Equine Herpes Virus
Equine herpesvirus-1 is one of a large group of viruses that causes potentially serious disease in horses.
EHV-1 infection may occur subclinically or be manifest in three clinical forms that can occur independently or concurrently. The most common form is the respiratory form but the virus can also cause abortion in mares (abortogenic form), or neurological problems in adult horses (neurologic form) secondary to damage to the spinal cord and/or brain. This latter form, now referred to as Equine Herpes Myeloencephalopathy (EHM) is of particular concern because (1) it results in a high death rate, (2) it is resistant to prevention by vaccination, and (3) it affects horses of all breeds and vaccination status. It has the potential to cause catastrophic losses to both the health of horses and the economy of the horse industry.
EHV-1 occurs throughout the world and indeed almost all horses older than 2 years of age have been exposed to it, similar to the herpes simplex type 1 virus in humans, which affects about 85% of the world population some time during childhood. Following initial exposure, EHV-1 has the ability to develop into an inapparent, latent infection—that is, it remains in a dormant state and does not produce any clinical signs. This ability to reside as a silent and persistent infection in horses provides a reservoir of virus that may play a role in transmission.
EHM is not a new disease, but the evidence currently supports the observation that it is seen more commonly now than it was in the past. Recent research has identified a specific strain of EHV-1 (the so-called neurotropic or neurogenic strain) that is more likely to cause neurologic disease because it multiplies to higher levels in the blood than “non-neurotropic” strains and is thus more likely to infect neural tissues and produce a fulminating disease (one that occurs suddenly and with great intensity) that can be fatal. It should be noted, however, that this “neurotropic” strain is not a new mutant. It has been in circulation for many decades but the molecular tools to differentiate it from other strains have only recently become available. It should also be noted that the “non-neurotropic strain of EHV-1 can also cause EHM. Approximately 15% of clinical cases are caused by non-neurotropic strains, whereas 85% are caused by neurotropic strains. Even in the face of outbreaks of EHM, the virus appears to be restricted to latency in some horses and does not cause clinical signs, much like the herpes simplex type 1 virus in humans.