EVA Outbreak in New Mexico Wrapup

An outbreak of Equine viral arteritis (EVA) occurred on a Quarter Horse breeding farm in New Mexico in June of this year and the ensuing dissemination of infection to farms both within and outside the state raises significant issues and conclusions. These issues include the management and veterinary procedures employed by the Quarter Horse industry to maximize reproductive performance by extensive use of artificial insemination and embryo transfer, which contributed to the spread of EVA.

When an infectious disease outbreak occurs, it is imperative that a prompt and accurate diagnosis is obtained. Preventive measures can then be immediately implemented to restrict the spread of infection and mitigate serious disease and economic losses. Accepting that EVA has now become established within the Quarter Horse population, it will be necessary to introduce a preventive program of vaccination accompanied by appropriate monitoring to determine the extent of this infection within the population.

In early June, the farm owner, a veterinarian, became concerned that mares previously identified as pregnant were, upon re-examination, found to be “empty.” By June 16, the increasing number of losses prompted the owner to seek advice as to the possible cause. Sera and semen samples were submitted to the Maxwell H. Gluck Equine Research Center at the University of Kentucky. The Gluck Center made the recommendation, which was accepted, that shipment of semen from stallions on the farm be halted. On June 23, results revealed very high antibody titers to equine arteritis virus in the majority of the sera, and by June 26, the virus had been isolated from the semen of two stallions. These findings provided very strong circumstantial evidence of recent exposure to the virus, which was later confirmed by examination of paired sera from individual animals.

The owner promptly informed clients that had recently received semen from the infected stallions as well as those who had moved the many donor and recipient mares on and off the farm. The state veterinarian for New Mexico was informed, and the farm was placed under quarantine, prohibiting movement of animals. Following extensive communication and submission of numerous samples to the Gluck Center by farms that had recently received semen or mares from the index farm, it was confirmed that equine arteritis virus infection had become widely disseminated to farms both within the state and in six other states.

With considerable historical information provided by the index farm, it was determined that infection was most likely introduced in late May, with four stallions becoming infected in early June, three of which began to “shed” virus in their semen. Serological examination of over 200 animals on the farm confirmed an extremely high prevalence of infection, with every mare, foal, and stallion on the farm found seropositive. A third of the yearling colts were also positive, with the yearling fillies being serologically negative. Despite the high level of exposure, the reported clinical signs were minimal.  A few animals were reported to have fevers, dependent limb edema, and mild respiratory signs, with the majority of animals experiencing sub-clinical infection. However, the number of pregnant mares losing their pregnancies during early gestation was very high.

The initial spread of infection was considered to have been through aerosol transmission from direct contact with animals in the acute stage of infection. This transmission was compounded by the high concentration of animals on the premise. Secondarily, it was thought to be spread by venereal transmission once stallions became semen “shedders” and carriers of equine arteritis virus.

Vaccination of non-exposed yearlings using the modified live vaccine ARVAC (Fort Dodge Animal Health) was undertaken on the farm, and other animals considered at risk on other farms involved in the disease occurrence also were vaccinated. Because of previous low demand, limited supplies of vaccine were available within the United States, and these supplies were quickly used up, creating an immediate lack of vaccine availability. The supplier, Fort Dodge, has undertaken to manufacture a large batch of vaccine, which was scheduled to be available in October, in time to initiate a vaccination program prior to the onset of the 2007 breeding season.

Over the last two years, there has been increasing evidence of equine arteritis virus infection within the Quarter Horse population when compared to the results of the National Animal Health Monitoring Survey (NAHMS) published in 1998. That survey indicated that the prevalence of infection within the breed was as low as 0.5%.

At a meeting of Quarter Horse breeders, owners, trainers, and veterinarians held at Ruidoso Downs in New Mexico on Aug. 17, coinciding with the annual sales, the feasibility of embarking on a vaccination program of stallions and possibly mares was addressed. Currently this proposal is under consideration. If there is agreement on a policy of vaccination, this policy will need to be implemented prior to the commencement of the 2007 breeding season.

For more information contact Dr. David G. Powell, 859/257-4757, dgpowe2@uky.edu, or
Dr. Peter Timoney, 859/257-4757, ptimoney@uky.edu, at the Maxwell H. Gluck Equine Research Center at the
University of Kentucky in Lexington.

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Equine Disease Quarterly

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