Equine Herpesvirus (EHV): What Horse Owners Need to Know

Reviewed by Dr. Ali Ehtisham, DVM

Equine herpesvirus is one of the most widespread and significant viral diseases affecting horses worldwide. Virtually every adult horse carries one or more strains of equine herpesvirus, and the virus has the capacity to cause a devastating range of diseases — from common respiratory infections in young horses to sudden abortion in pregnant mares to a progressive, potentially fatal neurological disease that can sweep through entire competition yards within days. Understanding what EHV is, what it does, and how to manage it responsibly is essential knowledge for every horse owner.

What Is Equine Herpesvirus?

Equine herpesvirus (EHV) is a family of herpesviruses that affect horses, donkeys, and other equids. The family includes at least five members numbered EHV-1 through EHV-5, but the two of greatest clinical importance to horse owners are EHV-1 and EHV-4. Like all herpesviruses, these are DNA viruses characterised by their ability to establish lifelong latent infection in their host after the initial acute infection resolves — a fundamental biological feature that makes EHV impossible to eradicate once a horse is infected.

EHV-4: The Respiratory Virus

EHV-4 is the most common cause of equine rhinopneumonitis — viral respiratory disease in horses, particularly young horses and those in high-density environments such as competition yards, sales, and training facilities. Clinical signs resemble those of equine influenza: fever (often the first sign, typically 38.5–41°C), nasal discharge that may progress from clear to mucopurulent, reduced appetite, lethargy, and coughing. Most otherwise healthy adult horses recover within one to three weeks, though recovery is slower in heavily worked horses or those with secondary bacterial infections.

EHV-4 very rarely causes the neurological form and is not associated with abortion with anywhere near the frequency of EHV-1. However, it remains significant because of the welfare and economic impact of widespread respiratory disease in competition horses, and because identifying EHV-4 infection requires laboratory testing — clinical signs alone are not sufficient to distinguish it from equine influenza or other respiratory viruses.

EHV-1: A More Dangerous Virus

EHV-1 is the more clinically serious member of the pair. While it can cause the same respiratory disease picture as EHV-4, it is also associated with two additional and far more serious disease presentations: abortion in pregnant mares and equine herpesvirus myeloencephalopathy (EHM) — the neurological form.

Abortion: EHV-1 is one of the most important causes of infectious abortion in horses worldwide. Mares infected during pregnancy may abort spontaneously weeks or months after the initial infection, without showing obvious systemic illness. Abortion typically occurs in the last trimester, from around seven months of gestation onward. The aborted material — foetus, placenta, and fluids — contains very high viral loads and is highly contagious to other horses. Any aborted foal should be treated as a potential biohazard: isolate the mare, contain the aborted material, and contact your veterinarian immediately for testing and guidance.

Neonatal foal death: EHV-1 can infect foals in utero, producing foals born alive but severely compromised, dying within hours to days of birth with profound weakness and respiratory distress.

Latency and Reactivation

One of the most clinically relevant features of all herpesviruses — including EHV-1 and EHV-4 — is their ability to establish lifelong latent infection. After the primary acute infection resolves and the horse appears fully recovered, the virus retreats to specific sites in the body — particularly nerve ganglia and lymphoid tissue — where it persists in a dormant, non-replicating state for the remainder of the horse's life.

A latently infected horse shows no signs of disease and will not test positive on routine diagnostic swabs. However, under specific trigger conditions, the virus can reactivate: it begins replicating again, is shed from the respiratory tract, and can infect other horses. Common reactivation triggers include: long-distance transport, competition stress, concurrent illness, pregnancy, heavy training loads, weaning, and mixing with unfamiliar horses. This means that every adult horse in the population should be considered a potential EHV carrier capable of shedding virus during periods of stress, regardless of vaccination status or apparent health. This latency is precisely what makes EHV so difficult to control at the population level.

Equine Herpesvirus Myeloencephalopathy (EHM)

EHM is the neurological form of EHV-1 disease and the most feared manifestation of equine herpesvirus infection. It is caused specifically by neuropathogenic strains of EHV-1, though why some strains cause neurological disease while others primarily cause respiratory disease or abortion is an area of ongoing research. EHM can affect individual horses or cause explosive outbreaks affecting multiple horses simultaneously — a major multi-country outbreak in 2021 linked to equestrian competition events in Europe caused widespread disruption to the competitive calendar and illustrated the global threat the disease represents.

How it causes neurological disease: Unlike the respiratory form, EHM results from the virus infecting the endothelial cells lining the blood vessels of the spinal cord and brain. This causes vasculitis — inflammation of the blood vessel walls — and ischaemic damage to the spinal cord and brain tissue as the blood supply is disrupted. The resulting clinical signs are those of spinal cord and brain dysfunction, not respiratory illness.

Clinical signs: EHM typically begins with fever (38.5–41°C), which may be the only sign for the first few days and may be missed entirely if temperature monitoring is not being carried out. As neurological damage accumulates, the horse develops hindlimb ataxia (wobbliness and incoordination), hindlimb weakness that may progress to include the front limbs, and bladder dysfunction — inability to urinate or urinary incontinence indicating loss of bladder tone. In severe cases, the horse becomes unable to rise from the ground. A characteristic feature of EHM outbreaks is the rapidity with which multiple horses at a yard can develop signs, sometimes within 24 to 48 hours of each other.

Prognosis: Horses with mild hindlimb incoordination that remain standing throughout have a reasonable prognosis for survival and return to work with appropriate supportive care. Horses that go down and remain recumbent have a significantly worse prognosis — many require euthanasia on welfare grounds. Even horses that survive may be left with lasting neurological deficits.

Diagnosis and Treatment

Diagnosis of active EHV infection is confirmed through nasopharyngeal swabs and whole blood samples submitted for PCR testing — the most sensitive diagnostic method for acute infection. Post-mortem samples from aborted foetuses or affected horses provide definitive confirmation. Serology (antibody testing) can indicate recent exposure but is less useful in acute management decisions.

There is no specific cure for EHV infection. Treatment is supportive: nursing care, anti-inflammatory medication (NSAIDs such as flunixin meglumine are commonly used), intravenous fluids, and management of secondary complications. Antiviral drugs — particularly valaciclovir, which is bioavailable in horses — have been used in some EHM cases and there is some evidence for benefit in reducing viral replication, though clinical trial data is limited. Bladder management in horses with urinary dysfunction (manual expression or catheterisation) is an important nursing consideration in EHM cases.

Vaccination: Benefits and Limitations

Vaccines are available against EHV-1 and EHV-4, and vaccination is recommended as part of standard preventive healthcare. EHV vaccines reduce the severity of respiratory disease, shorten the duration of illness, and reduce the amount of virus shed from the respiratory tract. Vaccination of broodmares during pregnancy — typically at five, seven, and nine months of gestation using a specific killed EHV-1 vaccine — provides meaningful protection against abortion.

The critical limitation is that available vaccines do not provide reliable protection against EHM. This has been consistently demonstrated in outbreak situations where vaccinated horses have developed the neurological form alongside unvaccinated horses. The reasons relate to the different disease mechanism of EHM compared to respiratory disease, and the limitations of current vaccine-induced immunity in preventing the viraemia that precedes neurological damage. Vaccination remains recommended — it is not without value — but it should never be considered sufficient protection on its own, particularly during an EHV alert or outbreak situation.

Biosecurity During Outbreaks

When an EHV outbreak — particularly involving suspected EHM — occurs at a yard or event, the response must be immediate. Complete isolation of the affected horse or horses; immediate cessation of all horse movement into and out of the yard; twice-daily temperature monitoring of all horses on the premises; disinfection of shared equipment, buckets, and tack; and notification of the relevant veterinary authority and national equestrian federation are all essential steps. Good quality virucidal disinfectants effective against herpesviruses include products containing quaternary ammonium compounds, accelerated hydrogen peroxide, or aldehyde-based formulations. For broader guidance on equine health management, consult our horse health resources.

Frequently Asked Questions

Is EHV-1 the same as equine herpesvirus myeloencephalopathy?

No — EHV-1 is the virus; EHM is a specific neurological disease caused by neuropathogenic strains of EHV-1. Not all EHV-1 infections result in EHM. EHV-4 very rarely causes the neurological form.

Does the EHV vaccine prevent the neurological form EHM?

No — this is a critical point. Current EHV vaccines do not reliably prevent EHM. They reduce respiratory disease severity and viral shedding, and the broodmare vaccine reduces abortion risk, but vaccination alone does not protect against the neurological form.

How does EHV spread between horses?

Primarily through direct nose-to-nose contact, shared equipment and buckets, contaminated hands and clothing, and inhalation of viral particles shed from infected horses' respiratory tracts. Aborted foetal material also carries very high viral loads.

What are the signs of EHM in horses?

Fever followed by hindlimb incoordination, hindlimb weakness, bladder dysfunction, and in severe cases inability to rise. Multiple horses at a yard may be affected simultaneously. Any horse showing these signs after a fever requires immediate veterinary assessment.

Should I stop competing during an EHV outbreak?

Yes — follow your national federation's guidance closely. Movement restrictions during outbreaks are essential for containing spread. The risk to your horse and to others' horses is not worth any competitive benefit.

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Disclaimer: This article is for educational purposes only and does not constitute veterinary advice. Always consult a licensed equine veterinarian for diagnosis and treatment.