EHV1 (Equine herpes virus) – should we panic?
The recent confirmed case of EHV1 has sent shockwaves through the Aberdeenshire Equine community and triggered a 10 day shutdown on yards across the area. Is this action justified and will we have to repeat it every time there is a confirmed case of EHV1 in Aberdeenshire? A lot of the reaction is down to a lack of knowledge about a complex subject so after a review of the literature we aim to address this.
EHV1 has become headline news since a horse from the Beaufort Hunt Stables was confirmed with the neurological form of the disease recently. There have been a cluster of cases of EHV1 in Gloucestershire and hunting in the area has stopped temporarily. But what is EHV1 and why has this happened.
How widespread is EHV1?
EHV1 is one of several types of Herpes Virus to affect horses and has probably been around as long as the horse has. EHV is widespread in the horse population and most horses are believed to have been infected with EHV by the time they are 2 years old. A recent European study found EHV1 in 88% of horses1. So if virtually all horses are infected why should we be concerned?
EHV1 has three principal manifestations, a mild or asymptomatic respiratory infection, (these cases will often go undiagnosed but if we start looking for them we will find EHV1) secondly abortion in mares (EHV1 is the commonest diagnosed cause of equine abortion in the UK) and thirdly and perhaps most worryingly a paralytic form of the disease, Equine Herpes Myeloencephalopathy (EHM). EHM cases often occur in clusters and there is currently a cluster of EHM cases in Gloucestershire. In EHM the virus affects the spinal cord causing incoordination, paralysis and recumbency with sometimes fatal consequences. However cases can and do recover fully.
It is believed that a specific strain of EHV1, D752, may be more common in cases of EHM but it is believed that all strains of EHV1 can and do cause EHM2! The incubation period for EHV is short so symptoms can develop from 48 hours to a week after exposure but if infected horses can actively shed virus for at least 21 days even if their symptoms resolve.
How is it spread?
EHV is common in horses under 2 years of age. They often present with a nasal discharge sometime over the autumn and winter period and usually self-resolve without treatment. There may be a benefit in horses acquiring EHV at this time as they are less susceptible to infection in later life.
Although EHV and the horse more or less co-exist the horse does not seem to develop full immunity to EHV. Under periods of stress the virus can recrudesce (a bit like cold sores or shingles) and horse will show signs of infection such as a nasal discharge, cough, swollen limbs and a raised temperature. This can happen at any time and repeatedly in individual horses and they are infectious at that time. Even though many horses have probably been previously exposed the infection can still spread but possibly due to partial immunity it does not spread as quickly as Equine Influenza. However previously unexposed adult horses can and do succumb to EHV1.
The virus can spread through the air about 5 – 10m and also via an intermediary including personnel and grooming brushes; the biggest risk is feed buckets and bridles so don’t share these. Older horses seem more likely to get EHM and outbreaks are associated with very fit horses – hunt horses, polo ponies and racehorses – also these horses generally have much closer contact with each other.
Do we need to restrict movement of horses?
This is where it gets really tricky. As EHV1 is so widespread if we start testing every horse with a slight runny nose we will be identifying EHV every week and so outwith the affected yard the answer must be no!
Within an infected premises this will depend on a number of factors and should be decided in conjunction with the veterinary surgeon involved in the outbreak. In any case of EHV1 abortion or EHM the yard should be closed for a minimum of 28 days after the last case. If a single mild respiratory case occurs this horse must be isolated and kept in isolation for 28 days after its symptoms have resolved. If no new cases are identified and it doesn’t progress to EHM or abortion, quarantine of the remaining horses beyond 7 – 10 days (allowing for the extent and severity of infection within the premises to be determined) is possibly difficult to justify. Where multiple cases occur or for polo and racing yards where horses are routinely in much closer contact 28 days is sensible and is part of the HBLB code of practice 3.
Steps for protecting from the effects of EHV1
The commonest source of infection is an infected horse so you should have sensible quarantine precautions in place for new horses. However the virus can also come from almost any horse already resident on your yard subject to stress.
The risk of picking up a new strain of EHV1 is ever present but very small. Obviously you need to be aware of the local disease situation and be prepared to adapt your plan if a situation changes but in general terms the risk of EHV1 is no greater today than it was 12 months ago or is likely to be in 12 months’ time. Use common sense when mixing with other horses and people; don’t take any horse out if it may be showing or has recently been in contact with an infectious respiratory condition; monitor your horses carefully so if any signs of respiratory disease are found they can be diagnosed and appropriate control measures put in place quickly.
Vaccination against EHV1 and 4 is available. It will not provide full protection against any strain or manifestation of EHV but may reduce severity and virus shedding so if you are really worried vaccinate your horse. The vaccine is currently out of stock across the UK so please let us know if you want to vaccinate your horse and we will let you know as soon as it’s back in stock.
We know that stress and the introduction of new horses are key factors in any outbreak. Good yard management can reduce this significantly and the following steps may be a wise precaution with or without EHV. Keep horses in small settled groups – this is especially important for pregnant mares. Where possible separate the groups with gaps of at least 10m between groups. If stress is minimised and the horses are kept in good physical condition the risk of any disease is reduced. If infection occurs it may be easier to limit it to one group of horses.
For large livery yards a discussion amongst owners to create some form of segregation for horses that compete regularly, accepting that all of the group going to compete share a risk of picking up some infection from somewhere, may reduce the risk of introducing anything to the resident horses. But there is no such thing as zero risk; you just have to keep it in perspective
Is it safe to take your horse out?
90% of problems with horses like accidents with people probably occur at home so if you like competing go and do it.
If the sun is shining and the grass is green and I had the chance to take my best horses to polo today would I do it? Absolutely!!!
You must make up your own mind.
1. Edington N, Welch HM, Grifﬁths L. The prevalence of latent equid herpesviruses in the tissues of 40 abattoir horses. Equine
Vet J 1994;26:140–142
2. D.P. Lunn, N. Davis-Poynter, M.J.B.F. Flaminio, D.W. Horohov, K. Osterrieder, N. Pusterla, and H.G.G. Townsend. American College of Veterinary Internal Medicine Consensus Statements Equine Herpes virus-1 J Vet Intern Med 2009;23:450–461
3. HBLB Codes of Practice 2013 :31 - 39