EQUINE Viral Arteritis (EVA) is a contagious viral disease of horses caused by the Equine Arteritis Virus (EAV) that occurs worldwide.

In Ireland, the last recorded outbreak occurred in 2010. In April and May 2019, the UK authorities reported the identification of four EAV infected carrier stallions, occurring in non-thoroughbred animals on closely linked premises in Dorset and Devon. In July 2019, a further unrelated outbreak of the disease was confirmed in a fifth non-thoroughbred stallion in Shropshire, who was subsequently castrated.

EVA is a notifiable disease in Ireland, this means that any suspect clinical cases or positive/inconclusive laboratory test result must be notified to the Department of Agriculture, Food and the Marine (DAFM) immediately.

Transmission

Infection can be transmitted between horses in four main ways:

1) Close contact via the respiratory route, for example coughing and sneezing.

2) Through venereal spread in the semen of acutely infected or long-term carrier stallions at mating or by artificial insemination.

3) From mare to foal in utero as well as from contact with aborted foetuses.

4) Indirectly due to contact with virus-contaminated objects, for example breeding shed equipment, clothing and staff.

Clinical signs

Horses infected with EAV may demonstrate few if any clinical signs, which if present are clinically indistinguishable from those of other common equine respiratory diseases including those caused by equine influenza or equine herpes virus.

When clinical signs are present, they can vary widely in range and severity, and last up to 14 days. Acutely infected animals can show one or more of the following signs:

Critically, a large proportion of stallions infected with EAV can become long-term carriers of the virus, which can be economically devastating to their breeding value.

In these animals, the virus remains in the reproductive tract and is shed into semen whenever the stallion is bred or has semen collected for artificial insemination. Only sexually mature colts or stallions can become carriers, and the carrier state does not occur in infected geldings, mares or immature colts. There is no treatment for carrier stallions.

Diagnosis

When a horse becomes infected with the virus, it produces antibodies to fight against the infection. Laboratories can use different serological tools to screen blood samples, including the serum neutralisation test (SNT) and enzyme linked immunosorbent assay (ELISA). For EAV infection, suspicion of disease is often based on laboratory test results alone.

Control

If EVA is suspected in a horse and after notification of the disease to DAFM, an official veterinarian from DAFM’s regional veterinary office will visit the horse.

Owners will be asked to have the horse’s passport, including completed vaccination pages, and any previous EVA test results available for inspection. Based on the findings of the visit, an appropriate follow-up plan will be agreed with the owner, and this may require restrictions being imposed until disease can be satisfactorily ruled out.

In the case of suspect stallions, in order to rule out the carrier state, semen must be investigated for the presence of the virus.

This can be done directly by testing a semen sample for the virus or indirectly by checking if antibody negative mares return a positive test result after being bred to the stallion.

Prevention

There are several measures that owners can take to protect their horses and prevent an outbreak of EVA in Ireland. These include:

1) Follow relevant equine industry codes of practice. Ensure all breeding stallions are correctly vaccinated against EVA. Vaccine manufacturers advise that following a primary vaccination course, boosters should be administered every six months. Stallions should only breed to mares that have been blood-tested prior to mating and are free from active infection.

2) Before importing breeding animals, prospective owners and keepers should consult with their private veterinary practitioner about the appropriate testing requirements.

3) In Ireland, the inactivated EVA vaccine Equip Artervac (produced by Zoetis) is available for stallions and teasers under special licence from DAFM. Applications can be made by contacting the virology division at the central veterinary research laboratory. All applications must be accompanied by the appropriate form and a blood sample for antibody measurement.

For more information, consult your veterinarian or contact virology@agriculture.gov.ie. To notify DAFM about an EVA suspect please contact your local regional veterinary office www.agriculture.gov.ie/contact/regionalveterinaryoffices or the DAFM out of hours exotic disease hotline on 1850-200-456.