Equine Herpes Virus (EHV):
What You Need to Know to Protect Your Horse

By: Dr. Lydia Gray

What is EHV?

EHV refers to Equine Herpes Virus and it is a family of viruses named by numbers that commonly infects horses worldwide. In fact, the great majority of horses are exposed to EHV-1 early in life and develop a latent infection, meaning they carry the virus their whole lives without showing signs of illness. The virus seems to be “reactivated” from its dormant status under periods of stress such as travel and competition. Nine different equine herpes viruses have been identified and numbered, but the focus is on EHV-1 and EHV- 4 because those are the two that can result in serious disease in the horse. 

Both are a common cause of mild respiratory disease known as equine viral rhinopneumonitis or simply “rhino” and both can cause abortions, especially in unvaccinated mares. Also, an infection of either EHV-1 or EHV- 4 (but EHV-1 more commonly) can result in the development of EHM or Equine Herpesvirus Myeloencephalopathy. This is the rare neurological form of equine herpesvirus that is associated with EHV infections and, since about 2000, seems to be on the rise in the US.

Experts aren’t sure what causes some EHV-infected horses to develop the serious, even fatal, neurological form. It also isn’t clear if there is truly an increase in the prevalence of herpesvirus neurological disease or if increased reporting and high-profile outbreaks just make it seem more common. To be on the safe side, the USDA has declared neurologic EHV-1 to be a potentially emerging disease worth additional study, especially now that a genetic variant has been described that is more commonly associated with neurologic disease.

Signs & Symptoms

Equine herpesvirus can cause disease in three separate systems: respiratory, neurologic, and reproductive, and therefore the clinical signs can vary. In the case of respiratory disease, a fever in two phases is common. That is, on Day 1 or 2 of the infection and again on Day 6 or 7, rectal temperature may reach 102 – 107 °F. Next, the horse may develop a clear and colorless discharge from the nose as well as the eyes. The horse may or may not cough. Many develop swollen lymph nodes, lose their appetite, and act depressed or lethargic. Swelling of the limbs has also been observed.

Fever nearly always occurs before a horse comes down with the neurological form of herpesvirus, with some horses also displaying mild respiratory signs. EHM appears suddenly and progresses rapidly, with clinical signs reaching their peak intensity just 1-2 days after onset. Signs of neurologic problems primarily affect the back half of the body and include:

  • Weakness, stumbling, incoordination (“ataxia”)
  • Leaning against a fence or wall to maintain balance
  • Dog-sitting
  • Loss of skin sensation around the tail and hindlimb areas
  • Loss of tail tone
  •  Bladder dysfunction (urine dribbling or inability to urinate)
  • Lying down and being unable to rise (“recumbency”)
  • Complete paralysis

Mares infected with the equine abortion virus, as EHV-1 is sometimes known, may suddenly lose their foals between 7 – 10 months of gestation (the last third of pregnancy) with no warning and no other clinical signs. Live foals born to EHV-infected mares may only live a few hours or days. 

Diagnosis & Treatment of EHV

Diagnosis of equine herpesvirus is relatively straightforward and is based on clinical signs along with virus identification from either a nasal swab or a blood sample. Alternatively, two blood samples taken 2-3 weeks apart can also be used to diagnose an EHV infection based on a significant increase in antibody titer. Because it is important to know what a horse, herd, or barn is infected with in order to provide appropriate, early treatment as well as to limit an outbreak, it is recommended to contact a veterinarian for physical examination and diagnostic testing upon discovering any FUO (fever of unknown origin), respiratory signs, or neurological signs. The vet can also provide guidance on isolating sick horses and implementing other biosecurity protocols on the farm.

Since EHV is a viral, not bacterial, disease, treatment of both the respiratory form and the neurologic form mainly involves rest and good nursing care instead of antibiotics. This means keeping the horse comfortable (such as with non-steroidal anti-inflammatory drugs), hydrated (using intravenous fluids if necessary), and eating. Horses with secondary bacterial infections may be prescribed antibiotics, and those with more severe neurologic disease may need around-the-clock nursing care that includes assistance with urination, defecation, and even standing. There are certain situations when anti-viral medication may be of use, which is a topic for discussion with the horse and farm’s regular veterinarian.

The prognosis for a horse infected with EHV depends on which system was affected and how severely. For example, mares that lost a foal usually recover completely with no long-term effects on their reproductive health. Uncomplicated respiratory cases usually also recover completely in a few weeks, at which time they can be gradually returned to full work. However, horses with the neurological form of equine herpesvirus have more variable recovery rates and success depending on the severity of their clinical signs. For example, a horse that just had some weakness, stumbling, or incoordination is more likely to recover completely or with a slight deficit than a horse that was down for several days. Horses that are unable to rise for an extended period of time have a much worse prognosis – the fatality rate for EHM can be as high as 30%.

 Management & Prevention of EHV

Preventing an outbreak of the respiratory, neurologic, or reproductive forms of equine herpesvirus relies on a two-pronged approach that is developed with the farm veterinarian: 1) appropriate vaccination and 2) common-sense biosecurity. 


While there are several EHV vaccines labeled for use in horses in the US, some offer more protection than others although none carry a claim of preventing the neurological form of the disease. Therefore, it is critically important to work closely with the local veterinarian to design a vaccination program that takes into account the horse’s age (young or adult), use (performance or breeding), and exposure to other horses as protocols can differ widely in their timing and frequency. Vaccinating against equine herpesvirus may reduce the severity and duration of the respiratory and reproductive forms – as well as reduce the concentration of virus in the blood and shedding of the virus in nasal secretions – so there is value even though it may not prevent neurologic disease directly. Competitors in US Equestrian-recognized events should be familiar with the “vaccine rule” that took effect December 1, 2015.


The importance of vigilantly practicing good biosecurity both on and off the farm cannot be overemphasized when it comes to preventing the spread of equine herpesvirus. EHV can be passed from horse-to-horse through direct contact and so avoiding co-mingling of new horses with resident horses as well as one farm’s horses with another farm’s horses at an event is essential. Since the virus can easily survive 7 days in the environment (longer under ideal conditions), it can also be passed through indirect contact with premises (walls, floors, doors, and gates), trailers, feed and water buckets, tools and equipment, and even human hands and clothing. Fortunately, EHV is easily killed by most common disinfectants, as long as the area or item is washed and rinsed first to cut down on dirt and filth that inactivates bleach, Lysol, and other products. Handwashing between horses with regular soap and water – or the use of alcohol-based hand sanitizers – is a must.

Besides avoiding the mixing of horses (direct contact) and not allowing the virus to spread on surfaces (indirect contact), another component of biosecurity is monitoring rectal temperature. In order not to miss a fever spike, handlers are advised to take temperatures twice daily of horses in high-risk situations such as clinics and shows, as well as any horses that may be in quarantine before being added to an existing herd. Should a horse break with a fever or signs of respiratory, reproductive, or neurological disease, the veterinarian should be called immediately for diagnostic testing and best isolation practices. In addition to officially reporting an EHV outbreak to state and federal authorities, he or she can provide guidance on how long individual horses should remain in isolation as well as how long movement of any horses on and off the farm should be restricted. With a typical incubation period of 2-10 days that can be as long as 14 days, two weeks of isolation is the absolute minimum recommended with three and four weeks being more common.

Equine herpesvirus is a challenging disease to prevent in horses and there is much that is still unknown about it, but strict attention to hygiene as well as strategic vaccination are a horse owner’s best friends.

Additional Resources

SmartPak strongly encourages you to consult your veterinarian regarding specific questions about your horse's health. This information is not intended to diagnose or treat any disease, and is purely educational.

Article First Published August 2018