Lakewood Animal Hospital

4882 Portal Dr
Tallahassee, FL 32303


Equine herpesviruses are very common DNA viruses in horse populations worldwide. The two most significant are EHV-1, which causes respiratory disease, abortion, and neurologic disease; and EHV-4, which primarily causes respiratory disease and only occasionally can cause abortion or neurologic disease.

Equine herpes viral respiratory disease is usually caused by EHV-4 and is most commonly seen in weaned foals and yearlings, often in autumn and winter. Older horses are more likely than younger ones to transmit the virus without showing signs of infection. Although EHV-1 is the principal cause of outbreaks of viral abortion, some strains of EHV-4 have been associated with sporadic cases of the disease.

EHV-1 myeloencephalopathy (EHM) results from damage to the vasculature in the CNS. Certain strains of the virus are endotheliotropic; therefore, this results in vasculitis, thrombosis, and areas of infarction that lead later to foci of malacia in neurologic tissue. Herpesvirus myeloencephalopathy cases occur singly or as outbreaks affecting 20-50% of the at-risk population. Occurrences may or may not be preceded by a febrile episode or signs of respiratory disease. See ACVIM Consensus Statement.

Clinical Signs

Fever is often biphasic and can be transient. The initial febrile phase precedes infection of the upper respiratory tract. The second febrile phase (6-7 days) often precedes a systemic viremia. Fever may go undetected and may be the only clinical sign noted in an infected horse. Temperature monitoring twice a day is recommended.
Respiratory disease
• Fever (A horse whose body temperature is >101.0 or 1.5 degrees greater than the horse's normal body temperature is considered febrile.)
• Coughing
• Nasal discharge
• Variable enlargement of the mandibular and/or retropharyngeal lymph nodes
• Lethargy, anorexia
• Conjunctivitis
• Ocular disease including uveitis and keratitis
• Neonatal foals infected in utero are usually abnormal from birth and exhibit any combination or all of the following:

  1. Fever
  2. Lethargy
  3. Weakness
  4. Jaundice
  5. Respiratory distress/stridor/pneumonia
  6. CNS signs (occasionally)
  7. Death commonly occurs within 3 days
  8. Older foals: nasal discharge is the most common sign of disease


Most often, no warning signs of impending abortion in the mare. Typically occurs in late pregnancy (7+ months); very occasionally as early as 4 months.

Neurologic disease


  1. Incoordination of the hind (and occasionally fore) limbs
  2. Ataxia or wobbly gait
  3. Urine retention/dribbling
  4. Bladder atony
  5. Recumbency with inability to rise
  6. Neurologic signs are often preceded by fever and/or respiratory signs

Incubation Period

After exposure by any route, incubation period may be as short as 24 hours but is typically 4-6 days, or longer.

Note: EHV-1 abortion can occur from two weeks to several months following exposure to the virus with mares showing no clinical signs.

Risk Factors

    • • Evidence of transmission of EHV-1 virus
    • • Strain of EHV-1 virus detected
    • • Number of horses potentially exposed (Areas of high commingling of horses such as racetracks, hospitals, show grounds, etc…)
    • • Immune status of exposed horses, i.e. hospital or geriatric, horse rescue (Stress or immunosuppression: transport, hospitalization, training, showing, weaning, high doses of steroids)
    • • Testing results of exposed and clinically affected horses
    • • Movement of horses once released from restrictions/isolation


    Respiratory transmission

    (most common route of exposure)

    Inhalation of droplets from coughing and snorting. (Note: EHV is not believed to spread by this route as efficiently as equine influenza virus.)

    Mares which have aborted, or whose foals have died, can transmit infection via the respiratory route.

    Shedding by the respiratory route typically lasts for 7-10 days, but can be much longer. Therefore, based on a thorough risk analysis of the particular outbreak or case, a period of 14 to 28 days after resolution of clinical signs may be necessary before release from movement restrictions/isolation. EHV-1 testing of horses considered exposed or infected provides increased confidence in the release of restrictions/isolation period prior to 28 days.

    Direction transmission

    Aborted fetuses, fetal membranes and/or fluids are significant sources of the virus.

    Infected foals are highly contagious and can transmit infection to other horses via the respiratory route through shedding virus into the environment.

    Indirect transmission

    Virus can be viable for several weeks in the environment once it has been shed by the horse.

    Virus contaminated fomites are a significant factor in EHV spread.

    Shedding Time of Virus Following Resolution of Clinical Signs

    Possibly up to a week, but may be longer in exceptional cases.

    Recovered horses typically develop latent infections and are capable of shedding virus following reactivation (with or without signs of clinical disease) particularly under conditions of stress, for the remainder of their lives.

    Horses with residual stable neurological signs are not considered when determining the countdown to release from isolation.

    Environmental Persistence

    Environmental transmission plays a minor role in the maintenance of virus in the horse population since environmental persistence of EHV-1 is short, estimated to be no more than 35 days under ideal conditions and probably less than 7 days in most practical field situations.

    Specific Control Measures


    Please view and follow the AAEP biosecurity recommendations.

    Clinically normal horses housed within the primary perimeter may be permitted segregated exercise periods outside the perimeter. Precautions should be taken, and may include:

    • Exercise scheduled

    after general population's exercise period to avoid potential virus transfer to unaffected horses/barns by exercise riders.

    • • Access to starting gate or similar equipment denied.
    • • Restricted use of ponies/outriders' horses—horses housed within the primary perimeter may only be escorted by horses housed within the same facility.
    • • Direct horse-to-horse contact is to be avoided.
    • • Prompt post-contact use of alcohol hand sanitizer by individuals having contact with horses during exercise.


    Booster vaccination of healthy animals in both primary and secondary contagion control perimeters may have some value, and is not known to lead to complications. If animals are unvaccinated prior to an outbreak there is unlikely to be time to administer an effective vaccination series during the risk period unless a modified nasal vaccine is used. Do not vaccinate clinically ill animals. Please see AAEP Vaccination Guidelines.

    Release of Animals from Isolation

    Maintain isolation procedures for 14-28 days after last clinical signs are detected, basing the release date on risk analysis. A shorter quarantine period may be justified, such as 21 days, if during this time:

    1. No horse had a fever (temperature taken at least one time every 24 hour period and without any treatment of non-steroidal anti-inflammatory drug)

    2. No horse had an abortion

    3. No new cases of neurologic disease (Note: Neurological clinical signs are considered to be resolved when they stabilize, i.e. residual neurological signs are not considered in determining a day 0 for countdown of release of restrictions/isolation.)

    4. All exposed horses have been tested and have given a negative result based on testing nasal swabs for EHV-1 by real-time PCR. EHV-1 testing of horses considered exposed or infected would allow for increased confidence in the release of restrictions/isolation prior the 28 day time period. There should be compliance with requirements by state animal health officials for duration of quarantine and testing.

    Biosecurity Issues for Receiving Animals

    Horses having been housed within primary perimeter:

    Isolate from general population for 28 days

    Horses having been housed within secondary biosecurity perimeter:

    After having determined its level of risk-aversion, the recipient facility may consider the following:

    1.  Vaccination Requirments for entrance into facility

    2.  Health certificate specifications

    3.  Test findings (a negative PCR result on a nasal swab)

    Coypright AAEP - Revised 2012