Period of excretion of equine herpesvirus 3 (EHV-3) from a stallion before showing clinical signs of equine coital exanthema and the effect of acyclovir treatment on the duration of EHV-3 excretion

Abstract

In 2017, two Thoroughbred stallions, A and B in Farms A and B, respectively, in Hokkaido in Japan showed clinical signs of equine coital exanthema (ECE). In 2020, stallion C in Farm B showed clinical signs of ECE. Eighteen mares were mated within five days before stallion A developed ECE. Ten mares that mated within 3 days before onset showed clinical signs of ECE on the external genitalia. Equine herpesvirus 3 (EHV-3) was isolated from vaginal swabs from three mares that mated within 2 days before onset. Swabs from 12 mares that mated within 4 days before onset were real-time PCR (rPCR)-positive and nine of those mares had an increased EHV-3 antibody titer. The three stallions were administered valaciclovir orally and topical acyclovir ointment was applied. Treatment started on the next day after onset in stallion A and on the day of onset in stallions B and C. EHV-3 was firstly isolated from penis swabs of stallions A and B before treatment and from penis swabs of stallion C 2 days after treatment. EHV-3 was not isolated after 8, 5 and 8 days from onset in stallions A, B and C, respectively. However, swabs were rPCRpositive for at least 12, 9 and 15 days after onset of stallions A, B and C, respectively. EHV-3 was excreted from the stallions at least within 4 days before the onset of ECE, and acyclovir treatment resulted in the termination of excretion within 8 days after onset.

Equine Coital Exanthema: New Insights on the Knowledge and Leading Perspectives for Treatment and Prevention

“…Valacyclovir (VACV; ACV prodrug) was also evaluated in vitro by a plaque reduction assay compared with ACV, and though both showed the same effectiveness against EHV-3 infection in vitro, when oral VACV (27 mg/kg/eight hours for two days and 18 mg/kg/12 h for eight days) was administered to stallions showing clinical ECE, the treatment did not reduce the severity and the duration of clinical disease [ 54 ]. Recently, treatment with an incremented oral dose of VACV (35 mg/kg/eight hours for three days and then 25 mg/kg/12 h for nine days) together with the application of a topical ointment of ACV on the penis of three naturally infected stallions, demonstrated a complete inhibition of EHV-3 replication five and eight days after initiation of the treatments [ 60 ]. However, it is difficult to arrive to a conclusion regarding the effectiveness of oral VACV and topical ACV, together or separately, against EHV-3, from the previous studies, since these studies were carried out in the face of natural outbreaks without untreated infected animals as controls, and since the concentration and frequency of application of the ACV ointment was not reported.…”