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Equine viral arteritis (EVA) is a highly contagious venereal disease affecting horses. This infection is relatively widespread in Europe but we have precautions in place in the UK to prevent it.

If UK breeders fail to comply with the Horserace Betting Levy Board (HBLB) guidelines, EVA could get in to this country and cause havoc to our breeding industry.

EVA is caused by a virus and is spread between horses in four ways:

  • Through venereal contact between mare and stallion at mating
  • Through droplets from the respiratory tract, spread by coughing and close contact between horses
  • From stallion to mare through infected semen at AI
  • Contact with aborted foetuses, fluids and placentas of mares which aborted due to EVA.

About 30% of infected stallions never eliminate the infection and remain as persistent shedders, spreading it for the rest of their lives. Alarmingly, many of these show no outward signs and their fertility is unaffected.

Mares mated or inseminated by a shedder will be infected and may become ill themselves. The virus may cause abortion in pregnant mares and severe respiratory disease in foals. There is no cure for EVA.

In an alarming outbreak in Leicestershire in 1993, a sport horse stallion was imported from Poland apparently with clear documentation that was later said to be incorrect. He arrived one autumn, covered his first mare next spring and infected her.

She in turn spread the infection to other mares, which were subsequently covered by other stallions on the stud, which also became infected. Artficial insemination caused further spread and soon more than 100 horses had EVA.

The consequences of EVA

The consequences were devastating for the individual stud and alarming for the whole equine industry. Careful control measures stopped that outbreak and other subsequent cases, but in the UK horses have little immunity and are at risk of developing the disease should the virus be imported again.

Although mares and geldings can be infected, only stallions are potential long-term shedders of the EVA virus. So the greatest risk to the British horse population is from importation of either an EVA-infected stallion or his semen. Those stallions could then go on to infect many more mares via breeding activities. There is also a risk that a mare or youngstock with EVA but few, if any, clinical signs might enter a stud and infect the stallions via the respiratory route.

Official import controls may be insufficient to prevent the disease coming in, particularly if documentation is inadequate and, as is usual, a stallion appears healthy. Breeders must be responsible and ensure their horses are safe, especially if they choose to import a breeding prospect. Anyone importing a stallion should ensure he is tested, even if they have no intention of breeding from him at that time.

Clear guidelines have been devised by the HBLB within its standard codes of practice to prevent further EVA outbreaks. These codes have been effective in stopping the disease in its tracks in this country so far.

The need for testing

The difficult concept to grasp is that the horse can look well but still be infected and be a spreader or shedder of disease. It is important that all stallions and mares are thoroughly screened each season before breeding starts. The precautions recommended are straightforward and logical.

Breeding stallions and teasers in the UK should be blood tested for EVA after 1 January but before mating, teasing or collecting semen. The results should be recorded in the horse’s passport. The risk with mares can vary, but the safest thing is to blood test prior to sending them to stud or for insemination to ensure they are clear. Ask your vet about the most appropriate action for your case.

Particular care should be taken if importing horses, semen or equine embryos. EVA virus survives in chilled and frozen semen and is not affected by the antibiotics added. Specialist veterinary advice should be taken before proceeding. In general:

  • Before any importation, blood samples should be taken and sent to a recognised laboratory, not necessarily in the country of origin of the horse
  • When a horse arrives from abroad, it should be isolated for 21 days
  • A vet should take blood samples after arrival
  • If semen is imported, blood test results for the stallion at the time of semen collection should be properly checked. A straw of the frozen semen should also be tested. This is called the “one straw” test
  • Stallions that compete abroad can be infected at any time by the respiratory route, even if they do not cover mares while travelling.

What if EVA is confirmed?

If EVA is confirmed, you will have to stop breeding operations, seek immediate veterinary advice and inform DEFRA because EVA is a notifiable diseases in certain circumstances.

By following the HBLB guidelines rigorously and responsibly, we can avoid problems that the equine industry can well do without.

  • This veterinary feature was first published in Horse & Hound (26 January, ’06)

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