Peter J. Timoney, MVB, PhD, William H. McCollum, PhD and Mary L. Vickers, PhD
Reprinted with Permission by Dr. Peter J. Timoney, MVB, PhD
Equine viral arteritis (EVA) continues to be a source of widespread concern and at times, significant controversy among many horse breeders and owners because of the potential risk of virus-related abortion in mares and establishment of the carrier state in stallions. Much of the current awareness and concern over EVA have stemmed from the restrictions that were imposed on the international movement of horses from the USA following the 1984 epidemic in Thoroughbreds in Kentucky.
Studies carried out since 1984 have confirmed the importance of the carrier stallion in the epidemiology of equine arteritis virus (EAV) infection. Many stallions continue to harbor the virus in the reproductive tract for extended periods of time. Since such animals shed EAV constantly in the semen, they are very effective transmitters of the virus and readily infect seronegative mares to which they are bred by natural service or artificial insemination.
There is growing evidence that a significant percentage of the frozen semen currently imported into the USA is infected with EAV. The use of semen from certain carrier stallions has on occasion, been directly linked to the occurrence of outbreaks of EVA, some of which have been associated with abortion and deaths in neonatal foals. At the present time, the USA is unique among all other horse breeding countries in having no federal requirements whatsoever relating to the importation of carrier stallions or infective semen. In the majority of instances, horse owners and breeders are breeding their mares with imported semen in total ignorance of its infectivity status for EAV and the potential risks involved.
In view of the significant role of the carrier stallion in the epidemiology of EVA, every effort should be made to minimize frequency of the carrier state through immunization of susceptible stallion populations. Furthermore, vaccination of sexually immature colts between 6 and 9 months of age is recommended as a means of preventing establishment of the carrier state in the event of possible exposure to EAV at a later age. Such a vaccination strategy would, in a matter of a few years, reduce the carrier reservoir of the virus in those breeds in which the virus is currently endemic.
In light of the very real risk of introducing EAV into a susceptible horse population through the use of infective fresh-cooled or frozen semen, appropriate measures should be adopted to prevent possible outbreaks of EVA resulting from insemination of mares that are seronegative and unprotected against infection with the virus.
Current programs for the prevention and control of EVA have been directed primarily at restricting spread of the infection in breeding populations both to minimize the risk of virus-related abortion and prevent establishment of the carrier state in the stallion. Kentucky and New York are the only states in the USA which have formulated specific control programs for this infection; the programs in both states, however, apply specifically to their respective Thoroughbred breeding populations. In late 1996, the Dutch Warmblood Registry of North America instituted a requirement that all stallions be tested prior to registration to establish their carrier status for EAV and also, that all imported semen be examined to determine its virus infectivity status.
The impact that EVA continues to have on the international movement of horses, underscores the need for an effective and practically feasible policy for the prevention and control of the disease at the national level. This will require testing and identification of imported carrier stallions and infective semen and acceptance of approved guidelines for the control of EVA by the U.S. horse industry. An appropriate set of guidelines has been developed under the aegis of the American Horse Council which has received the full endorsement of the USAHA and AAEP. The need to implement a national control program for EVA based on these guidelines is long overdue.
Copyright © 1999 American Horse Council