There are various types of herpesviruses, and some can pose serious risks to the animals we care for. Currently, officials are monitoring an increasing outbreak of neurological illness affecting working horses nationwide.
On Monday, the Equine Disease Communication Center (EDCC) provided an update regarding the outbreak of equine herpes myeloencephalopathy, commonly referred to as EHM. So far, at least 29 cases have been confirmed across seven states, with the outbreak believed to have started at a rodeo and racing event in Texas earlier this month.
A worrying virus
EHM is triggered by an infection from the equine herpesvirus-1 (EHV-1).
EHV-1 is widely present and highly contagious. Most horses contract it at some point, generally through direct contact, airborne transmission over short distances, or indirect contact with contaminated surfaces, hands, or clothing. Typically, EHV-1 causes acute respiratory illnesses, after which it becomes dormant in the body—something that is characteristic of many herpes viruses. Yet, there are instances where the virus can travel to the brain and spinal cord, inflaming blood vessels and leading to dangerous clotting, which results in EHM.
Though EHM is mostly associated with particular strains of EHV-1 that lead to neurological infections, it can also be triggered by a respiratory strain in rare cases. Additionally, the virus has the ability to reactivate from dormancy and spread to the nervous system, resulting in EHM.
The EDCC noted that the outbreak likely began during the Women’s Professional Rodeo Association (WPRA) World Finals and Elite Barrel Race held in Waco from November 5 to 9. Several horses that participated in this event subsequently developed EHM, leading to warnings from animal health and racing officials across multiple states. Last week, officials decided to cancel the remaining events of the Barrel Futurities of America (BFA) World Championship in Oklahoma after two horses from the state, having recently traveled to Waco, were diagnosed with EHM.
At this stage, the full impact of the outbreak is uncertain, but it could be extensive. One of the most significant known outbreaks of EHM/EHV-1 occurred in 2011 during a cutting horse event in Ogden, Utah, which exposed around 2,000 horses across 19 states, infected at least 90 animals, and resulted in 13 deaths.
“The full extent of this EHV-1 outbreak is not yet clear. EHV-1 has presented acutely in affected horses, with quick progression and significant clinical severity. A concerning number of deaths in horses suspected of having EHM have been reported,” stated the Texas Animal Health Commission in an update last week.
A recurring concern
Even though this outbreak is alarming, most EHM cases are not fatal, especially if detected early and managed with supportive care. There is a vaccine available for EHV-1, designed to prevent the respiratory strain and reduce the risk of miscarriage in pregnant horses. While the vaccine doesn’t directly prevent EHM, it might help limit outbreak spread by lowering the potential viral load in infected horses.
Currently, officials are working with sponsors of the WPRA event, horse owners who participated, and veterinarians to monitor and control the outbreak. Vets who suspect a potential case are encouraged to report it to the EDCC.
