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Appendix 5

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Identifying EAV shedding stallions

When a seropositive stallion is identified, it is vital to establish whether he is shedding the equine arteritis virus (EAV) in his semen. If so, he is a primary source of infection. If the serology result cannot be fully explained by vaccination, further testing to determine the presence of EVA in semen has to be carried out through either of the methods below while the stallion is kept under strict isolation. If the initial veterinary assessment concludes that the stallion is likely to have been infected the local Field Service APHA office has to be notified.

Detecting virus in semen

The virus isolation (VI) test for EAV in semen is the test prescribed for international trade but for non-export screening purposes PCR (Polymerase Chain Reaction) testing may be used to provide an initial indication of EAV in semen.

A whole ejaculate of semen should be sent to a laboratory; a second whole ejaculate should be collected at least seven days later and sent to the same laboratory. Transport requirements (eg cooling) should be arranged with the laboratory. If EAV is detected in either sample, the stallion is a shedder. He must be kept in isolation and not be used for any breeding activities while he is still shedding, unless permitted under an official licence issued by Defra.

In the event of negative results for both semen samples, experience has shown that it is advisable to confirm these results by test mating. 

Test mating

This must be done in strict isolation and under veterinary supervision. The stallion and mares must have no contact with other horses. The following procedure should be followed:

  • Identify at least 2 seronegative mares;
  • Take and store blood samples from each and then isolate the mares;
  • Consult the testing laboratory about storage conditions;
  • Mate each mare twice a day with the stallion on 2 consecutive days;
  • Keep the mares in isolation;
  • After 28 days, take blood samples and send them, with the pre-isolation samples, to the laboratory.

If the mares remain seronegative, the stallion is unlikely to be a shedder and can be released after a clinical examination.

If one or more mares become seropositive, the stallion is a shedder. He must be kept in isolation and not be used for breeding activities while he is shedding, unless permitted under an official licence issued by Defra.

Seropositive mares must remain in isolation until they have a stable or declining antibody level in two sequential blood tests taken at an interval of at least 14 days.