As many are aware, the neurological form of Equine herpesvirus-1, referred to as Equine Herpes Myeloencephalopathy (EHM) has recently been confirmed in multiple horses across the state and region stemming from an indexed event in Waco, Texas, in early November.  EHM is a highly contagious variant of EHV-1 and is among a larger group of 9 strains of Equine Herpes Virus. EHV-1 is also referred to by the name Rhinopneumonitis.  

Some distinct features of Equine Herpes Virus are its ability to lie dormant in its host and recrudesce in the horse later, under opportune conditions, like a cold sore in people. EHM is an upregulated strain of the virus that can reproduce rapidly and overwhelm the immune system with viral load. Viral load in the bloodstream is called a viremia and can often be accompanied by a fever. In EHM, the viremia is significant enough that it causes blood vessels in the brain to swell, leading to neurological clinical signs. The incubation period is between 2 and 14 days, and horses can shed the virus for up to 28 days. Finding the actual virus’s DNA in the nasopharyngeal region or blood of the horse is key to its diagnosis, and the use of qPCR or rtPCR in the lab can help differentiate what EHV is present.

Substantial outbreaks in horse populations seem to spike in the fall months and often correlate with times when horses begin to comingle for futurity and winter events. Significant aggressive outbreaks have occurred in the past, including an NCHA event in 2011 in Ogden, UT, where 428 horses were exposed, 13 known horse deaths occurred, and a dispersal of the virus to well over 1,500 horses across 10 states. The outbreak started in April and wasn’t declared contained until the summer, with more than half of the cases occurring from secondary exposures.  

Aggressive outbreaks often require strict biosecurity protocols, limitations on horse movement, surveillance, and immunizations with booster vaccines.  While there are vaccines for Equine Herpes Virus, there are none specific to prevent EHM.  Some may increase serum neutralizing antibodies to a higher level or provide specific antibody isotypes that may be helpful in reducing viral shedding and disease; as always, consult with your veterinarian.

Additional Resources:

• Texas Animal Health Commission Statement
• Fact Sheet on Equine Herpes Myeloencephalopathy
• Horse Owner's Guide to Biosecurity
• Women’s Professional Rodeo Press Release on EHV-1/EHM