EVA Discussed at Veterinary Convention

An engaging Table Topic discussion on Equine Viral Arteritis (EVA) was held at the 2009 American Association of Equine Practitioners Convention in Las Vegas, Nev. Approximately 40 equine practitioners attended the discussion, which touched on some current topics regarding the management and control of EVA.

Topics covered in this session ranged from vaccination of the pregnant and post-partum mare to import and export regulations of virally infected semen and stallions.

A disparity is found between countries regarding stallions. A few countries do not distinguish between seropositive, non-infected stallions and seropositive, persistently infected stallions, while a majority of the countries do differentiate between the two situations. A similar discussion touched on state-specific requirements involving semen and/or stallions persistently infected with equine arteritis virus.

A major point raised in this session is the lack of uniformity of procedures and protocols between states within the USA for dealing with this disease, the shipment of semen, and the handling of persistently infected stallions.

The implications of EAV infection with respect to advanced reproductive techniques were also discussed. EAV can certainly be transmitted through chilled or frozen semen, and indeed this is how the disease is often spread. Several participants in the discussion mentioned they have managed client mares that were bred with EAV-positive semen, and that with appropriate quarantine measures, significant morbidity (Illness) did not occur. Some preliminary research data was shared that suggests EAV virus might be associated with embryos collected from mares bred with EAV-positive semen.

State and federal regulations relating to EAV were also discussed. Testing for EAV prior to or upon importation is not currently required by the USDA; individual states might or might not not require testing for EVA, although most do not appear to have any mandatory regulations. The USDA's EVA Uniform Methods and Regulations was briefly mentioned, but it was made clear that this is a document containing recommendations to states, not requirements. If testing before importation to a state is desired by the equine industry, the industry should share this desire with their state regulatory animal health officials.

When faced with the question as to the best time to vaccinate the mare, some preliminary research data was shared that suggests that vaccination of pregnant mares in the last two months of gestation should be avoided if possible, but in the face of an EVA outbreak, the risk of vaccine-associated abortion might be less than the risk of wild type virus-associated abortion.

Immediate post-partum (immediately after foaling) vaccination of the mare does not appear to be associated with adverse affects in the mare or foal. Additional discussion focused on biosecurity requirements for farms breeding mares with virally infected semen and the vaccination of mares at risk.

When considering the vaccination of weanling colts, the question arose as to the best time to vaccinate and document the seronegative status of the colt before vaccination. Colts should be serologically tested for antibodies to equine arteritis virus (EAV) before vaccination, and this testing should commence between six and nine months of age, when EAV maternal antibodies have waned. It is critical that the seronegative status is documented before vaccination.

The need to vaccinate prepubertal colts was aimed at the eventual elimination of the viral reservoir--the persistently infected stallion. It was reiterated in this session--as in past sessions--that the American Quarter Horse Association maintains a vaccination and serology documents repository for the American Quarter Horse breed.

A final concern was raised regarding the future availability of EVA-specific vaccine. Fort Dodge Animal Health, the sole source of an approved vaccine (Arvac), has recently merged with Pfizer. Whether or not this vaccine will continue to be produced was not known. (Editor's Note: Pfizer will continue to manufacture the Arvac and have it available through licensed veterinarians, see TheHorse.com/ViewArticle.aspx?ID=15137.)

Submitted by moderators Reed Holyoak, DVM, and Charlie Broaddus, DVM

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