Equine Herpes Virus (EHV)

01st March 2021

Equine Herpes Virus (EHV) are a group of relatively common viruses that affect horses.


The most common strains of EHV are EHV1 and EHV4.  Typically, they cause respiratory signs varying from sub-clinical (showing no signs of infection) through to fever, lethargy, loss of appetite, nasal discharge, coughing and occasionally pneumonia.    EHV-3 is a venereal disease (Equine Coital Exanthema) that causes pox-like lesions on the penis of stallions and the vulva of mares. EHV-5 is a virus that is associated with Equine Multinodular Pulmonary Fibrosis a rare but serious lung condition.


EHV1 and 4 can also cause abortion epidemics in pregnant mares or result in the birth of dead or very weak, not viable foals.  There is a vaccine available which is licensed for use in pregnant mares which helps prevent this devastating result of an EHV infection and we recommend that pregnant mares are vaccinated in month 5, 7 and 9 of their pregnancy.  A genetic variant of EHV1 can also sporadically cause a paralytic neurological disease called Equine Herpes virus Myeloencephalopathy (EHM).   The clinical signs of EHM include incoordination of the hind and sometimes the front limbs, fever, bladder paralysis, lying down and inability to stand.


Horses usually become infected with EHV 1 and 4 as a foal and the virus then becomes latent living in the horse’s upper respiratory tract. The EHV becomes activated in times of stress, particularly in weanlings, yearlings and when the horse first enters training or is brought into a new yard. It is endemic in most horse populations however, most mature horses seem to develop a degree of immunity to the respiratory form of the disease but can still act as a carrier to infect other younger horses.  Unfortunately, horses don’t seem to develop any significant immunity against the abortion causing or neurological forms which can affect any age of horse including older horses.


The disease is transmitted mostly by the respiratory route through coughing and sneezing of nasal secretions or by handlers spreading mucus on clothes and equipment.  Aborted fetuses, fetal membranes and fluids are also highly infectious.  The virus can live for 7 days in the environment but is easily killed with disinfectants. All horses can be carriers and shed the virus when stressed, without showing clinical signs and it is considered that a large percentage of the equine population is doing this.  It is also possible that up to 5-10% of horses carry the genetic mutation that causes EHM without ever showing any signs of illness.


It is unclear whether the neurological variant of EHV1 is becoming more common or whether we are just diagnosing it better and communicating more effectively to get a worldwide perspective on the condition.  Vaccinations are available to protect against EHV1 and EHV4 however, these are only licensed to provide protection against the respiratory and abortion causing forms of the disease.  There is currently no licensed vaccine that will provide protection against the neurological version.  While neurological herpes is actually quite rare, outbreaks can occur if care is not taken.


Good biosecurity can stop the spread of contagious disease!


At home

  1. Isolate new horses coming into a yard for 3 weeks – monitor temperature daily.
  2. Call a vet if a horse in isolation becomes unwell


Pregnant mares

  1. Pregnant mares should not be mixed with other groups of other animals especially youngstock
  2. Avoid travelling or stressing mares in late pregnancy
  3. Do not add new pregnant mares to existing groups
  4. Ensure stables and equipment are cleaned and disinfected after each foaling


When out and about

  1. Do not share buckets and equipment at shows.
  2. Do not allow horses to graze at show venues.
  3. Do not stand nose to nose with other horses in collecting rings.
  4. Do when you arrive at the show venue – keep your horse on the box until warm up time and return to the box straight after competing and cool down.
  5. Do keep yourself to yourself.
  6. Clean & disinfect your horse box and equipment after each show
  7. Do monitor your horse’s temperature daily if you go to shows.
  8. Do be vigilant as to how your horse may be feeling so early signs of a problem can be detected.