Equine Herpesvirus-1

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Also Known As

Equine Herpesvirus Myelocencephalopathy (EHM), Equine Protozoal Myeloencaphalitis (EPM), Equine Viral Rhinopneumonitis


Equine herpes viruses are viruses that are commonly found in horses in nearly every country in the world. Fortunately, many horses that have been infected with the virus have no serious side effects. It is unknown what causes some horses to develop the serious neurological forms that may be fatal.

Equine herpes virus infection in horses can cause respiratory disease, abortion in mares, neonatal foal death, and neurological disease. The virus is not transmissible to humans. Damage to the blood vessels in the brain and spinal cord cause neurological signs to appear.

Equine herpes myelocencephalopathy is most often due to the neuropathogenic strain of EHV-1 and may occasionally be caused by the non-neuropathogenic strain of the virus.

The disease is easily spread and typically has an incubation period between 2-10 days. Respiratory shedding of the virus occurs for 7-10 days, but may last longer in infected horses.


  • Fever
  • Nasal discharge
  • Incoordination
  • Hind limb weakness
  • Loss of tail tone
  • Lethargy
  • Urine dribbling
  • Head tilt
  • Leaning against stationary fixtures, such as a fence, to maintain balance
  • Inability to stand


Equine Herpes Virus is caused by a family of viruses, with EHV 1,3, and 4 posing the most serious health risks.

EHV-1 creates four manifestations of disease in horses, including the neurological form, respiratory disease, abortion, and neonatal death.

EHV-3 causes a venereal disease called coital exanthema that affects the external genitalia.

EHV-4 causes a nonfatal upper respiratory tract disease in foals and is uncommonly associated with abortion and rarely with neurological disease.

EVH-1 disease is highly contagious and is spread by direct horse-to-horse contact with nasal secretions. It may also be spread by the inhalation of infected air particles, but it is not known how far distant the airborne virus can spread under typical horse management and environmental conditions.

The disease can also be spread indirectly through contact with physical objects contaminated with the virus, such as human hands and clothing, equipment, tack, trailers, grooming equipment, contaminated feed, and water buckets.


Horses suspected of having EHV infections should be examined by a veterinarian and tested for virus shedding. Infected horses should be isolated immediately and all contact with other horses should be avoided. Exposed horses should also be isolated.

Two main methods of prevention can be implemented to avoid EHV outbreaks. Vaccinations and biosecurity protection are both important in preventing EHV diseases and in reducing exposure.

The goal of vaccination is to induce resistance to infection prior to exposure by producing a strong and durable immune response without inducing clinical signs of the disease in the horse. Several vaccines are available for protection against both respiratory disease and abortion as a result of EHV-1 infection, but currently no equine vaccine is licensed that has a label claim for protection against the neurological strain of the virus.

Biosecurity measures include doing everything possible to reduce the chances of an infectious disease being carried onto your property by people, animals, equipment, or vehicles, either accidentally or on purpose. Anything that touches an infected horse and secretions shed by sick horses have the potential to transfer pathogens to other horses.

The immediate separation and isolation of identified suspect cases is key to disease control. Ideally, a person caring for a sick horse should not also work with or be around healthy horses. If this is impractical, it is recommended that healthy horses should be worked with first and sick horses last.

Wearing disposable gloves and changing them between horses or the use of hand sanitizers between horse contacts are alternative recommendations. Thorough cleaning and disinfection of foot wear between entry into barns and stalls can also minimize risks of disease spread.

Herpes viruses are susceptible to many disinfectants, including a 1:10 dilution of bleach in water. Both alcohol and bleach disinfectants are inactivated by organic matter, so all soil and manure should be cleaned away before using these products. Soap and detergent can be used effectively before applying a disinfectant.

A disinfectant that retains activity in the presence of organic matter is recommended In barn environments where organic material cannot be completely eliminated. Phenolics, such as 1 Stoke Environ or SynPhenol-3, or accelerated hydrogen peroxide products, such as Virkon, have this property. Always follow manufacturer's recommendations when using these products.


A veterinarian should be called to make a diagnosis at the first sign of symptoms of EHV. Nasal swabs and blood samples will be collected from the symptomatic horse. Recent advancements in EHV-1 diagnostic testing enable laboratories to differentiate the non-neuropathic and the neuropathic strains of EHV-1.

Unfortunately, there is no cure for the equine herpes virus. It may remain dormant in the horse's body, re-emerge at any time, and is resistant to the horse's immune system defenses. However, there are medications, such as antibiotics, that can help with the secondary infections and symptoms the horse may suffer from.

Horses infected with the EHV-1 virus need plenty of rest and a proper, well-balanced diet, while those with the subtype affecting the respiratory system will require fresh, clean air. These horses may also require medication to break up the mucus that accumulates and clogs their airways.

Treatment is mostly supportive and includes anti-inflammatory medications, such as corticosteroids. Some horses require IV fluids if they cannot maintain their hydration. Horses should be isolated in a safe, well-bedded stall, especially if they are severely ataxic and have trouble rising. Recumbent horses may benefit from the use of a sling.

Horses that have difficulty urinating may need to be catheterized several times daily and may benefit from medication to prevent bladder infection.

In cases of the EPM form of the disease, horses usually begin to display clinical signs, some of which are consistent with other neurologic diseases. It is important that a veterinarian diagnose the disease as soon as possible. A neurological exam along with a blood or cerebrospinal fluid test is usually done.

Unfortunately, these tests determine only if the horse has been exposed to a causative agent. The IFAT test has been shown to help establish a probability of whether a horse has EPM, but no test is 100% accurate.

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EquiMed staff writers team up to provide articles that require periodic updates based on evolving methods of equine healthcare. Compendia articles, core healthcare topics and more are written and updated as a group effort. Our review process includes an important veterinarian review, helping to assure the content is consistent with the latest understanding from a medical professional.