Equine Herpes Myeloencephalopathy (EHM) is a complication of the respiratory form of Equine Herpes Virus-1 (EHV-1) disease. To completely understand EHM you must also understand the respiratory form of this disease so this is a companion article to Rhinopneumonitis: Equine Herpes Virus (EHV-1/4) which should also be studied as it contains additional information on aspects of this disease.

Behind Equine Protozoal Myeloencephalitis (EPM), the second most diagnosed cause of myeloencephalopathy and the resulting incoordination and weakness is equine herpesvirus-1 (EHV-1). EHV-1 is associated with upper respiratory disease, abortion, and the neurologic form of the disease in horses. Though the neurological form of EHV-1 has long been with us there is a perception that the incidence of the disease is growing and in recent years there have been highly publicized outbreaks at large horse show venues and equine hospitals. These outbreaks have been devastiating on those owners who have lost horses and the economic impact on those around the outbreaks has been huge. In spite of this many important questions remain difficult to answer. This article tackles these questions and includes information on transmission, clinical signs, diagnosis, treatment, quarantine, and prevention of EHV-1 infection.

The neurological form of EHV-1 infection (EHM) is a complication that occurs in about 10% of the horses infected with the respiratory form of the disease. In EHM the vascular endothelium of the central nervous system becomes infected by the virus. This resulting vasculitis can be quite extensive and seriously damages the vessels resulting in ischemia (poor blood profusion and lower oxygen available to the tissues) and neurological damage. Why the neurological form of the disease develops remains unclear but three factors are associated with an increase incidence of EHM:

• Strain of EHV-1: A single nucleotide polymorphism (SNP) in the DNA polymerase (DNApol) gene that has been labeled D752 is associated with an increase incidence of EHM.
• Levels of virus in the blood: In all cases of EHV-1 infection the virus invades the blood stream using white blood cells as there transport method. Horses with higher levels of viremia seem to be more likely to develop EHM.
• Immune status of the individual infected: Older horses (> 20 yrs) are more likely to develop EHM which suggests some that parts of the immune system effected by age may be involved in protection from the neurological form.

It should be noted that none of these factors is absolute and exceptions to the three predisposing factors exist. It would be logical, though not proven, that in cases where none of three factors exist EHM is quite rare but where all three exist together EHM common. Most cases are somewhere between.