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    Equine Herpesvirus

    Overview

    Equine herpesvirus (EHV) refers to a group of viruses that affect horses and are identified by different numbers—EHV-1, 2, 3, 4, and 5. Among these, EHV-1, EHV-3, and EHV-4 present the greatest concern for domestic horses. EHV is a widespread DNA virus found in horse populations around the world, with EHV-1 and EHV-4 being the most frequently seen.

    EHV-1 and EHV-4 typically cause respiratory illness, which is often mild and resolves on its own, but both can also lead to abortion in pregnant mares. In some cases, EHV-1—and very rarely EHV-4—can trigger a severe neurologic condition known as equine herpesvirus myeloencephalopathy (EHM), which damages the spinal cord and can be fatal. EHV-3 is responsible for coital exanthema, a sexually transmitted infection that affects the external reproductive organs.

    Horses may carry EHV without showing symptoms and still spread it to others. The main tools for controlling the virus are testing, isolating affected horses, and enforcing quarantine. Vaccines can help reduce disease severity but do not fully prevent infection or transmission.

    Biosecurity practices for EHV focus on minimizing horse-horse transmission through aerosol particles from nasal discharge or through contaminated surfaces including people, clothing, feed and water, implements, and stalls. This includes extensive disinfection of surfaces and equipment that come in contact with infected horses, as well as isolation of any horse that tests positive on nasal swab or blood.
     

    Effective December 8, 2025, a temporary quarantine order is in effect, carrying additional documentation requirements for Interstate Certificates of Veterinary Inspection for the transport of horses into Pennsylvania. The order includes documentation requirements to attest that a horse has not been exposed to EHM or had a suspected or confirmed diagnosis of EHM. The order reiterates documentation of temperature check requirements. 

    Quarantine Order (PDF)

     

    Resources

    Clinical signs

    • Fever may be intermittent throughout the course of disease, and the absence of fever does not rule out EHM. 
    • Nasal discharge 
    • Lethargy 
    • Distal limb edema 
    • Incoordination 
    • Hind limb weakness 
    • Loss of tail tone or bladder tone (urine dribbling or inability to urinate) 
    • Dog sitting position 
    • Leaning against a fence or wall to maintain balance 
    • Recumbency (inability to rise)
       

    Prevention

    There is currently no vaccine labeled for the prevention of the neurologic form (EHM). Management practices for preventing EHV-related disease include maintaining current vaccinations on all horses on the property, practicing biosecurity while traveling and showing, and quarantining any new horses (or horses returning to a farm after travel) for at least 21 days before integration into the farm herd. Horses at shows should have their temperatures monitored twice daily. 
     

    Biosecurity

    Biosecurity practices for EHV focus on minimizing horse-horse transmission through aerosol particles from nasal discharge or through contaminated surfaces including people, clothing, feed and water, implements, and stalls. This includes extensive disinfection of surfaces and equipment that come in contact with infected horses, as well as isolation of any horse that tests positive on nasal swab or blood.

    EHM

    If a horse tests positive for EHV and has neurologic signs, the diagnosis is EHM, and the horse is quarantined and given supportive care.

    While EHM is a serious disease, the majority of cases are not fatal. Veterinary evaluation and isolation of exposed or affected horses is crucial.

    Horses carrying EHV1 and EHV4 viruses may not show signs at all. But it’s important to remember that stress can weaken a horse’s immune system, and cause illness. Cold, moist, indoor environments can enable the virus to live longer on surfaces, and proximity to other horses can cause its spread. Recommended preventative measures include:

    • Minimize stress by limiting transport of horses.
    • Use separate tack and feed buckets.
    • Allow horses to be outside on pasture as much as possible.
    • Develop and put into practice a biosecurity plan specific to your farm, your trailers, and any event you attend.

    EHM does not affect humans.
    Veterinarians are required by Pennsylvania and federal law to report any suspected cases of EHM to the Department of Agriculture at 717-772-2852.