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A Brief Review of EHV-1 Neurological Disease: EHV-1 Myeloencephalopathy

About EHV-1

Equine herpesviruses (EHV) are found in the majority of horses. Most horses have been exposed to the virus and have had no clinical signs or side effects. There are nine EHVs that we have been able to identify worldwide, but EHV 1, 3, and 4 are the strains that have caused clinical disease in horses.

EHV-1 specifically has multiple manifestations of disease in horses: neurological form, respiratory form, and can cause abortion and neonatal death. There is also some evidence that it may cause chorioretinopathy (a disease process in the eye). Research has shown that greater than 80% of horses may be latently infected with the EHV-1 virus, but not all horses will develop clinical signs.

The Science Behind EHM

EHV-1 myeloencephalopathy (EHM) is still being studied today and we are constantly acquiring new information. There are multiple reports of theories describing how the virus causes neurological disease. A single point mutation in the DNA polymerase gene has been associated with neurological disease, causing the presence of aspartic acid (D) or asparagine (N) at position 752. AAEP reports that 80-90% of neurological disease cases are caused by D752 isolates, and 10-20% by N752 isolates.

Recently, there has also been a proven association with a single nucleotide polymorphism at position 2254 in the DNA polymerase gene and the occurrence of EHM.

The virus can become reactivated and furthers cell-associated viremia delivering the virus to endothelial cells in the uterus and central nervous system. Clinical signs occur due to the vasculitis and cellular damage caused by the virus.

Infection of Horses

Horses are affected through contact with respiratory secretion, aborted fetuses/placentas, or by fomites (infected objects, such as grooming tack). Following infection, a viremia is established as the virus circulates the body in infected cells. Following this, virus-infected cells are latently infected. Horses can shed virus in nasal secretions as early as day 1 of infection, and can continue to shed the virus for approximately 28 days. The virus can cause disease in infected horses or can re-activate from latency causing clinical disease in the horse. The virus can typically survive in the environment for up to 7 days, but this can vary based on the environmental conditions.

Signs of EHM

The incubation period of EHV is typically 4-6 days, but has the potential to be longer. Some clinical signs associated with the neurological disease may include:

  • Fever

  • Nasal discharge

  • Hindlimb weakness

  • Incoordination

  • Loss of tail tone

  • Lethargy

  • Urine dribbling

  • Head tilt

  • Inability to rise

Diagnosing EHM

Clinical signs (neurological signs and fever) and isolation of the virus are how we confirm EHM. Isolation of the virus can be done using quantitative polymerase chain reaction (qPCR) from nasal swabs and blood collection.

Treating and Preventing EHM

After confirming EHM, implementing a strict quarantine and biosecurity measures under supervision of local and state veterinarians is key. Treatment options include anti-inflammatories and supportive care.

Vaccination and biosecurity protection are the two ways to help avoid disease outbreak. No current EHV-1 vaccine has a claim to prevent EHM. Some EHV-1 vaccines can reduce nasal shedding and possibly viremia. By vaccinating your horses, you induce a strong immune response to the virus without inducing clinical disease.

It is important to clean tack, equipment and the environment to inactivate the virus. Cleaning first with a detergent or soap to allow for removal of organic material (such as soil) is recommended in order to prevent disinfectants becoming inactive when they make contact with organic material. There are multiple effective disinfectants that will kill the virus, including 1:10 dilution of bleach to water.

Other key points for understanding EHM prevention include:

  • Immunity following infection or vaccination offers limited protection

  • Boosting (routine vaccination) contributes to herd immunity to help protect individual horses

  • It is difficult for the body to establish immunity due to “evasion” properties of this virus

Early recognition and diagnosis is key

  • Quarantine and close monitoring of suspected cases is important

  • Exposed horses should have their temperature taken twice daily

Please note that this virus is still being investigated and new research is constantly being release as we try to better understand EHM.

Additional resources:


  • Allen GP. Risk factors for development of neurologic disease after experimental exposure to equine herpesvirus-1 in horses. American Journal of Veterinary Research 69,         1595–600, 2008

  • Diallo IS, Hewitson G, Wright L, Rodwell BJ, Corney BG. Detection of equine herpesvirus type 1 using a real-time polymerase chain reaction. Journal of Virological Methods          131, 92–8, 2006

  • Diallo IS, Hewitson G, Wright LL, Kelly MA, Rodwell BJ, Corney BG.            Multiplex real-time PCR for the detection and differentiation of equid herpes- virus 1 (EHV-1)          and equid herpesvirus 4 (EHV-4). Veterinary Microbiology 123, 93–103, 2007

  • Dunowska M. A review of equid herpesvirus 1 for the veterinary practitioner. Part A: Clinical presentation, diagnosis and treatment. New Zealand Veterinary Journal, 62,         171–78, 2014a

  • Dunowska M. A review of equid herpesvirus 1 for the veterinary practitioner. Part B: Pathogenesis and epidemiology. New Zealand Veterinary Journal, 62, 179– 88, 2014b

  • Equine Herpesvirus (Rhinopneumonitis). (n.d.) Retrieved March 7 2016, from

  • Goodman LB, Loregian A, Perkins GA, Nugent J, Buckles EL, Mercorelli B, Kydd JH, Palu G, Smith KC, Osterrieder N, Davis-Poynter N. A point mutation in a herpesvirus      polymerase determines neuropathogenicity. PLoS Pathogens 3, e160, 2007

  • Pusterla, N., & Hussey, G. (2014). Equine Herpesvirus 1 Myeloencephalopathy. Veterinary Clinics of North America: Equine Practice, 30(3), 489-506.


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