Equine Herpes Myeloencephalopathy (EHM) is caused by neurologic strains of Equine alphaherpesvirus 1, also known as Equine Herpes Virus-1 (EHV-1). Two main strains of EHV-1 have been isolated, explaining some puzzling clinical features of EHV-1 infection. The D752 strains are correlated to outbreaks of neurological disease, whereas outbreaks associated with abortion and other symptoms are more closely related to strains of N752. However, not all horses infected with D752 strains develop neurological disease; conversely, some horses infected with N752 strains will develop neurological disease. Both strains are associated with respiratory disease.

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 devastated those owners who have lost horses, and the economic impact of the outbreaks on those around them has been immense. This article includes information on transmission, clinical signs, diagnosis, treatment, quarantine, and prevention of EHV-1 infection.

This is a companion article to Rhinopneumonitis: Equine Herpes Virus (EHV-1/4) which focuses on the respiratory forms of the Equine Herpes Disease complex.

The neurological form of EHV-1 infection (EHM) is a complication that occurs in about 10% of the horses that develop 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 pretty extensive and seriously damages the vessels resulting in ischemia (poor blood perfusion 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 increased incidence of EHM:

• D752 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 as compared with the N752 strain.
• Levels of virus in the blood: In all cases of EHV-1 infection the virus invades the blood stream using white blood cells as its transport method. Horses with higher levels of viremia seem more likely to develop EHM. These may be the horses with higher fevers.
• Immune status of the individual infected: Older horses (> 20 yrs) are more likely to develop EHM, suggesting that parts of the immune system affected 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 that in cases where none of the three factors exist, EHM is quite rare, but EHM is common where all three exist together.