Equine herpes virus (EHV-1) is known to be prevalent in 60-80% of horses by age two. It remains latent in the reticuloendothelial system and the trigeminal nerve. Equine herpes myeloencephalopathy (EHM) develops through a leukocyte-associated viremia in the bloodstream that causes vasculitis within nervous tissue due to damage associated with thrombin and platelet activation.
The efficacy of a combination of valacyclovir and heparin to treat EHM was presented by Megan Marchitello, DVM, at the 2019 AAEP Convention. Past studies demonstrated that each of these drugs showed benefit to managing EHV-1:
- Heparin and phosphodiesterase inhibitors block EHV-1 thrombus production to decrease incidence of the virus;
- Treatment with valacyclovir decreased nasal shedding and viremia regardless of the timing of treatment. Treatment of an EHM horse for two weeks with this medication decreased the degree of ataxia.
The retrospective study was examined from results on three farms that combined both medications. The objective was to identify if there was an effect of the combination drugs on incidence of EHM or associated mortality.
During a six-month outbreak of EHM in 2017-2018, nasal secretions and whole blood were obtained from horses to conduct PCR testing for EHV-1. Horses that shared air space or husbandry with positive horses were considered as exposed. Those that developed a fever greater than 101.5 degrees Fahrenheit and/or developed limb edema were isolated in quarantine for 21 days.
Of 111 horses exposed, 23 developed clinical signs consistent with EHV-1. Of those 23 horses, 10 developed EHM.
Of the 23 EHV-1-positive horses, 19 received the combined treatment and survived. Six EHM horses were given the combined treatment and survived with eventual return to their level of work. Four EHM horses that did not receive heparin or valacyclovir developed advanced neurologic signs of EHM and were euthanized.
The retrospective study demonstrated that targeted treatment during an EHV-1 outbreak can significantly decrease development and severity of EHM.