What is EHM?

By Wendy Talbot on 14 February 2020

What is EHM?

Equine Herpes Myeloencephalopathy (EHM) is a rare but serious neurological disease caused by a type of Equine Herpes Virus (EHV), EHV-1. Infection with EHV-1 usually causes respiratory signs but in some animals the virus can travel to the brain and spinal cord causing paralysis, which usually carries a poor prognosis. It’s unknown why EHV produces the neurological form in some horses and thankfully it is still a rare occurrence.

How is EHV-1 transmitted?

EHV-1 is transmitted by direct horse-to-horse contact such as nose-to-nose touching. Coughing can spread the virus through the air over short distances. It can also be spread by sharing infected equipment, including wheelbarrows, brushes and buckets and via the hands and clothes of people who have been in contact with infected horses. EHV-1 can also cause abortions and aborted mares and infected aborted material, which is not correctly disposed of, can be a source of infection.

EHV infection is common

EHV infection is actually very common and in most cases it will not cause the severe disease associated with EHM. Just like human herpes viruses (e.g. the virus causing cold sores) once a horse has the virus it is likely to remain a carrier for the rest of its life. It can shed the virus even when showing no clinical signs, especially when under periods of stress such as during transport. It isn’t possible to predict when or why the disease may develop into EHM.

Signs of EHM

Although EHM is very rare, it is life-threatening so it’s good practice to be aware of the clinical signs:
  • Fever
  • Nasal discharge
  • Depression
  • Incoordination
  • Hind limb weakness
  • Loss of tail tone
  • Urine dribbling or inability to urinate
  • Dog sitting position
  • Leaning on a support to maintain balance
  • Inability to rise

How is EHM diagnosed?

Your vet will perform a clinical examination and if EHV is suspected collect a blood sample and a nasal swab. A horse with typical neurological signs that tests positive for EHV-1 will be considered positive for EHM. In rare cases horses with EHM may recover but may retain neurological problems.

How can I prevent EHV?


Vaccination against the respiratory and abortion forms of EHV is possible but vaccines are not licensed to protect against the neurological form. Vaccination of horses directly exposed to infected horses during an outbreak of EHM is not advisable as it may make the situation worse and there is no clear benefit to doing it at this late stage. Horses not already exposed to the outbreak may benefit from vaccination to reduce the possibility of viral spread within the population.


Control of EHV requires stringent biosecurity measures, especially when introducing new horses to the premises, when travelling and when at shows or events. This may sound complicated but simple tips like washing hands or using an alcohol gel between horses can make a big difference. Find out more about biosecurity here https://www.horsedialog.co.uk/Health/Equine-biosecurity-what-you-need-to-know.aspx

What should I do if I think my horse has EHM?

If you think your horse may have any signs of EHM or EHV respiratory disease you should isolate them immediately and contact your vet to discuss the next course of action.


Equine Disease Communication Center. Equine Herpesvirus Myeloencephalopathy (EHM) Equinediseasecc.org

Slater J. What is Equine Herpes Virus? Accessed January 2020.https://www.horsedialog.co.uk/Health/WhatisEHV.aspx

Ivens P, Rendle D, Kydd J, Crabtree J, Moore S, Neal H, Knapp S, Bryant N, Newton JR. Equine Herpesviruses: A Roundtable Discussion. UK Vet Equine, July/Aug. 2019

Lunn, D. P., et al., J Vet Intern Med, 2009.

Epidemic disease caused by equine herpesvirus-1: recommendations for prevention and control. Allen, G. P. Equine Veterinary Education; 14(3):136-142. 2002.

Allen GP. Respiratory Infections by Equine Herpes Virus Types 1 and 4. International Veterinary Information Service. 2002.




Wendy graduated from Bristol University in 1999. She then went on to complete a residency at Liverpool University and holds a European Diploma in Equine Internal Medicine. After working in practice for 13 years, she joined Zoetis in 2012 as the National Equine Veterinary Manager.

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