Equine herpesvirus type-1 (EHV-1) myeloencephalopathy (EHM) generated more interest on TheHorse.com in 2003 than any other infectious disease, due in part to several fatal outbreaks in the news. Many questions about treatment were raised during these outbreaks, so researchers stepped forward and looked into the efficacy of one prominent treatment that was described. At the 2003 American Association of Equine Practitioners' convention, Pamela A. Wilkins, DVM, MS, PhD, Dipl. ACVIM, ACVECC, of the University of Pennsylvania's New Bolton Center, summarized acyclovir and its use in the treatment of EHM.

The EHV-1 organism can cause several forms of disease, including rhinopneumonitis (a respiratory disease found mostly in young horses), abortions in pregnant mares, and neurologic disease. "EHM is a virus-induced central nervous system (CNS) vascular necrosis (blood vessel tissue death) with associated thrombosis (blood clotting), ischemia (oxygen deprivation in the tissues), and malacia (softening)," wrote Wilkins and other collaborators on the research project. Probably the most prominent EHM outbreak last year was one at the University of Findlay in Findlay, Ohio, and included respiratory signs as well as EHM.

In the past, EHM therapy has been limited to supportive treatment (keeping the animal alive and comfortable while the disease takes its course). Anti-inflammatory drugs have been used, mainly corticosteroids such as dexamethasone and prednisolone. However, these drugs might be associated with some risk, because corticosteroid administration has been shown to reactivate latent (dormant) infection in horses (and latency often occurs after infection). Judicious use of anti-inflammatories is reasonable. Wilkins mentioned that Steve Reed, DVM, PhD, Dipl. ACVIM, of The Ohio State University, who helped care for the Findlay horses, has used dimethyl sulfoxide (DMSO) as a treatment option, and Wilkins noted that interferon-alpha 2a (used extensively as an antiviral agent) has also been tried in EHM treatment, but without much success.

Acyclovir is an anti-viral drug with a high activity and selectivity for herpesviruses. It has been used in equine outbreaks, even though past studies have not established a clear-cut benefit of using the drug. Acyclovir is of potential interest to equine practitioners since it recently came off patent and "is potentially an affordable directed therapy for EHM," wrote the researchers. It can be given orally or intravenously (IV), but is much more expensive to administer IV