibodies to the foal. If it is her first time being bred to a carrier stallion she should be isolated for 21 days due to a small amount of virus shedding that could put other pregnant mares at risk.  After any breeding all mares should be isolated for 24 hours to prevent mechanical spread of the virus. 

            When breeding a mare, one should always find out the EVA status of the stallion so that the appropriate precautions can be taken if necessary.  Remember, the virus can be in the semen, so protocols apply even if breeding by artificial insemination. 

There can also be a small amount of shedding after vaccination which means other horses in contact can be inadvertently "vaccinated."  This is not a concern if the pasture mates are geldings, non-pregnant mares, vaccinated stallions or carrier stallions.  It is a concern for pregnant mares, stallions that have not yet been tested or stallions that require a negative blood test for exporting reasons.  A newly vaccinated horse should therefore be quarantined for 21 days from the latter group. 

Following these guidelines for controlling EVA is the best way to prevent spread of the disease and ensure a good breeding year with healthy stallions, mares and foals.