Equine Herpesvirus-1 Myeloencephalopathy: On the Rise?

In recent years it seems owners and practitioners are increasingly confronted--both directly and indirectly--with cases of equid herpesvirus-1 myeloencephalopathy (EHM).

Lutz Goehring, DVM, MS, PhD, Dipl. ACVIM, associate professor of equine medicine at Colorado State University's College of Veterinary Medicine and Biomedical Sciences, discussed characteristics of this neurologic form of herpesvirus-1 and why it might be on the rise, at the International Conference on Equine Infectious Diseases Practitioner's Day, held Oct. 21 in Lexington, Ky.

Goehring explained that this infectious disease typically spreads when one horse inhales viral particles from an infected horse. Viruses attach to the respiratory tract epithelium (lining), are transported to the lymph nodes, and then travel through the bloodstream (also known as viremia) to their specific destinations: vascular endothelial cells lining the blood vessels of the pregnant uterus, central nervous system, or eye.

Viremia typically causes a high fever and occurs prior to clinical signs of EHM. Veterinarians commonly see neurologic gait abnormalities like ataxia (incoordination), paresis (impaired movement), and dysmetria (a high-stepping gait yet limited joint movement) and bladder dysfunction, said Goehring.

A key step to preventing development of neurologic disease is to prevent endothelial cell infection. Thus, Goehring said EHM therapeutics aim to lower viremia and general viral replication using antivirals (e.g., valaciclovir) and, potentially, block endothelial cell infection using simple drugs such as non-steroidal anti-inflammatories. Goehring recommended administering these two therapies in combination on the first day a horse exhibits fever during an EHM outbreak.

As for disease prevention, "vaccination is important," said Goehring, "but it won't work alone without good husbandry and management such as separating horses and practicing good hygiene and disinfection."

In describing EHM's epidemiology, Goehring noted that its likelihood and severity differ between horse breeds, ages, and genders. He said potentially mules and definitely ponies appear less susceptible, while tall breeds such as Thoroughbreds, Quarter Horses, and Warmbloods are most susceptible. Adult horses (3 years and older) are most susceptible, while foals, weanlings, and yearlings are least susceptible. Females and aged horses that contract EHM are generally affected more severely.

One challenging aspect of this disease is that a variable fraction of horses is latently infected (not displaying clinical signs themselves but carrying "hibernating" virus). But Goehring said "the perfect storm" is still needed for an outbreak to occur: one infected and virus shedding horse, many horses housed close together, correct breeds and age groups, and potentially stressors that compromise immunity.

Goehring said the disease is also most prevalent from October to May. "This may be due to more indoor activity, competitions, and closed up barns (during this time of year)," he explained.

But are we really seeing more EHM outbreaks? Goehring seemed to think it's possible. "Commingling of horses is the biggest risk (for disease transmission)," he said. "And there are now more competitions; bigger and longer show circuits; more large-scale boarding facilities; and a growing horse population, which helps with rapid spread of virus so that more horses are exposed during a shorter period of time."

Added to the equation are increased ways to spread news (e.g., social media) and greater media attention of EHM outbreaks. This means more people are talking about EHM outbreaks than ever before.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.