Abstract Background Equine herpesvirus-5 is commonly isolated from the lungs of horses with EMPF, suggesting an etiological link. Valacyclovir is used empirically to treat EMPF; however, no data is available concerning its impact on EHV-5 viral kinetics. Objectives To determine the effect of oral administration of valacyclovir on EHV-5 viral load measured by qPCR in blood, nasal secretions (NS) and BALF in horses with EMPF. Animals Six horses diagnosed with EMPF. Methods A prospective clinical trial was performed. Horses received 10 days of PO administered valacyclovir (loading dose 30 mg/kg, maintenance dose 20 mg/kg). Blood, NS, and BALF were collected for EHV-5 viral kinetics analyses during treatment. Blood and NS were collected every other day. BALF was collected on day 0 and day 10. Results There was no statistical difference in median EHV-5 viral load between day 0 and day 10 for all samples tested. In blood median EHV-5 viral load was 7676 (range 575-39 781) on day 0 and 6822 (range 1136-18 635) glycoprotein B (gB) gene copies per million cells on day 10. For NS median EHV-5 viral load was 2.944 × 106 (range 184 691-3.394 × 109) on day 0 and 8.803 × 106 (range 251 186-9.868 × 108) gB gene copies per million cells on day 10. For BALF median EHV-5 viral load was 59,842 (range 61-315 655) on day 0 and 185 083 (range 3562-542 417) gB gene copies per million cells on day 10. Conclusions and Clinical Importance Valacyclovir might not be an effective short-term antiviral treatment but efficacy in treatment of EMPF is unknown.