Circulation and characterisation of Equid herpesvirus-3 strains in France between 2010 and 2021

Background: Equine coital exanthema (ECE) is an acute viral infection of horses caused by equid herpesvirus-3 (EHV-3). The presence of the virus and its economic impact is well described in different countries such as Argentina or Japan. However, even though it is included in the HBLB code of practice (UK) and in the list of diseases monitored by the RESPE (FR), only a few studies have been performed in Europe. Objectives: To determine the circulation of EHV-3 in France and measure the efficacy of new antiviral compounds against EHV-3. Study design: Retrospective field study and in vitro experiments. Methods: 71 field samples from suspected ECE cases, and collected between 2010 and 2021, were analysed by qPCR (DNA polymerase gene) and phylogenetic analysis (gG gene) was performed on 10 positive samples. Antiviral compounds (aciclovir, ganciclovir, valganciclovir, decitabine, idoxuridine, pritelivir, cidofovir, aphidicolin, vidarabine) previously described with antiviral properties against EHV-1, were evaluated using Real-Time Cell Analysis (RTCA) on equine dermal cells with the reference strain EHV-3 VR3522, cytopathic effect monitoring (microscopy) and viral load quantification (qPCR). Results: 16/71 (22%) suspected ECE cases were confirmed positive by PCR with 0 to 3 cases per year. French strains were clustered into two of the three observed phylogenic groups, but there was no correlation between the phylogeny and the time of collection or location. Aphidicolin was the most effective compound with an EC50 value of 2.43±0.02 μM (RTCA) and 1.63±0.48 μM (qPCR). Pritelivir and cidofovir were less potent molecules against EHV-3 (EC50 >50 μM). Main limitation: Antiviral compounds were tested against the reference strain only. Conclusions: This study demonstrates for the first time the presence of EHV-3 over a twelve-year period in France. In vitro studies show the antiviral efficacy of aphidicolin.