Equine Viral Arteritis

The disease

Equine viral arteritis (EVA) is a highly contagious equine disease with a wide distribution throughout the world. It is caused by a virus which can cause fever, depression, and oedema (swelling) especially of the limbs and inflammation around the eyes. The virus may cause abortion in pregnant mares and severe respiratory disease in young foals. Although the disease occurs widely in many countries including mainland Europe, resulting in high horse population immunity and low grade signs of disease this is not the case in the United Kingdom. Except for a small and controlled outbreak that occurred following the importation and use of a Polish stallion in 1995, the UK has been relatively free from EVA. Infection occurs only sporadically with prevalence varying between different breeds. This means that the UK horse population has little immunity and is at risk of disease, should the virus be imported again.


In the UK EVA is a notifiable disease. Any person who suspects that a stallion or mare may be infected with EVA following natural mating or artificial insemination within the previous 14 days, should notify the Divisional Veterinary Manager of the Ministry of Agriculture under the Equine Viral Arteritis order 1995. The stallion and/or semen concerned are then subject to breeding restrictions and requirements pending the results of a veterinary enquiry.

Current guidelines on the control of EVA are included in the Horserace Betting Levy Board’s Codes of Practice on the control of equine venereal diseases. These have been adopted widely by the UK Thoroughbred breeding industry and are recommended for all types of horse breeding. The purpose of this information sheet is to highlight the particular risk of spread of EVA through semen used for artificial insemination (Al).

The greatest risk to the UK is from imported EVA-infected stallions, which should be serologically tested pre-purchase or pre-importation. There are also substantial risks from imported equine semen for use with Al, irrespective of whether the semen is chilled or frozen.

An inactivated virus vaccine is licensed for use in horses in the UK and has been used widely in Thoroughbred stallions to provide some immunity against EVA. The protective efficacy of this specially produced vaccine has not been proven in the face of a field outbreak. Veterinary opinion is that unless the disease becomes uncontrollably widespread in the UK, it is preferable not to vaccinate mares in our horse population so that their exposure status can be monitored accurately.

Methods of virus transmission

EVA is unusual in that it is spread via both the respiratory and venereal route. If stallions become infected their internal genital organs may become persistently infected and they may shed the virus in their semen for the rest of their lives i.e. they become permanent carriers of infection and thereafter permanently unsuitable for breeding purposes. This may render them commercially valueless. Mares mated with, or inseminated by, infected stallions may become infected and may then spread the infection to others in contact with them on the same premises, by the respiratory route. In contrast to stallions, mares, geldings and immature colts eliminate virus within a few weeks of infection and they do not become long-term carriers.

AI transmits the infection effectively

Venereal transmission of EVA can occur not only by natural mating but also by Al using fresh chilled or frozen semen. Neither chilling’ nor freezing will destroy the virus which is preserved in the semen. Antibiotics commonly used in semen extenders have no effect on the survival of the virus. In recent years there has been an increase in the use of Al and there is no doubt that the UK is now at greater risk of acquiring EVA via this means than ever before.


Import Controls

Official Control measures for importation of equine semen from EU Member States may not be adequate to prevent importation of infective semen. This has been highlighted by the introduction of EVA into South Africa through frozen semen imported from mainland Europe, in spite of full official government certification from the countries involved.

The UK horse industry must protect itself

UK horse breeders must voluntarily take responsibility for eliminating the risks of importing contaminated semen. No matter how difficult or inconvenient, importers of equine semen must establish the status of the donor stallion at the time when the semen was collected. After its arrival in the UK, frozen semen can and should be tested for the presence of the EVA virus.

Recipient mare owners and their veterinary surgeons must he fully aware of the risks and take adequate steps for prevention and monitoring of disease. This should include asking the agent supplying the semen for evidence of the stallion’s EVA status at the time of collection of the semen. This is the only safeguard possible for imported chilled semen.

Illegal importation of semen

The Animals and Animal Products (Import and Export) Regulations I995 and the Products of Animal Origin (import and Export) Regulations 1996 make it an offence to import from other EU Member States and none EU countries respectively, equine semen which does not meet the requirements of EU law. Powers exist to seize and destroy illegally imported semen or require it to be re-exported.

Illegal importation of equine semen carries an unacceptable risk, not only for the importer but also for the whole UK horse population. Personal assurances about the safety of such material, derived from countries with immune horse populations, are not relevant for the UK. Horse breeders are strongly urged not to obtain semen in this manner and veterinary surgeons should not inseminate illegally imported semen. Reliable veterinary health accreditation must be checked carefully.

Further Information

Further information about EVA and its prevention is available in the Horse Race Betting Levy Board’s Codes of Practice on the control of equine venereal diseases. This document is available from the Horse race Betting Levy Board, 52 Grosvenor Gardens, London SWIW OAU.



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