Unlike mares and geldings that will easily clear the infection, 30-60% of infected stallions will become persistently infected.  The virus persists in the stallion's reproductive tract--an ideal location for the virus to disseminate itself to susceptible mares via direct or indirect venereal contact.  This means that if your stud is a carrier he can transmit the virus to mares in situations of both natural service and artificial insemination!

Testing Your Stud for EVA

To confirm that your stud is not persistently infected with equine arteritis virus some laboratory diagnostic tests need to be performed on his blood and semen.  To begin, a blood sample should be taken and then analysed.  If the bloodwork shows evidence of antibodies to the virus without a history of vaccination, a semen sample needs to be analysed.  Two semen samples (can be collected on the same day) should be obtained by collection into an aritificial vagina or condom.  A less ideal method of collection would be to obtain a dismount sample after breeding to a mare.  It is imperative that the external genitalia of the stud has not been cleansed with antiseptic or disinfectant.  After collection, the samples should be shipped on ice to the diagnostic lab for evaluation.  Alternatively,
test breeding the stallion to two mares serologically negative for antibodies to equine arteritis virus at least twice on each of two consecutive days (four covers) and the mares checked for the development of serum antibodies to the virus 28 days after breeding is another appropriate method of detecting the carrier state. 

The AAEP has outlined a protocol for vaccination of stallions in their VACCINATION GUIDE.

Managing a Shedding Stallion:

Shedding stallions can still be used for breeding provided that proper precautions are taken.  It is important to
disclose your stud's shedding status to those interested in booking their mares to your stud.  Shedding stallions should only be bred to seropositive/vaccinated mares to minimise the spread of the disease.  Shedding stallions should be physically isolated from seronegative or unvaccinated horses.  While managing a carrier stud is possible, the costs and benefits of  continuing to breed such an animal must be carefully considered. 

Import Restrictions/Guidelines

The causal organism of EVA has a global distribution. Many countries deny entry to carrier stallions and EAV-infected semen in an attempt to control the spread of the disease and to mitigate the risk of disease.  Currently, the USA does not have an import policy regarding the entry of carrier stallions or the import of infected semen.  For a thorough look at restrictions/guidelines currently in place in several countries, please see the OIE's TERRESTRIAL ANIMAL HEALTH CODE.


  • OIE Terrestrial Animal Health Code.