Recent Outbreaks

The neurologic disease caused by equine herpesvirus, known as myeloencephalopathy, occurs relatively infrequently. However, several recent outbreaks of this form of the disease have caused serious concern in the horse industry and caused some to wonder whether neurologic equine herpesvirus is an emerging disease (18).

neurologic Symptoms: Neurologic signs include ataxia (failure of muscle coordination), urinary bladder atony (incontinence), and reduced tail tone. In severe cases, the affected horse may be unable to stand (18).

Outbreaks: Seven outbreaks occurred in five different states of the United States in 2005, and eleven outbreaks occurred in eight different states in 2006. In comparison to the typical none to few outbreaks identified annually prior to 2003, this is a sharp increase. Most of these outbreaks were in the eastern United States. There were also significant outbreaks in 2005 in Canada, Ireland, South Africa, Switzerland and the United Kingdom (18).

Why now? It is thought that the increase in prevalence of the neurologic form in recent years may be due to a strain of the virus with a mutation that allows it to reproduce rapidly with a predilection for nervous tissue. One factor involved in the emergence of this new strain may include the increased movement of animals, since movement could result in the virus being introduced to a new population, and the stress of transport plays a role in allowing the reactivation of the virus from the latent stage. Vaccination is not thought to play a role, since many of these outbreaks have occurred in well-vaccinated horses (18).

Risk: One study in the Netherlands found that female horses, aged horses, and specific breeds (Standardbreds, draft horses, and hispanic breeds) were at greater risk to develop the neurologic form of the disease. Most outbreaks occur during the late fall or early spring (19).