Equine Herpes Virus (EHV)

The two most important equine herpes viruses are EHV-1 and EHV-4. These viruses usually cause respiratory disease, but can cause neurological disease and abortion. EHV-1 is much more commonly associated with neurological disease and abortion than EHV-4. Most horses in the UK have probably been affected by this virus at some point in their lives and can harbour ‘latent’ virus, which can then start shedding at times of stress, making it impossible to eradicate this disease.


  • Respiratory
  • Can be very mild
  • Cough
  • Nasal discharge
  • Off colour
  • High temperature (easily missed as can go up and down)
  • Inappetence
  • Lethargy


  • Usually in mares after 5 months.
  • High temperature (easily missed as can go up and down)
  • ‘Red bag’ delivery
  • No warning signs
  • Can also cause death of new-born foals
  • Inappetence
  • Lethargy


  • Variable severity, from mild hind limb weakness to completely recumbent and unable to stand.
  • High temperature (easily missed as can go up and down)
  • Inappetence
  • Lethargy


If a case is suspected, it should be isolated from the others immediately and the vet called. Stringent biosecurity measures should be put in place until the lab results have come back or until the vet says it is ok to relax them. Horses that have been in contact with the affected horse should be isolated from all others in a further group and monitored very closely with twice daily temperature monitoring. All movement on and off the yard needs to be stopped. Virucidal disinfectants, PPE and biosecurity barriers are required. Further information on this would be provided by the vet at the time and/or can be found on the Equibiosafe app (free to download) and on the HBLB codes of practice.


Vaccination of all horses on a yard reduces viral shedding significantly; therefore reducing the circulating infectious virus significantly. Vaccination of your horse will not prevent EHV but it will reduce the risk and reduce severity.

Pregnant mares should be vaccinated at 5, 7 and 9 months of gestation and all others should have a primary course 4 weeks apart followed by a booster every 6 months.

If there is an outbreak we do NOT recommend the vaccination of in-contact horses, but the vaccination of horses in the wider area is advised. It is best if horses are up-to-date with vaccinations prior to an outbreak!

There are no requirements for EHV vaccination currently in the UK, but it is recommended that competition horses and racehorses are vaccinated due to increased stress, higher risk of coming into contact with unknown horses and the negative effect on performance that would be seen with respiratory disease.

All horses on a yard/premises with pregnant mares should be vaccinated against EHV.

Reference: Equine herpesviruses: A roundtable discussion. UK-Vet Equine Vol 3 (4) 2019.


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