Equine viral arteritis: understanding the risks

Equine viral arteritis may be rare in the UK, but we cannot afford to be complacent, warns Karen Coumbe MRCVS

WITH vague signs and the ability to spread undetected via silent “shedder” stallions, the infectious disease equine viral arteritis (EVA) has the potential to cause havoc within the UK breeding industry.

EVA is caused by the equine arteritis virus and is found virtually worldwide, including in mainland Europe where there is high horse population immunity and low-grade signs of the disease.

Evidence suggests the global incidence of EVA is increasing. While the UK has remained relatively unaffected, this means our equine population has little or no immunity and would be highly vulnerable to infection when or if the virus recurs here again.

EVA is unusual in that it is spread by both respiratory and venereal routes. The disease is rarely life-threatening to healthy adult horses, but can result in abortion in broodmares and, infrequently, death in young foals.

The most serious issue is that some stallions are apparently healthy carriers of infection, shedding the virus in their semen. Mares mated with or inseminated by such shedder stallions may become infected and spread the disease to others by the respiratory route.

The disease can be transmitted by all forms of artificial insemination (AI); neither chilling nor freezing will destroy the virus, which is preserved in the semen. Antibiotics commonly used in semen extenders have no effect on its survival. The consequences of an outbreak could be devastating, both for individual studs and the wider equine industry.

How the equine arteritis virus spreads

Through venereal contact between mare and stallion
From stallion to mare at AI, through infected semen
Via droplets from the respiratory tract, spread by coughing and snorting, through any close contact, such as in yards, turnout or at shows
Through contact with aborted foetuses, or fluids and placentas of mares who aborted due to EVA, or during foaling

High-risk situations

MECHANISMS are in place to prevent the introduction of equine viral arteritis to the UK via semen imports.

Any semen imported should be certified by an official vet from the country of origin, to confirm that the stallion is free from infections – including EVA.

In the UK, only vets and certified AI technicians are allowed to inseminate mares. It is their responsibility to notice the lack of certification, whether paper or electronic, to acknowledge this is an illegal import that should be rejected, and to report the infringement appropriately.

The British horse industry must do all it can to protect itself from any further outbreaks. EVA is classified here as an exotic notifiable disease, which means that a case must be reported to the authorities if it is identified in stallions, or mares that have been bred or inseminated in the past 14 days.

High-risk situations for spread of the disease include any import and also the covering or insemination of mares, and when they are later foaling down, or whenever there is movement of horses between premises.

As horses which have EVA and are capable of passing on the disease do not necessarily show clinical signs, control and prevention is based upon blood testing to identify carriers.

Owners and breeders need to be aware of the potential risks of EVA. Equally, vets must remain vigilant. The disease should be considered a possibility until it can be ruled out by diagnostic testing of any suspects.

It would seem that most of the recent EVA infections in the UK have been carrier stallions, first detected by routine serological (blood) testing conducted for international trade, pre-purchase or pre-breeding purposes, and not identified through investigation of clinical disease.

Be cautious, however, and monitor your inseminated mare for signs of illness, which is one way the disease has previously been detected.

Physical clues include fever, lethargy and depression, conjunctivitis (pink eye), nasal discharge, nettle rash-type skin lesions and swelling of the eye socket, lower limbs, scrotum or udder.

All of these signs are vague, so lab tests are required to confirm the diagnosis. Proper investigations can identify the presence and level of antibodies by testing blood, and can also screen for the actual virus in blood, semen and other samples.

When a mare aborts her foal, or if a newborn foal’s death could be EVA-related, the mare’s clinical history and blood samples should be sent to the lab along with samples from both the placenta and the dead foal.

Prevention via vaccination

There are reliable, readily available tests to control equine viral arteritis. The main method of prevention is vaccination, particularly for stallions, accompanied by blood tests on all horses prior to breeding to confirm that they are free from infection.

Semen can be tested to ensure it is safe for insemination, and also to identify whether a stallion is shedding the virus. More than one EVA outbreak, such as that in South Africa, has been caused by semen sent from Europe that subsequently was reported to be infected.

In the UK, the vaccine is used to protect stallions but not mares. The timing of the vaccine is crucial, so it should be administered under strict veterinary guidance. A blood test must be taken beforehand to show that the stallion has no antibodies, because post-vaccination tests cannot distinguish whether antibodies present are from infection or immunisation.

It is vital for breeding, sale and potential export purposes that any such blood test results are received and properly documented in the horse’s passport before vaccination – and that the vaccination is administered every six months.

Certainly, no mare or stallion should be used for breeding until there is a definite clear lab test signalling that it is safe to go ahead. A test is likely to cost less than £50 and can save a huge amount of hassle.

Even if a stallion is destined for competition and not breeding, it is still wise to arrange a basic blood test before purchase and importation – and to double-check the horse after arrival.

Treatment for equine viral arteritis

THERE is currently no treatment for EVA. While some people have suggested that anti-testosterone medication can reduce shedder status in stallions, this is unreliable and not recommended.

The problem will be eliminated if the horse is castrated, as only stallions are persistent shedders. Otherwise, such horses must be isolated under Government movement restriction orders and managed extremely carefully. Some are re-exported.

Currently, the UK Government publishes details of restricted infected horses on its website. The same website has a review of the 2019 UK outbreak, which reveals the capacity for spread of infection by respiratory means, rather than just through covering, and the relatively large number of horses involved.

The review also highlights the importance of making sure that horses coming from countries where EVA commonly occurs (Spain, for example, in the 2019 outbreak), into countries such as the UK, where it is not routinely present, should be properly tested as negative before importation and retested after arrival. Quarantine is also an important factor to consider, along with being aware of where your horses have travelled, so that any spread of infection can be tracked.

The health of all of our horses is put at risk if these vital checks are ignored. Insist on clean certificates for all breeding horses at the start of every breeding season, be extra vigilant and consider quarantine with new arrivals – especially those that are imported. This is a serious equine disease, which has the potential to be a major impediment to international horse travel.

Detailed Codes of Practice for 2021 can be found at the Horserace Betting Levy Board (HBLB) website: codes.hblb.org.uk

H&H 11 February 2021

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