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Equine Viral Arteritis (EVA)

Equine arteritis virus was first isolated in 1953 during an outbreak of respiratory disease and abortion in Ohio, U.S.A. Since then outbreaks have occurred all over the world and although exotic to Ireland, we have experienced an occasional incursion. The clinical signs associated with equine viral arteritis (EVA) are extremely variable and range from subclinical disease with reduced fertility to severe disease with abortion in pregnant mares and death of young foals. The most commonly observed clinical signs include fever, depression, a decrease in appetite, limb oedema (swelling) especially of the hind limbs, stiffness in gait, inflammation of conjunctiva (“pink eye”), skin rashes and ocular and nasal discharge. Oedema of the mammary gland is frequently seen in mares and the majority of stallions develop scrotal oedema. The predominant signs vary during different outbreaks.

Viral transmission is primarily by venereal infection of mares by stallions during mating or when artificially inseminated with semen from a carrier stallion. There is no evidence of the carrier state in mares but after exposure a mare may shed virus in all her bodily fluids for up to 24 days. At the end of this period she is no longer infectious and is safe to mate. However for the few weeks that she is shedding virus she may infect other horses (mares, foals, geldings and stallions) by direct contact or by aerosol (infected droplets from the respiratory tract). Up to 50% of the stallions that are exposed to virus in this way may become long term or even, permanent venereal shedders. Such stallions shed virus in the sperm rich fraction of their semen and may infect every mare they cover. This has a devastating effect on their economic value. EVA is a Notifiable disease in Ireland and to-date no carrier stallion has been permitted to breed here.

Less than 1.5% of our Thoroughbred mares have antibodies against EAV. The introduction of the virus by the importation of a carrier stallion or his semen for AI, or a mare that has recently been exposed to virus could have serious consequences for our highly susceptible horse population. All horse owners need to protect their livelihood by testing and vaccinating stallions and by sending mares to studs that require them to be blood tested for EAV in accordance with the Code of Practice i.e. within 4 weeks before mating.

The serum neutralisation test (SNT) is considered the gold standard test for EAV by the OIE (World Organisation for Animal Health) and by the Department of Agriculture. There are ELISA tests available which do not require the expertise and the laboratory facilities necessary for the SNT, but they are not considered as reliable. The SNT is the test that is used by the IEC.

Turnaround Time
Serum Neutralization test (SNT)
2 - 3 days
Polymerase Chain Reaction (PCR)
2 days
Virus Isolation
1 - 2 weeks
Equine Breeding Profiles Lab Tests
Turnaround Time
2 - 3 days
7 days
7 days
Sample submission instructions:
  1. A 10ml clotted blood sample should be submitted for SNT. The same sample may be used for EIA testing. The name of the horse should be clearly written on the sample submission form. This allows the laboratory to check the computer records for the horse. If the horse tested positive before, the previous sample will be retrieved from storage and tested with the current sample to ascertain that the antibody titre is stable. If the horse is an entire male this should be clearly indicated on the submission form along with the vaccination status.
  2. Although an SNT result may be available within 48hrs this cannot be guaranteed and samples should be submitted well in advance of the covering date. The laboratory should be notified if the sample is urgent. This is particularly important early in the year when there is very high sample throughput and samples are processed on a first come/ first serve basis. Occasional samples are toxic to the cells used in the test and the result cannot be conclusively determined. In such circumstances a second sample will be requested.
  3. In accordance with the Code of Practice a whole ejaculate of semen should be submitted to the laboratory to determine the carrier status of a stallion. Virus detection is performed on the sperm rich fraction of ejaculates. The detection rate from dismount samples is not of equivalent accuracy and the testing of the pre-sperm fraction is likely to lead to false negative results.