EVA Outbreak in Argentina

An outbreak of abortion associated with equine arteritis virus (EAV) infection occurred on a Thoroughbred breeding farm in San Antonio de Areco, Buenos Aires Province, in March 2010. On March 31, EAV was detected by reverse-transcription polymerase chain reaction (RT-PCR) and virus isolation from an aborted equine fetus and placenta, which was the third abortion to have occurred on the premises. The finding was reported to the National Health Authorities (SENASA) in Argentina, and the farm was placed under quarantine.

Based on information provided by the veterinarian, the pregnancy losses associated with this virus reached approximately 50%. This total included abortions at around five to six months of gestation and also mares that had been previously identified as pregnant and were subsequently found to have lost their pregnancies.

The Thoroughbred mares (n=40) commingled in the same paddock with sports horse mares (n=16) that had been inseminated with semen from five stallions (four standing outside of the country and one domestic stallion). Two of the four stallions’ imported semen had been used on the farm for the first time this year. Evidence pointed to semen from one of these two stallions as the source of EAV infection in the sports horse mares, which in turn, were believed responsible for spreading the infection to the pregnant Thoroughbred mares on the premises. Based on serologic testing of mares on other farms inseminated with semen from the second stallion's imported semen, none of them were carriers of EAV.

On April 14, straws containing frozen semen from the five stallions were submitted for virological examination. Equine arteritis virus was detected and isolated from the semen of one of the stallions. This semen had been imported from the Netherlands; testing carried out by the Animal Health Authorities at time of entry had given negative results for EAV.

Three mares at this farm had been inseminated with semen from this stallion. The first was inseminated on January 7, and it appears that this inseminated mare was the index case.

On April 3, a 45-day-old foal showing weakness and incoordination of the hind limbs died within 48 hours of the onset of clinical signs. The foal had not exhibited any antemortem evidence of pneumonia or enteritis. Samples of lung, liver, thymus, and spleen were collected and submitted for laboratory examination. EAV was isolated from all the tissues tested.

On April 5, blood samples were obtained from all the horses (n=140) on the farm. These consisted of 54 mares, 1 stallion, 34 yearlings, 14 foals, and 37 other (work) horses. Serological examination confirmed a very high prevalence of infection in the mares and foals and a very low prevalence in the yearlings that had been physically separated from the mares.

The infective semen was traced, and all the straws still available from the stallion were confiscated. Since four other farms had also used this semen, they too were put under quarantine. Serological surveillance of the horses on these farms confirmed that all the mares inseminated with the infective semen were seropositive.

In addition, a very high prevalence of EAV infection (97 out of 120 horses) was found in an equestrian club located in downtown Buenos Aires, where two mares had earlier been inseminated with the infective semen. When the veterinarian to this facility became aware of the results, he indicated that a few months previously several horses had developed signs of respiratory disease accompanied by fever and limb edema; these signs were not associated with equine viral arteritis at the time.

Argentina's Department of Agriculture notified the Office International des Epizooties (OIE) on May 7 of confirmation of equine viral arteritis in the country. Furthermore, it declared a health alert for horses throughout the country. Movement of horses was prohibited in Buenos Aires Province, where all the major equestrian clubs, horse breeding farms, and racecourses are located.

Since there had been significant movement of horses off the five primary affected premises, all such movements were traced from early January to May 7, 2010. A statistically representative blood sampling was carried out at the main racecourses and equestrian clubs in the country, with approximately 10,000 horses of all breeds tested. Seven additional affected premises were identified as a result of this surveillance. Dissemination of EAV had occurred as a result of the movement of horses that were either incubating the infection or subclinically infected with the virus. All the 12 affected premises identified (five as a consequence of the use of infective semen and seven because of the movement of infected horses) were located in Buenos Aires Province.

The serological survey revealed that apart from the involvement of Thoroughbred mares on the index premises, infection was limited to sports horses. Thoroughbred, polo, Criollo, Arabian, and other breeds evidently were not exposed to EAV infection during this occurrence of equine viral arteritis.

Approximately 30 stallions of a jumping breed (Silla Argentina) became seropositive during this outbreak. Investigations are ongoing to determine whether any of them are semen shedders and carriers of EAV. The genetic characterization of the isolates is also being undertaken. The feasibility of applying preventive measures to protect the country's valuable horse industry against the possible risk of future introduction of this disease was considered, and on July 13 a vaccination program for Thoroughbred stallions was officially approved. To this point, approximately 100 Thoroughbred stallions have been vaccinated with a commercial modified live vaccine against EVA (Arvac, Pfizer Animal Health). Consideration is also being given to the need to include other breeds in the vaccination program.

Maria Barrandeguy, DVM, Laboratorio de Virus Equinos, Instituto de Virología, Buenos Aires, Argentina

Reprinted from the Equine Disease Quarterly, October 2010, University of Kentucky, Department of Veterinary Science.

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