On January 7, 2023, The Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) notified our clinic of a confirmed case of equine herpes myeloencephalopathy (EHM), caused by equine herpesvirus-1 (EHV-1) infection, at an equine facility in Waterloo County. It is our understanding that some horses at this facility have unfortunately died from the disease.
Of the nine different herpesviruses that can infect horses, EHV-1 and 4 are known for causing the most morbidity (sickness). Both EHV 1 and 4 can cause respiratory disease. EHV-1 is also associated with abortion in mares, and in rare circumstances, such as the current outbreak, it can cause severe neurologic disease. As with human herpesviruses – such as those that cause cold sores – equine herpesviruses can live dormant, or latently, within the horse for all of its life. During periods of stress or immunosuppression, a chronically infected horse can start shedding virus again.
A fever (greater than 101.5 °F or 38.6 °C) may be the first indication of infection. The temperature of potentially exposed animals should be monitored and recorded twice daily for 21 days and any abnormalities discussed with a veterinarian. Neurological signs, if they develop, may include loss of balance, hind-limb weakness, difficulty urinating, decreased tail tone, depression and being down and unable to rise. It is important that a veterinarian assess horses demonstrating neurological signs since it can be difficult to distinguish this from other serious diseases such as rabies.
EHV-1 infection is easily spread to other horses by nose-to-nose or close contact with an infected horse, by sharing contaminated equipment including bits, buckets and towels or by the clothing and hands of people who have recently had contact with an infected horse.
Prevention is the best medicine. Biosecurity is key to preventing the spread of EHV-1. Implementing routine biosecurity measures is the best way to minimize viral spread and should be in place at all times to prevent a disease outbreak. Such measures include hand hygiene and basic cleaning and disinfection practices. Fresh clothing and boots should be worn by all boarders when going to the stable. A footbath at the entrance to the stable is also a great way to ensure no pathogens get dragged in. Horses that have been shipped long distances should be segregated for 14 days prior to entering the general population. While this current outbreak is active, it would be wise for other stables in the area to restrict their boarders from visiting other stables, as much as possible. At a minimum, fresh, clean, clothing and boots should be worn between stables.
EHV-1 vaccines marketed for prevention of respiratory disease may reduce viral shedding but are not protective against developing the neurological form of the disease in the vaccinated animal. This includes the vaccines used by the Milverton-Wellesley Veterinary Services, including Vetera Gold, Vetera 2XP, and Calvenza.
If EHV-1 is diagnosed at a stable, it is important to restrict movement for 21 days of both people and horses. Any person moving between equine facilities should change their clothes, shoes/boots and wash their hands. Facility owners should also inform all service providers that have attended the facility within the previous week, including, but not restricted to, veterinarians, farriers, feed suppliers and transporters, of the presence of the virus at the facility so they can take appropriate precautions.