The equine herpes virus is always present in the British horse population, but the spotlight is on the disease again after the Mediterranean Equestrian Tour in Oliva Nova, Spain in February 2023 was cancelled as a result of positive cases on site.
The equine herpes virus (EHV) is widespread among the horse population in many countries, including Britain. It is thought most horses have been exposed to the disease when young, usually by the age of two and then become latent carriers of the virus. A horse can therefore be infected for life, and can intermittently discharge the virus from the nose and infect susceptible horses. The virus lays dormant in these horses before becoming active again at times of stress. It is important to understand that horses do not always show clinical signs while they are shedding the virus.
EHV has nine strains, with five (EHV-1 to EHV-5) affecting horses.
The two most common strains are EHV-1, which can cause respiratory problems, abortion in pregnant mares and, in rare cases (including an outbreak in southern Britain in January 2020 and a major outbreak linked to a showground in Valencia, Spain that began in February 2021 and spread widely around Europe), neurological problems leading to paralysis and in some cases death; and EHV-4, which most commonly causes mild respiratory problems and very occasionally a single case of abortion in a pregnant mare.
EHV is not a notifiable disease by law in the UK, although the Horseracing Betting Levy Board’s code of practice for the disease calls for thoroughbred breeders to notify the Thoroughbred Breeders’ Association if EHV abortion or neurological diseases occur. Other breeders should inform their relevant breed association. Previously the Animal Health Trust also kept track of cases, but they can now be followed through the International Collating Centre (ICC).
There is a vaccine available for EHV, but effective biosecurity is equally important, alongside quarantine during an outbreak. Regular vaccination can lead to a reduction in infectious virus circulating in the population and it is likely to have helped, along with preventative management measures and biosecurity, in reducing the number of outbreaks of disease reported. There is evidence of reduced respiratory disease and abortion in vaccinated animals, however the efficacy of vaccination to prevent the neurological form of the disease is unclear.
It is not recommended to use the vaccine on any horses that may have recently been exposed to the infection while an outbreak is ongoing. That said, vaccination may be advisable for the horses, outside the immediate geographical location of the outbreak, to reduce the long term viral shedding and improve herd immunity. There are currently no official requirements for vaccination against equine herpes virus in the United Kingdom, but it is required for thoroughbreds racing in France.
Types of EHV
EHV-1 can cause respiratory disease in young horses, abortion in pregnant mares and paralysis in horses of all ages and types. The neurological form that causes paralysis is called EHV-1 myeloencephalopathy (EHM).
EHV-2 rarely leads to clinic signs, but it can suppress immunity and make the horse more prone to other infections as well as causing respiratory disease in some cases.
EHV-3 is a venereal disease that causes lesions on the penis of stallions and the vulva of mares.
EHV-4 usually causes low-level respiratory problems, but can occasionally cause abortion. It is also known as equine rhinopneumonitis virus.
EHV-5 is associated with pneumonia and lung scarring (equine multinodular pulmonary fibrosis) in adult horses
Is it serious?
While most cases of EHV result in a relatively mild respiratory disease, it can cause either a single or group of pregnant mares to miscarry their unborn foals without warning, or a horse to show mild abnormalities when walking and within hours be unable to stand.
The risks should not be under-estimated and a vet should be contacted for any horse showing signs. Data from recent outbreaks suggest the virus may be becoming more virulent, i.e. more likely to spread. This is another reason why effective biosecurity is so important.
Following the major outbreak in Europe in spring 2021, the FEI put in place new horse health requirements to help protect horses competing at international events.
What are the signs of equine herpes virus?
Most often seen in groups of young horses (weaned foals and yearlings), the signs of the respiratory form of the disease may include:
- Raised temperature
- Watery nasal discharge that can become thick and infected
- Lack of interest in food
- Swollen glands around the throat
Signs of the neurological disease may include:
- Hind limb weakness and a lack of coordination
- Head tilt
- Inability to lift the tail
- Dribbling urine/difficulty in urinating
- Leaning against a fence or wall
- Inability to stand
- Neurological signs may be preceded by fever and respiratory signs
In abortion cases, the mare will unexpectedly lose her unborn foal, or a foal carried to term may be born weak, with breathing problems and die within the first few days.
Respiratory disease caused by EHV is most often seen in weaned foals and yearlings, typically in autumn and winter. However, older horses can also pick up the virus without showing obvious signs and then transmit it to others. The ability to shed the virus without showing signs is why EHV can spread widely leading to abortion in pregnant mares and cases of EHV neurological disease in horses of all types and ages.
How is it spread?
A horse becomes infected with EHV by inhaling the virus that has been expelled into the air by an infected horse coughing or sneezing, or it can be transmitted via infection in the local environment, or directly from horse to horse.
Unlike equine flu, EHV does not travel long distances in the air (only up to about 50 metres), but it is thought to be increasingly infectious and can spread quickly within groups of horses that are living together either in paddocks, open barns or America barn-style stable yards, or when they come into contact with other horses at shows or training events.
The incubation period may be as short as 24 hours, but more typically is approximately four to six days, and horses are most infectious in the early stages of infection (when their temperature is raised). This is why regular twice daily monitoring of temperatures is a wise protective precaution.
When mares abort due to the EHV infection, the fetus, fetal membranes and fluids can release large quantities of infectious virus into the local environment, which can then be inhaled or spread indirectly by those caring for the mare and the equipment they use. This is a particular concern when horses are stabled with shared air space, which allows the virus to spread, such as within a barn, without the wind being able to dissipate the infection as it would in an outdoor paddock. The virus can survive in the local environment for up to a month after being shed by a horse.
Diagnosis of equine herpes virus
Your vet will base their diagnosis on clinical history and signs, followed by tests to isolate the virus on nasal swabs taken as early as possible after infection is suspected.
In addition blood samples will be taken to measure the levels of antibodies in the blood when the horse is first examined by the vet and then again two to three weeks later. An increase in the levels of antibodies seen in the blood will confirm recent exposure to the virus.
Treatment of equine herpes virus
There is no specific treatment for EHV currently available, so careful nursing is key to helping horses recover. Because EHV is a virus, antibiotics are not effective against it, but your vet may prescribe them where a secondary infection is present. Non-steroidal anti-inflammatory medication can help reduce high temperatures and give pain relief where necessary. Intravenous fluids may be required in some cases. Occasionally anti-viral medications are prescribed, although these are not licensed for use in horses.
Horses showing signs of a respiratory infection should receive complete rest in a dust-free environment.
Horses showing neurological signs should be cared for in an appropriate location, such as a well-bedded stable, barn or paddock with feed and water within easy reach. If a horse becomes unable to stand and remains recumbent, nursing care becomes challenging and regrettably euthanasia may be recommended on welfare grounds. There are a few reports of some recumbent horses that have recovered with intensive care, careful nursing and support, for instance, with purpose-designed slings.
Mares that have aborted due to EHV usually appear well in themselves and do not require any special nursing, but they should be isolated for 28 days to reduce the chance to spreading the infection further and kept separate from other pregnant mares for eight weeks.
What to do during an equine herpes virus outbreak
If a case of EHV is confirmed on your yard, then all horse movement on and off the premise should cease. It is recommended that movement restrictions are kept in place for at least 28 days after the last case and that the local equestrian community should be informed.
Horses that are confirmed with EHV should be isolated with strict barrier nursing and biosecurity measures put in place to reduce the likelihood of the virus spreading. Ideally separate staff should attend to the infected horse(s) using suitable protective clothing and biosecurity protocols to reduce the risk of spread of infection.
The use of virucidal disinfectants and heat can destroy the virus so stables, equipment and transport should be cleaned, steam cleaned and then disinfected with an approved disinfectant. Bedding should be removed and destroyed. Where possible virucidal disinfectant should be allowed to dry naturally on suitable surfaces to maximise the chance of destroying the virus.
If there is an outbreak of EHV in your local area, it may be wise to stop hacking out or going to external training, shows or hunting to minimise the risk of coming in contact with the virus. This will help to contain the outbreak and prevent it spreading to other yards.
Can you vaccinate against equine herpes?
There is a equine herpes vaccine available with evidence of reduced respiratory disease and abortion in vaccinated animals, however the efficacy of vaccination to prevent the neurological form of the disease is unclear.
In non-pregnant horses, the primary vaccination course involves two vaccinations four to six weeks apart, followed by booster vaccines every six months. Pregnant mares are typically vaccinated at five, seven and nine months of gestation to reduce the risk of abortion associated with EHV.
It is not recommended to vaccinate any horse that may have recently been exposed to the virus. Horses that have no risk of having been exposed to the virus can receive the vaccination while an outbreak is ongoing, provided they are not going to be exposed in the near future, e.g. they are going to remain in a closed yard and are not going to be travelling. It is not recommended to vaccinate any horses in the vicinity of any suspected outbreak.
How to prevent an equine herpes virus outbreak
Good biosecurity and hygiene practices are key. Even if your horse is vaccinated, you should still be strict in observing effective biosecurity measures.
Biosecurity recommendations include:
- When away from your own yard, do not allow your horse to touch other horses, share drinking water or graze
- Avoid touching other people’s horses or sharing any equipment
- Practice hand washing in between handling horses
- Keep separate feed buckets, brushes, towels, and tack/equipment for each horse. In short, maintain social distancing – think coronavirus precautions!
- Know your horse’s normal temperature and take it twice a day, especially after your horse has been away to a show or external training session
- Have a plan for isolating new arrivals or potentially infected horses
- Early identification and reporting of ill horses is vital in order to trace possible points of exposure and to aid in the prevention of further spread of the disease. There is a view that the management in the early stages following identification of a potentially infectious horse frequently determines the extent and scope of transmission and potential outbreak.
Always ensure you are ready for rapid implementation of an isolation plan at a competition grounds and/or at your home yard. Although not all yards have such a facility, it makes sense to isolate horses at first signs of illness and contact your vet immediately for advice.
When any new horse comes on to your yard, it should be isolated for three to four weeks to avoid bringing a new infection into an established group of horses. You should also consider what precautions to apply when a horse is returning to the yard from a show, training or veterinary treatment.
Due to being at particular risk from EHV, it is recommended that pregnant broodmares be kept separate from horses that are attending competitions or external training sessions.
If you have any concerns or questions, always consult your own vet.
Latest update: 28 February, 2023
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In an update today (1 March) the FEI confirmed four horses had died from the neurological form of EHV-1 in
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