Equine Herpes Virus 1 has been the main topic of discussion among horse owners over the last couple of weeks. The original outbreak was traced back to a National Cutting Horse Association Event in Ogden, Utah that was held a few weeks ago. Two horses that attended the event were diagnosed in Colorado as having the neurological form of EHV-1, otherwise known as EHM, and were subsequently euthanized. Since that event, there have been numerous horses diagnosed with EHV-1, but no other horses connected to the event have shown neurological signs (EHM.)
First, what is EHV-1? EHV-1 stands for Equine Herpes Virus 1. In addition to Equine Herpes Virus 1, EHV-3 and EHV-4 are also common in the horse population. The other acronym commonly associated with the current outbreak is EHM, or Equine Herpes Myeloencephalopathy - the neurologic form of EHV-1.
The Equine Herpes virus is endemic in the equine population, meaning that all horses are infected at some point from birth to one year of age through contact with other horses carrying the disease. Once infected, horses are lifelong carriers and usually show signs of the disease during times of stress; such as when traveling to a horse show. The virus typically manifests itself as an upper respiratory infection, but horses can shed the virus without showing clinical signs. Other clinical signs of EHV-1 are late term abortions in mares, early neonatal foal death, and less commonly, severe neurological dysfunction. Horses that are vaccinated with the rhinopneumonitis vaccine, or “Rhino”, are protected from all forms except EHM.
The form that causes equine herpes myeloencephalopathy (EHM) is a mutation of the EHV-1 normally found in the horse population, meaning current vaccines do not provide protection against EHM. The mutated version of the virus attacks the blood supply of the Central Nervous System which results in severe damage to the brain and spinal cord. Because the virus must mutate to cause neurologic signs EHM is rare in the horse population but, in theory, can be shed by any horse at any time. Research is ongoing to determine the factors that cause EHV-1 to mutate from a usually mild disease to an often fatal one.
Diagnosis of the disease is based on a combination of laboratory findings and clinical signs. If a horse has been exposed to another horse diagnosed with EHM, the current recommendation is to monitor the horse’s temperature twice a day. The normal temperature for a horse is 99-101 degrees Fahrenheit, but can vary depending on activity level and other factors, so it is important to determine normal body temperatures for individual horses. It is not recommended to swab clinically normal horses in an attempt to see if they have EHV-1 since healthy horses can shed the disease. If a suspect horse develops a fever, however, then a veterinarian should be called out to collect blood samples and nasal swabs, which should then be sent to UC Davis for further work-up.
There have been a lot of questions in recent days regarding what horse owners should do from this point on when considering summer equine activities. The advice for the past few weeks has been to “lay low” and let the dust settle. This has allowed time for any new cases of EHM to surface; no new cases linked to the original outbreak have been detected. While it was important in the short term to limit attending horse events, I would not find it necessary to cancel all plans this summer.
Knowing that the equine herpes virus emerges during stressful situations, it is important to minimize a horse’s stress level when traveling to events. In addition, if you are at an event and your animal goes off feed or spikes a fever, be responsible - separate your animal from all others and minimize contact between your suspect horse and other horses. With a little knowledge about the disease, we can all take steps to make sure that we minimize risk for all horses.
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