Equine Viral Arteritis (EVA)

What is EVA?

EVA is a contagious viral disease of horses. There are periodic outbreaks of EVA in horse populations throughout the world. The most recent outbreak in the United States occurred in Quarter Horses in New Mexico and Utah in 2006.

Clinical signs/symptoms

EVA can cause mild to severe respiratory disease, abortions in pregnant mares, and can result in some stallions becoming persistently infected. Clinical signs of EVA mimic many other equine diseases and include: fever, lethargy, nasal discharge, coughing, diarrhea, swelling of the legs, body, and/or head. Most horses make complete clinical recoveries.

How does a horse become infected?

Horses are infected by inhaling the virus, through breeding with infected semen from a carrier stallion (natural service, cooled, and frozen semen), or contact with contaminated objects (bedding, buckets, stalls, etc). The incubation period from exposure to onset of clinical signs is 3-7 days. Acutely infected horses shed the virus in their nasal secretions for up to 16 days. Mares that contract the virus from a carrier stallion will shed the virus via the respiratory route. Exposure of mares greater than 3 months in foal may lead to abortion or the foal may be born with pneumonia and die within a few days of birth. Carrier stallions contract the disease via the respiratory route. The virus may than persist in his accessory sex glands and be shed in the semen. Chronic carrier stallions do not shed the virus via the respiratory route, but continue to shed the virus in the semen for weeks, months, years, or even a lifetime.

Treatment

There is currently no specific treatment for EVA. Treatment consists of supportive/symptomatic care including: rest, anti-inflammatories, diuretics, and hydrotherapy. Treatment of foals with pneumonia caused by EVA has not been successful. Castration will resolve the carrier state in stallions but there is no non-surgical way to eliminate the virus from a stallion’s reproductive tract.

Testing

A blood test is available to determine if a horse has antibodies to the virus. Current tests are not able to distinguish antibodies that develop following vaccination from antibodies that develop following natural exposure to the virus. Therefore, all breeding stallions must be tested prior to vaccination. Stallions that test negative should be vaccinated within a few weeks of testing. Stallions that test positive should have their semen tested for viral shedding, to determine if they are a carrier. Mares to be bred to a carrier stallion should also be tested. It is imperative to keep records of testing and vaccination.

Vaccination Recommendations ( AAEP Guidelines for Vaccination of Horses)

 *There is only one commercially available vaccine for EVA in North America . Unfortunately it is in limited supply due to the 2006 outbreak. Vaccine availability is expected to increase in the next few months.

The vaccine is a modified-live vaccine and is safe, effective, and low-cost. Vaccinated stallions and mares should not be bred for at least 28 days or 21 days respectively and all vaccinated horses should be isolated from pregnant mares for at least 21 days following vaccination to prevent the minimal risk of the spread of vaccine virus. The vaccine is not recommended for use in pregnant mares.

Proper documentation of vaccination should include accurate horse identification, serological status prior to vaccination, and vaccination date.

Foals and Weanlings: Intact colts intended to be breeding stallions: One dose at 6-12months of age.

 Yearlings: Annual for colts intended to be breeding stallions.

 Performance Horses: Annual for colts intended to be breeding stallions.

 Pleasure Horses: Annual for colts intended to be breeding stallions.

 Broodmares: Annual for seronegative, open mares 21 days before breeding to carrier stallions; isolate mares for 21 days after breeding to a carrier stallion.

Stallions: Annual for breeding stallions and teasers, 28 days before the start of the breeding season; isolate for 21 days following vaccination.

Breeding to a Carrier Stallion:

Mares to be bred to a carrier stallion should be tested prior to breeding.

Negative mares should be vaccinated 21 days before breeding. They should be isolated for 21 days after vaccination to allow sufficient time for antibodies to develop and prevent the minimal risk of spreading the vaccine virus to other susceptible horses. Post-breeding, mares should be isolated for 21 days. Subsequent breedings do not require isolation.

Positive mares do not need to be vaccinated prior to breeding to a carrier stallion. They should be kept isolated from other susceptible horses for 24 hours following breeding to avoid transmission of the virus from voided semen.

Prevention