Like other issues that have formed the subjects for the Silent Killers series, Equine Viral Arteritis (EVA) is not currently present in the UK, the last cases having been reported in 2019. It is, however, a notifiable disease in the UK and, should it make its way back into the country, could pose a serious threat to equine breeding programmes.

EVA is a contagious disease caused by equine arteritis virus (EAV) and, while it is rarely life-threatening in otherwise healthy adult horses, it is of special concern to breeders because it can cause abortion in pregnant mares, death in young foals and render breeding stallions permanent carriers of the virus.

The virus can be transmitted in a number of ways, primarily through mating and stallions can carry the disease for extended periods without showing clinical signs. They can transmit it both through natural mating or if their semen is collected for artificial insemination as the virus can survive in chilled and frozen semen and is not affected by the antibiotics added to preserve semen during shipping. Stored in stallion reproductive organs, the virus can be shed in semen for weeks, months or possibly life and the fertility of shedder stallions is not affected.

Other risks include contact with aborted foetuses or contaminated equipment and materials. Droplets on the breath of infected animals, transmitted by coughing or snorting, can be infectious for up to two weeks.

Mares can spread the disease to their foals during pregnancy or through their milk, and to stallions if mated while infectious.

The clinical signs of EVA can be varied and in some cases there may be no signs of infection at all but it can still be transmitted by stallions shedding the virus in their semen when breeding. In rare cases, the disease can cause severe clinical signs or death in young foals. The main signs of EVA are fever and runny nose, lethargy and stiff movement, swelling of the lower legs, conjunctivitis (‘pink eye’), swelling around the eye socket, ‘nettle rash’, and swelling of the scrotum or mammary gland. An aborted foal could also be caused by the virus.

The variety, or possible absence, of signs of EVA, means that clinical diagnosis is not always possible so before breeding stallions should be tested as free from disease. Laboratory diagnosis can identify the presence and level of antibodies via blood tests which can be used to screen for the virus. Laboratories generally require blood serum for antibody detection and heparinised or EDTA blood or semen for virus detection.

Where an abortion or early foal death may be EVA-related, a detailed clinical history of the mare, together with blood samples from the mare and samples of the placenta and the foetus or carcase, must be sent immediately for laboratory examination.

The main ways of preventing EVA are vaccination and ensuring freedom from infection before any breeding activity starts, and the disease status of breeding stock by laboratory testing should be carried out at the beginning of every season. Ongoing good biosecurity is essential and breeders should follow guidance on isolation and testing when importing animals.

If you suspect the presence of EVA, it is essential that all breeding activity be stopped and the local Field Office of the Animal & Plant Health Agency (APHA) is contacted immediately. Infected horses should be isolated and treated according to veterinary advice, and any in-contact horses should also be isolated and tested. Repeat testing should be carried out after 14 days and then every 14 days thereafter until freedom from active infection is confirmed, using the same laboratory for repeat samples as for the first test. Disinfect all equipment used for breeding purposes.

Semen in store from stallions that prove to be infected should be checked by laboratory testing one straw from each ejaculate, with the same process applied for any in-contact stallions. If any straw is revealed to be infected, all straws from that ejaculate should be destroyed.

Pregnant mares must be isolated for at least 28 days after leaving the premises and any mares who become infected after they have conceived should be foaled in isolation.

And, finally, no breeding activity should be resumed until freedom from EVA has been confirmed in all animals.

Please visit RBST Equine Conservation Project 2024 | Rare Breeds Survival Trust for more information on Equine conditions, where handouts are available for print. 

Photograph : Hackney Horses by Linda Trotman