skip to content

Equine Viral Arteritis

The clinical signs of EVA can be varied, for example some horses can show signs of infection whereas others show none. However, even when there are no clinical signs, infection can still be transmitted. This means that stallions can shed the virus in their semen when breeding and spread disease. In pregnant mares, abortion may occur, and the disease can be fatal.


The main signs of EVA are fever, lethargy, depression, swelling of the lower legs, conjunctivitis ('pink eye'), swelling around the eye socket, nasal discharge, 'nettle rash' and swelling of the scrotum and mammary gland.

How is it spread?

Infection can be spread between horses in any of the following ways...

  • Direct transmission during mating or ‘teasing’ before mating
  • By inseminating mares with semen from infected stallions or semen which has been contaminated during semen collection or processing. The virus can survive in chilled and frozen semen and is not affected by the antibiotics added to preserve semen during shipping
  • Contact with aborted fetuses or other products after birth
  • Via droplets from coughing and snorting


The main ways of preventing EVA are vaccination and ensuring freedom from infection before breeding activities start. This involves checking the disease status of breeding stock before beginning 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.

How is it tested for and controlled?

Stallions must be tested as free from disease before breeding because the virus is stored in a stallion reproductive organs and can be shed in his semen for several weeks, months or years or possibly even for life. The fertility of shedder stallions is not affected and they show no clinical signs but they can infect mares during mating, or through insemination with their semen.

Mares may infect other horses via droplets from coughing and snorting.

If EVA infection is suspected in any horse, stop all breeding activities immediately, notify the local Field Office of the Animal & Plant Health Agency (APHA), isolate the horse(s) concerned and seek veterinary advice about the next steps.

Laboratory diagnosis, through blood testing, is essential. Antibodies are usually present in the blood 7-14 days after infection and remain present for the rest of the horse's life. Diagnosis can be made by with the Coggins or the ELISA test.

What do I do if EVA is confirmed?

This is a notifiable disease so the Animal and Plant Health Agency (APHA) will let you know what you need to do and restrictions on movements are likely to be imposed.