Recently the horse community has experienced an outbreak of Equine Herpes Myeloencephalopathy (EHM). Multiple outbreaks of confirmed cases of the neurological form Equine Herpes Virus – 1 (EHV-1) has recently resulted in the cancellation of all FEI sanctioned events for the remainder of the Spring in Europe, and has promoted stricter biosecurity measures in places like Ontario, Canada and Ocala, Florida.

While each outbreak does not appear to genetically relate to one another, they have all worked to foster collaborative networks among veterinarians and horse owners alike to limit the spread of EHM.

Equine Herpes Virus is prevalent in the horse population, and while there are five different strains of herpes viruses in horses, EHV-1 can cause frequent outbreaks of respiratory disease, abortion, neonatal foal death, and neurological deficits.  EHM is a specific mutation that can cause an intermittent fever (on day one to two and then again on day six to seven of infection) along with a sudden onset of neurological deficits that can emerge as quickly as 24 hours after the second fever phase.

Neurological signs can include: incoordination, hind limb weakness, loss of tail and anal tone, lethargy, urine dripping, a head tilt and/or leaning to combat imbalance.

EHV-1 is typically a disease that is contracted by foals and young horses, but mature horses are still susceptible to EHV-1 infection. The disease can lay dormant and become reactivated with physical and environmental stressors including, but not limited to, changing social structures within the herd, relocation, prolonged transport, poor nutrition, parasitism, environmental exposure and the weaning of foals.

This is similar to the chickenpox virus in humans causing shingles in adults.  EHV-1 subclinical infections are common and suggest frequent spread and high-risk exposure for the horse show community.

Current research is investigating the pathway and the rate at which an EHV-1 respiratory infection can mutate into the neurological form.  A primary respiratory infection, with or without clinical signs, can develop into a viremia, meaning a systemic viral infection that can then spread specifically to the uterus in pregnant mares or to the central nervous system of all horses.

While some evidence suggests that infection with a large number of viruses and for a long time is associated with the development of EHM from EHV-1, the exact mechanism and risk factors for EHM are largely unknown.  The development of EHM from EHV-1 is quite rare with approximately 10% of EHV-1 cases that present with clinical neurological signs.  Despite this seemingly low rate of positive cases, any positive EHM case should be taken seriously and should stimulate a re-evaluation of biosecurity measures and vaccination protocols.

Vaccination remains the best way to prevent EHV-1 infection in horses.  While some emerging data suggests that the modified live form of the herpes vaccination can prevent neurological signs from developing, there is no actual labelled vaccine against EHM infection.  The use of herpes virus vaccination does, however, help to reduce severity of respiratory disease and prevent abortion in pregnant mares.

The vaccine in general limits viral shedding, thus decreases the chances of viral spreading from horse-to-horse.   Currently, the American Association of Equine Practitioners (AAEP) suggests vaccinating horses with a herpes vaccine every six months, while some vaccine manufacturers suggest that maximum protection is achieved with an every 90-day vaccination protocol – depending on the horse’s risk of exposure.  Over vaccination, such as vaccinating with a herpes virus vaccine in intervals of every 60 days or less has resulted in adverse immune reactions.

Early detection is key to managing EHM infections.  Make sure to have a functioning thermometer on hand and check your horse’s temperature at the first sign of lethargy, or if they appear to be not-quite-right.  If their temperature is 101.5 degrees Fahrenheit or greater, isolate your horse from other horses in the barn by placing them in a stall or paddock at least 20 feet away from others.  Additionally, make sure to quarantine any new horses coming on the farm for at least 3 weeks in a separate paddock or isolated stall.  Continue to monitor new horses with daily temperature checks and monitor for any clinical signs such as nasal discharge or neurological deficits.

Upon the discovery of a fever, make sure to contact your veterinarian.  Your veterinarian can give you advice on how to manage the fever and they can conduct diagnostics to better determine the presence or absences of EHV, or another infectious agent, with nasal swabs and/or blood samples for biological testing. It is important to contact your veterinarian right away and to allow them to collect samples at this time, rather than waiting until the fever has broken or until your horse experiences the second-wave of fever.  The most viral particles are shed during the first fever and samples taken during that time yield the most accurate results.

Testing is available to determine whether a EHV-1 positive horse has the EHM mutation – this early detection can help your veterinarian create a therapeutic plan composed of antiviral medications and anti-inflammatories to limit viral replication to either reduce or halt clinical neurological signs before they develop.

Herpes virus can be spread by direct horse-to-horse contact, or can travel through the air via aerosol transmission.  It is difficult to say exactly how far viral particles can spread with airborne transmission, but the most common route of infection is from inanimate surfaces such as buckets, brushes, pitch forks, tack and even the inside of trailers during transport.  These inanimate objects are also known as fomites.

Herpes virus can live in the environment and on fomites for up to 7 days.  The best way to clean your environment to prevent further spread of the virus is first to clean surfaces with soap and water before using a disinfectant – the removal of organic debris allows disinfectants to work more effectively.

In the era of COVID-19, we have all become oh-so familiar with biosecurity restrictions.  It should come as no surprise that the pillars of an Equine Herpes Virus outbreak response are to: commit to a scheduled vaccination protocol, to be observant and responsive to EHM clinical signs, and to prevent the spread of disease with quarantine, cleaning and disinfectants.

Dr. Melissa Aceto hails from Orillia, Ontario, Canada, where she raised Quarter Horse and Appaloosa performance horses. She completed her Doctorate of Veterinary Medicine at the Ontario Veterinary College (OVC) at the University of Guelph in Ontario, Canada. She enjoys riding and showing in reining, western pleasure and western riding. Dr. Aceto is currently completing her year-long equine internship at Saginaw Valley Equine Clinic. You can email her at melissa@saginawvalleyequine.com or visit her at www.saginawvalleyequine.com.