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Animal and Plant Health Inspection Service
U.S. Department of Agriculture
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Equine Herpesvirus (EHV)

What is Equine Herpesvirus (EHV)?

Equine Herpesvirus (EHV), also known as Equine Rhinopneumonitis, is a highly infectious Alphaherpesviridae found virtually worldwide. There are currently 9 known EHVs. EHV-1, EHV-3 and EHV-4 pose the highest disease risk in the U.S. horse population. EHV-1 and EHV-4 can cause upper respiratory disease, neurological disease, abortions, and/or neonatal death. EHV-3 causes a venereal disease called coital exanthema. In recent years there has been an increase in the number of EHV-1 neurologic cases, also referred to as Equine Herpes Myeloencephalopathy (EHM). The virus incubation period is highly variable and can be as long as 14 days. Prognosis is good for horses not affected with the neurologic form and varies for those with neurologic signs. EHV has not been shown to be zoonotic.

Vaccines are available to prevent the respiratory and abortion forms of EHV, but none are labeled as effective against the neurologic form of the disease (EHM).  Vaccination may reduce the severity of clinical signs and may lead to reduced viral shedding.  In breeding operations the use of vaccination appears to be effective at reducing the risk of EHV-1 and EHV-4 abortions and its use is strongly recommended.  Pregnant mares with foals at their side should be kept in small groups and prevented from contacting new introductions.  Isolate new horses for a minimum of 21 days (30 is strongly recommended), this includes returning horses who have left the farm to attend any event.  Practice good hygiene and biosecurity.

Equine herpes virus (EHV-1 and EHV-4) is spread via nose to nose contact, contaminated equipment (water and feed buckets, tack and grooming supplies, and shoes) and respiratory secretions within stalls/stables. Aborted fetuses and after-birth can also contain the virus.  EHV-3 is spread through venereal transmission or contaminated equipment used for breeding.

  1. Call your veterinarian.

  2. Isolate affected horse(s) from healthy horses.

  3. Clean and disinfect all water/feed buckets, mucking equipment, tack and any other equipment that may have come into close contact with the affected horse.

  4. It is strongly recommended that each new horse introduction to the property adhere to a minimum 30 day quarantine/isolation period during which time the horse can be monitored for signs of infectious disease and be housed in a separate location on the property well away from resident horses.

Treatment consists of supportive care which may include IV fluid therapy and anti-inflammatories to reduce fever. In more severe neurologic cases treatment may require hospitalization with more intensive veterinary care. The majority of horses with uncomplicated infections recover.

Clinical signs may be non-specific and some horses may be non-clinical

  • Fever
  • Difficulty urinating
  • Nasal discharge and cough
  • Depression
  • Head tilt
  • Loss of tail tone
  • Stumbling or weakness in the hind limbs, may progress to all limbs
  • Down and unable to rise
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