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The main ways of preventing EVA are vaccination, particularly for stallions and teasers, and the establishment of freedom from infection before breeding activities commence. However, the 2019 UK outbreaks demonstrated the capacity for respiratory (non-venereal) spread of infection and the importance of making sure that horses imported from countries where EVA is endemic into countries where it is not are serologically tested negative for EVA before importation and quarantined, monitored and re-tested after arrival.

Establishing freedom from infection

This involves checking the disease status of breeding stock before commencing breeding activities each year. Veterinary surgeons should take blood samples from horses for testing in a laboratory to detect the antibodies that the horse generates in response to infection with the virus. The horse also generates antibodies in response to vaccination against EVA.

The laboratory detects both the presence and the level of antibodies in the blood (‘serological testing’).

If antibodies are not present (‘seronegative’), the horse is not infected and breeding activities may begin.

The presence of antibodies (‘seropositive’) may be the result of:

  • active infection;
  • previous infection;
  • vaccination.

In mares, a rising level of antibody in two or more sequential samples indicates active infection and the mare should not be used for breeding activity. A stable or declining antibody level indicates previous infection or vaccination and the mare can be used safely for breeding activity.

A stallion who is shedding virus in his semen is always seropositive but a seropositive stallion is not necessarily a shedder. Therefore, if a stallion returns a seropositive result, it is important to establish whether he is a shedder (see Appendix 5) before use for breeding activities.


Routine vaccination against EVA is particularly recommended for stallions and teasers. In the UK, routine vaccination of mares is not recommended and emergency vaccination might only be considered in exceptional circumstances involving widespread disease outbreaks. One vaccine, Equip Artervac (Zoetis), is available in the UK. Horses that were seronegative before vaccination will become seropositive afterwards. This positive status cannot be differentiated from positive status caused by infection. It is essential, therefore, for breeding and export purposes, to be able to demonstrate that the horse is positive because of vaccination and not infection. This is done by blood testing before vaccination to show that the horse was previously seronegative and keeping a record of the test result, certified by a veterinary surgeon, preferably in the horse’s passport. The vaccine should not be administered until the blood test result is available.

Veterinary advice should be sought on the timing and administration of the vaccine. The current datasheet requirement for the only inactivated vaccine against EVA used in Europe presently is for 6 monthly (not annual) boosters. See Appendix 8 for vaccine details.

All vaccinations (primary course and booster doses) must be recorded in the horse’s passport, by the veterinary surgeon who administered the vaccine. Details should include the date and place where the vaccine was given, and the name and batch number of the vaccine.

Recommendations for prevention - domestic mares

The risk associated with any mare can vary. Decisions regarding the testing of mares visiting stallions should therefore be made in conjunction with the attending veterinary surgeon, according to the circumstances of the individual premises and the mare’s history and contacts with other horses in the past year.

In any breeding season, the safest option is to blood test all mares whether intended for natural mating or AI after 1st January and within 28 days before use for breeding activities. The mare should not be used until the results are available.

  • If a mare is seronegative, breeding activities may begin.
  • If a mare is seropositive and had not previously been shown to be seropositive, she may be infected and must be isolated immediately. Repeat blood samples should be taken at intervals of at least 14 days and sent to the laboratory that tested the first sample. When the mare is no longer infectious, as indicated by stable or declining antibody levels, breeding activities may begin.
  • If a mare was seropositive in a previous year and her current test returns seropositive, breeding activities may begin if the antibody level in the current sample is stable or declining compared to the level in her last test (the laboratory that tested the previous sample should test the current sample). If there is any doubt about the comparison of results, a second test should be done at least 14 days after the first, using the same laboratory. If the antibody level is stable or declining, breeding activities may commence. If it has increased, isolate the mare and consult a veterinary surgeon immediately.

If any mare is seropositive unexpectedly, the in-contacts should be isolated and screened for EVA by blood testing. Any foster mares on the premises should also be tested.

Recommendations for prevention - imported mares

Before importing a mare, veterinary advice should be sought on the incidence of EVA in the exporting country and the following precautions taken when the disease is known or suspected to occur in that country:

  • Ensure that the mare is blood tested within 28 days before import and proceed only on the basis of a seronegative result or, if seropositive, of stable or declining antibody levels in at least one further test at an interval of not less than 14 days. Between blood testing and import, reasonable precautions should be taken to minimise the risk of infection, such as segregation from unvaccinated or untested stock.
  • Immediately on arrival, place the mare in isolation for at least 21 days. Blood tests should be done immediately and repeated at least 14 days later. If the results are seronegative, or seropositive with stable or declining antibody levels, natural mating or AI may begin. If the results are unexpectedly seropositive, or the antibody level is rising, keep the mare in isolation, do not use her for breeding activities and consult a veterinary surgeon about the next steps.

Recommendations for prevention - domestic stallions

After 1st January in any year, all unvaccinated stallions and teasers should be blood tested. Do not use the stallion for breeding activities until the result is available. If the result is seronegative, breeding activities may commence. If the result is seropositive, notify the Animal & Plant Health Agency (APHA) immediately and isolate the stallion while steps are taken to determine whether he is shedding the virus in his semen (see Appendix 5). He must not be used for breeding activities during this time. If he proves not to be a shedder, he may be used for breeding activities as long as any advice from the veterinary surgeon, and any conditions laid down by the APHA, are implemented. If he proves to be a shedder, he must remain in isolation until his future is decided. None of his semen should be allowed off the premises and previously released semen should be traced and the recipients notified.

Vaccinated stallions and teasers may be seropositive or seronegative, depending on when the last dose of vaccine was given and whether the horse might have become infected since the protection afforded by the vaccine declined. These horses should be blood tested after 1st January. Do not use them for any breeding activities until the results are available. If the result is seronegative, breeding activities may begin. If it is seropositive, the stallion’s history in the past 12 months - including dates of EVA vaccinations, results of pre-vaccination blood testing and any post vaccination testing and contacts with other horses since the last vaccination - should be reviewed in consultation with a veterinary surgeon.

The current datasheet requirement for the only inactivated vaccine against EVA used in Europe presently is for 6 monthly (not annual) boosters. If the EVA vaccination has lapsed or expired, the stallion may be susceptible to infection and seropositive results should be investigated. If the veterinary assessment concludes that the stallion’s seropositive status is likely the result of infection rather than vaccination, isolate the stallion and notify the Animal & Plant Health Agency (APHA) immediately. The stallion should then be tested further to determine whether he is shedding the virus in his semen (see Appendix 5). He must not be used for any breeding activities during this time.


Advice for Owners/Agents of stallions that may be exported either temporarily (shuttle) or permanently

At present, regulations regarding the export of EVA vaccinated stallions to countries outside the EU, such as Australia, are becoming stricter. AQUIS (Australian quarantine agency) require that stallions vaccinated against EVA comply with current OIE guidelines. At present these guidelines state: ‘The horse was isolated and a single blood sample taken not less than seven days after commencement of isolation and testing using a virus neutralisation test as described in the OIE Manual for equine viral arteritis with negative results.The horse was then immediately vaccinated against equine viral arteritis and remained isolated from other equids not of equivalent health status for 21 days immediately after vaccination and has been revaccinated regularly according to the manufacturer’s recommendations.’

Current Artervac booster vaccinations are recommended every 6 months, which may not be compatible with stallions shuttling to Australia and other countries where Artervac does not have a current licence. This can lead to problems with a stallion maintaining its correct vaccination history. Other countries such as New Zealand and Argentina have slightly differing import requirements at present.

Any vaccinated stallion that fails to meet import or re-import conditions may be subject to semen testing for EVA culture. It is very important to consult a veterinary surgeon as well as possible shipping agencies with regard to possible exportation of stallions to non-EU countries.

Stallion managers accepting mares for ‘walking in’ may wish to seek additional reassurance by requesting pre-visit negative serum EVA antibody test results (in addition to routine pre-season tests), where visiting mares are or may be in contact with ‘at risk’ horses. For Thoroughbred stud farms, this may apply to mares visiting from other stud farms where there is contact with non-Thoroughbred horses.

 Recommendations for prevention - imported stallions

The following applies to import of stallions normally resident overseas, returning shuttle stallions and stallions who are normally resident in the UK when they have been overseas for non-breeding purposes but will be used for breeding activities upon return to this country.

Using imported stallions for breeding activities increases the risk of spread of EVA because the disease occurs worldwide and is transmitted readily between horses via the respiratory as well as the venereal route. In the UK, the law does not require any official testing of stallions for EVA before importation from EU member states so voluntary testing to establish their EVA status should be undertaken. Official testing requirements exist for imported stallions from non-EU countries. However, they may not be adequate to prevent the import of infection. Also, horses can become infected via the respiratory route during transport with other horses. Additional voluntary precautions are therefore advisable.

Before importing a stallion, veterinary advice should be taken on the incidence of EVA in the exporting country. The importer should take the following precautions when EVA is known or suspected to occur in that country:

  • Ensure that the horse is blood tested no more than 28 days before import, and since he was last used for mating. If the result is seronegative, importation may proceed. If the result is seropositive, seek veterinary advice before proceeding. Between blood testing and import, reasonable precautions should be taken to minimise the risk of infection, such as segregation from unvaccinated stock.
  • Immediately on arrival, place the stallion in isolation for at least 21 days. Two blood samples should be taken, one immediately and the second at least 14 days later. They should both be sent to the same laboratory. If the results are seronegative, breeding activities may commence. If any result is seropositive, notify the Animal & Plant Health Agency (APHA) immediately, keep the stallion in isolation and consult a veterinary surgeon about the next steps. The stallion must not be used for mating, teasing or semen collection during this time.

Sport horse stallions

Where stallions are imported into the UK for competition purposes, their EVA status should be established if it is decided, after their arrival, to use them for mating or semen collection while they are in the country. The stallion should be isolated for at least 21 days, and blood tested immediately and again at least 14 days later, using the same laboratory each time. If the results are seronegative, breeding activities may commence. If any result is seropositive, notify the Animal & Plant Health Agency (APHA) immediately, keep the stallion in isolation and consult a veterinary surgeon about the next steps. The stallion must not be used for mating, teasing or semen collection during this time.

Recommendations for prevention - artificial insemination and embryo transfer

Semen should not be used from any stallion unless that stallion has been tested for EVA according to the previous recommendations for domestic (page 24) and imported (page 25) stallions.

When semen is collected from a stallion:

  • The stallion owner/manager must record the dates of movement of the stallion on and off the premises, collection and movement of semen and insemination of mares at the stallion’s premises.
  • The disease status of the stallion at the time when the semen was collected must be established by blood testing. If the stallion was seropositive, the semen must not be used unless it can be proved that he was not a shedder (see Appendix 5).

Under EU law, import of semen from shedder stallions is not permitted.

Mare owners planning to use semen from overseas stallions should check the EVA status first. Semen should be accompanied by documentation certifying that the stallion or the semen was tested negative for EVA shortly after the semen was collected in the country of origin. Frozen semen should additionally be tested on arrival in the UK. It is only necessary to test one straw from each ejaculate.If the result is negative, the semen may be used. If it is positive, all straws from that ejaculate should be destroyed. For practical reasons it is not possible to test chilled semen on arrival. Appropriate testing in the exporting country is, therefore, essential. When transferring embryos, whether conceived in the UK or overseas, the disease status of both the stallion and mare at the time of conception must be established. Mares should have seronegative status, or seropositive status with stable or declining antibody levels. Stallions should have seronegative status, or seropositive status with proof that they are not shedders.

1. AI - see AI Guidelines
2. 'Stallion' means mating stallions, teasers and stallions used for AI.
3. Under EU law, the importation of known shedder stallions is not permitted.
4. Equine arteritis virus survives in chilled and frozen semen and is not affected by the antibiotics added.