By Vicky Rowlands BVM&S CertEP MRCVS, Ashbrook Equine Hospital.
Equine herpes virus (EHV) is a contagious viral disease commonly found in horses. Despite this virus having had such a high profile recently, it has been continually endemic in the equine population.
Types of EHV
There are multiple different types of herpes virus that affects equines. EHV-1 is probably the highest profile of the EHV viruses as it causes the most dramatic signs, including abortion and neurological disease. EHV-4 is more commonly identified and is associated with respiratory disease. EHV-3 is responsible for a blister-like disease of the external genitalia called ‘coital exanthema’. For those in the Americas, EHV is more commonly known as ‘rhino’.
Spread of EHV
Spread of herpes occurs when healthy equines inhale or contact respiratory droplets, such as those released when infected horses cough. Infection can also occur from contact with aborted material. EHV is an unusual disease as once a horse is infected and recovered, the disease usually then stays latent or dormant for many months or even years. This infection can be re-activated at times of stress, such as high workloads and travel. Once it is reactivated, the horse can ‘shed’ infection again and the cycle of infection can then continue. The prevalence of EHV in the UK may be as high as 87.5% of horses.
If spread occurs in unvaccinated horses or those not previously exposed to the virus, disease quickly travels through that population; however, in those that have some protection from vaccination or previous exposure, disease will be less severe yet can still occur.
To give some context to EHV, it is worth remembering that coldsores in humans are a form of herpes. This disease stays dormant in the affected person and reactivates during stress resulting in visible, active coldsores which can then be transferred to other people.
Disease caused by EHV
Signs of EHV infection include a raised temperature, depression and a thin, watery nasal discharge which can then become thicker as a secondary infection occurs. Signs of disease are usually seen within 24hours to 6 days of infection, but this can be longer in some horses. Different types of infection have more specific signs to look out for.
EHV-1
EHV-1 infection is often seen as a mild viral cough and thin watery nasal discharge that quickly moves through a yard of young horses in training. Sometimes the symptoms may be so mild they are not observed and staff only notice a lack of performance in the horses.
EHV-1 infection may be initially seen as a fever and dry cough which quickly recovers. Sadly, EHV-1 has other, more significant forms which include abortion or neurological forms. Respiratory signs may or may not occur prior to a case of neurological disease or herpes abortion.
The neurological form of EHV-1 is called myeloencephalitis (EHM) and can occur in individual horses or in an outbreak. The neurological form of herpes causes progressive ataxia (wobbliness), which usually affects the hind end first. Horses may also have urinary incontinence and when severe, can become recumbent and unable to rise.
Abortion can occur in individual animals or as an outbreak ‘abortion storm’. When abortion occurs, it will typically happen in late gestation and may even occur many weeks or months after infection.
EHV-4
EHV-4 is more typically associated with respiratory infection only, although some forms of this have been associated with individual abortions. Horses affected by this will have a fever, cough, nasal discharge, occasionally eye discharge and swollen lymph nodes of the head. Although the disease itself is not severe, time off work and lost competition days are very disruptive, especially if it spreads through a yard of competition horses. In some cases, the respiratory infection can go on to form secondary infection and even pneumonia.
Diagnosis
Diagnosis can be achieved using swabs to sample the inside of the horse’s nasal passages and then sent to a laboratory for virus isolation. Blood samples can also be taken to assess for levels of EHV antibodies in the horse’s blood. Unless these levels are extremely high, two samples need to be taken 10-14 days apart to look for the increasing response which typically occurs after infection. Although the horse is usually recovering by the time of confirmation, it is important for management of the rest of the horses on the yard.
In the case of abortion, the placenta and aborted foetus can be sent to a laboratory for confirmation.
Treatment and recovery
Treatment is normally based on good nursing care and anti-inflammatories, as there are no specific licensed treatments for EHV. Recovery for simple respiratory cases will usually take a few weeks; sadly, many neurological cases carry a poor prognosis.
Prevention
As with any infectious disease, good biosecurity is the cornerstone of preventing an outbreak or infection with EHV. This is particularly pertinent when travelling, competing or working horses extremely hard as we know stress plays a large part in reactivation of latent infection. It is also important to know virus can persist in the environment for days to weeks so disinfection of stables and equipment is also vital.
If a horse is suspected of having an active EHV infection, this horse should be isolated until a diagnosis is made and twice daily temperature checks instigated for the other horses in the yard.
Pregnant mares should be kept in the same group to minimise stress and reduce the risk of new arrivals bringing in disease.
Vaccination
Vaccinations can reduce the spread of the disease and severity of infection should a horse encounters EHV. If a vaccination protocol is instigated, it should be maintained every 6 months. Pregnant mares should have booster vaccinations at months 5, 7 and 9 of gestation.
Although vaccination should protect against EHV infection, sadly there is no label claims or proven protection against the neurological form of EHV. Vaccination should also not be carried out when active infection is on a yard; it may even worsen potential symptoms.
Although EHV is an endemic disease of the equine population, there are many ways of reducing the risks to your horse. Your vet will be able to discuss reducing the risks, implementing good yard biosecurity protocols and provide advice regarding vaccination.
A good source of further reading can be found at
https://aaep.org/guidelines/infectious-disease-control/equine-herpesvirus-resources
