Equine Viral Arteritis in 2006

Last year was marked by the re-emergence of equine viral arteritis (EVA) on a widespread scale in the United States, with evidence of infection confirmed in 10 states. The occurrence was significant in that it represented the first major incursion of the disease into the Quarter Horse population, which previously appeared to have largely escaped exposure to the causal agent, equine arteritis virus (EAV).

Equine viral arteritis is an acute contagious disease of equids, principally characterized by fever, depression, loss of appetite, dependent edema (especially of the lower limbs), reduction in the number of circulating white cells in the blood, and frequently abortion in unvaccinated pregnant mares. The causal virus can establish persistent infection in the reproductive tracts of a variable percentage of infected sexually mature colts and stallions.

Results of the National Animal Health Monitoring System (NAHMS) 1998 study underscored how widely susceptible the U.S. Quarter Horse population was to EAV, with serologic evidence of infection in only 0.6% of a representative sampling of the national herd. Under such circumstances it is hardly surprising that the virus spread so readily on affected premises in 2006.

Of major importance in the dissemination of EAV among the Quarter Horse population was the interstate shipment of semen from a highly popular stallion on a breeding farm in New Mexico that was found to be a carrier of EAV. As events transpired, the farm turned out to be the index premises for the 2006 multi-state occurrence of EVA. Approximately 70% of the direct exposures to infection resulted from use of this stallion's semen.

Movement of donor/recipient mares from the index farm to premises either in New Mexico or in other states was responsible for the remaining 30% of direct exposures to the virus.

Industry-related practices that contributed significantly to the dissemination of EAV among and on breeding farms were the widespread practice of embryo transfer, proliferation in the number of recipient mare premises, and the highly intensive systems of broodmare management in the Quarter Horse industry.

On many premises, mares were maintained under essentially "feedlot" conditions (many horses kept in a small area) that greatly facilitated transmission of EAV by the respiratory route. None of the foregoing had previously been identified as playing a role in the epidemiology of EVA.

Of the 20 states with confirmed cases of EVA in 2006, Utah and New Mexico had the greatest number of affected premises. Clinical signs of the disease were a notable feature of some--but certainly not all--individual outbreaks of EVA.

Virus-related abortion rates as high as 50-55% were confirmed on certain farms. A limited number of infected stallions became carriers. No disease-related fatalities were reported in foals or older horses.

The 2006 multi-state occurrence of EVA was remarkable, considering the overall duration of virus activity. It was estimated that EAV had circulated for five to six months, from the time it was believed to have been introduced onto the index premises in New Mexico in May, to the last presumptive case of infection in Utah on Nov. 6.

A critical shortage of the currently available vaccine against EVA for much of this period undoubtedly contributed to the extended duration of the 2006 occurrence of the disease.

Last year's costly experience with EVA for the Quarter Horse industry serves to re-emphasize the need for greater awareness and education about the disease among horse owners, breeders, and veterinarians. The importance of EAV as a cause of contagious abortion in mares and establishment of the carrier state in stallions cannot be overstated.

Regrettably, the 2006 occurrence of EVA in the Quarter Horse breed caught the industry totally unprepared for the economic consequences of what transpired.

In light of current breeding/management practices, Quarter Horse owners and breeders must seriously consider a program of vaccination against EVA that would include not only stallions and colts, but also mares that potentially could be exposed to infection through insemination with EAV-infective semen or through coming into contact with an acutely infected cohort.

About the Author

Peter Timoney, FRCVS, PhD

Peter J. Timoney, FRCVS, PhD, Professor and Chair of the Gluck Equine Research Center at the University of Kentucky, received a MVB degree in veterinary medicine from National University of Ireland (U.C.D.), MS in virology from the University of Illinois, PhD from the University of Dublin, and Fellowship from the Royal College of Veterinary Surgeons, London. He has worked at the Veterinary Research Laboratory in Dublin, Ireland, Cornell University and the Irish Equine Centre, and has specialized in infectious diseases of the horse since 1972.

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