Combined treatment with valacyclovir and heparin reduced the incidence of equine herpesvirus myeloencephalopathy and improved survival in naturally occurring equine herpesvirus-1 outbreaks

Objective: Describe the effects and outcomes of valacyclovir and heparin administration in equine herpesvirus-1 (EHV-1)-exposed horses. Methods: A multicenter retrospective study identified 111 horses naturally exposed to EHV-1 from August 2017 to March 2018. Equine herpesvirus-1 infection was confirmed via blood or nasal secretion PCR. Horses that developed neurologic signs were diagnosed with equine herpesvirus myeloencephalopathy (EHM). Clinical signs, treatment, and survival were reported. A subset of horses was treated with valacyclovir and heparin. Prophylactic medication administration was performed prior to development of EHV-1 clinical signs (temperature > 38.6 °C, limb edema, nasal discharge, or neurologic deficits). Medication administration to clinical cases was defined as metaphylactic. Survival and EHM incidence were assessed between horses that received valacyclovir and heparin and horses that did not. Results: 22 horses were diagnosed with EHV-1, with 9 developing EHM. Fourteen EHV-1-positive horses received metaphylactic valacyclovir and heparin, with 5 developing EHM. Five EHV-1-positive horses received prophylactic valacyclovir and metaphylactic heparin, with 1 developing EHM. Four EHV-1 horses were not treated with valacyclovir or heparin; all developed grade 5/5 ataxia and were euthanized. All horses treated with valacyclovir and heparin survived. Conclusions: EHV-1 treatment with valacyclovir and heparin reduced the incidence of EHM and improved survival compared to horses that did not receive these medications. Clinical Relevance: In confirmed EHV-1 outbreaks, the administration of valacyclovir prior to the development of EHV-1 clinical signs and the administration of heparin after the development of clinical signs may improve patient survival.