Equine rhinopneumonitis, caused by Equid Herpesvirus 4 (EHV-4) and EHV-1, is one of the most frequent respiratory diseases experienced by horse breeders and trainers. The French equine infectious diseases surveillance network (RESPE) reports numerous outbreaks each year, which can significantly disrupt economic activities, as illustrated in France in 2018 when more than 200 equestrian events were cancelled. While the origin of this crisis could date back from the 2016 EHV vaccine shortage, no scientific evidence has been obtained to date. Research teams have often been less interested in the study of EHV-4, probably because EHV-1 is known to induce more severe forms of diseases (i.e. abortions, neonatal deaths and myeloencephalopathy). However, EHV-4 is responsible for approximately 6 times more epizootics (respiratory infection) than EHV-1 (1,2). Several cases of EHV-4 induced abortion have also been observed and reported in recent years (3,4). All of these data attest to the importance of the pathology associated with HVE-4 infection and demonstrate the need to better characterizing EHV-4 common characteristics and differencies with EHV-1. The objectives of this new project are to : 1 / Understand the importance of EHV-4 during the 2018 French crisis in a degraded context of vaccination. 2 /Characterise EHV-4 infection in order to improve pathophysiology knowledge, an essential step for future and effective prophylaxis. 3 / Characterise EHV-4 strains at the molecular level to identify future vaccine and antiviral targets.