Horse owners in the western United States are coming to grips with an equine herpesvirus myeloencephalopathy (EHM) outbreak, and the entire industry seems to be involved in the equine herpesvirus-1 (EHV-1) discussion.
I never thought it would come to the point where everyone looks at this virus as the public enemy and I have to become the public defender who provides the "psychological profile" explaining the virus' behavior so it will have a lesser sentence.
I have worked with equine herpesviruses for more than a decade now. My former PhD advisor once told me that in order to understand how a virus causes disease one has to see things through the eyes of the virus--what is in it for the virus if it causes a particular type of disease?
There’s a pattern to EHM outbreaks: They follow an event such as a horse show or competition.
We researchers recognize EHV-1 as a big, burly virus that does not like to travel far. It needs close contact to be transmitted from one horse to the other. Furthermore, EHV-1, like an undercover agent for an enemy organization, blends into the host’s society of cells and organs without proper authorities like the immune system recognizing it. EHV-1 replicates periodically in the respiratory tract, which causes typically mild disease; however, the virus' goal with this replication is to find another warm body after a "horizontal transmission" into another horse. EHV-1's sole purpose is to infect other hosts, colonizing respiratory tracts so the virus can replicate.
So, does EHM make any sense for EHV-1? Obviously not! It is a viral suicide mission: As soon as it hits the small blood vessels of the equine spinal cord (the microvasculature), a debilitating, strokelike disease results. This may result in euthanasia, but regardless it is a dead-end road that will not allow the virus to spread further.
As a graduate student working on EHV-1 and EHM in the Netherlands, my friends used to call me "the outbreak chaser." As soon as I got word of a neurologic outbreak I stepped into my car and drove all over the country. I went to megaboarding facilities with 200 horses, as well as 10-horse backyard operations in remodeled dairy barns. The most striking finding was that there was a pattern--EHM outbreaks followed an event: a horse show or competition. We also learned that the peak seasons for EHM are winter and spring, and EHM is more likely to occur in the tall breeds, Draft horses, Thoroughbreds, Standardbreds, Quarter Horses, and Warmbloods. EHM is less likely to occur in horses younger than 3 years old and in ponies, while many young horses and ponies clearly experience a primary respiratory EHV-1 infection and viremia (virus in the blood).
This is also something we notice during EHV-1 infection experiments, which we typically use to test vaccine efficacy. When ponies or yearlings are infected with an extremely high dose of an EHV-1 neuropathogenic (causing neurologic disease) strain we typically don't see the neurologic form of the disease. However, when you do this in horses over 20 years of age, we see about two-thirds of these horses develop neurologic abnormalities.
So in my opinion EHM is not a "virus-only" attack against the microvasculature of the spinal cord. It looks more like an extremely unfortunate chain of events, where host factors (as in genetics), viral factors, and potentially other yet-to-be-determined factors (environment, nutritional deficiencies, co-infections) all are necessary to orchestrate this nervous system attack.
Our EHV-1 group here at Colorado State is focusing on the pathogenesis of EHM. What is the chain of events that leads to EHM, and what can we do--and what makes sense--for therapeutic intervention?
More research is necessary for sure, but with all the currently combined international efforts to combat EHM, I feel we are closer than ever to finding out how this disease is caused in our horses. This outbreak is a wake-up call for show horse owners and trainers. As much as we love to see horses compete, a competition is a mixing and mingling place for horses coming from different backgrounds. Horses that return from shows should be received and housed in a separate barn or pasture (away from resident horses), and they should be monitored closely for signs of a contagious infectious disease. While this will not guarantee that disease spread is avoided completely, it will decrease the likelihood of disease propagation on a farm.
Lutz S. Goehring, MS, PhD, Dipl. ACVIM, is an assistant professor in equine medicine at Colorado State University's veterinary school.