Equid herpesvirus 1 is one of the most common viral pathogens in the horse population and is associated with respiratory disease, abortion and still-birth, neonatal death and neurological disease. A single point mutation in the DNA polymerase gene (ORF30: A2254G, N752D) has been widely associated with neuropathogenicity of strains, although this association has not been exclusive. This study describes the fortuitous isolation of a strain carrying a new genotype C₂₂₅₄ (H₇₅₂) from an outbreak in France that lasted several weeks in 2018 and involved 82 horses, two of which showed neurological signs of disease. The strain was characterised as U_L clade 10 using the equid herpesvirus 1 (EHV-1) multi-locus sequence typing (MLST) classification but has not been identified or isolated since 2018. The retrospective screening of EHV-1 strains collected between 2016 and 2018 did not reveal the presence of the C₂₂₅₄ mutation. When cultured in vitro, the C₂₂₅₄ EHV-1 strain induced a typical EHV-1 syncytium and cytopathic effect but no significant difference was observed when compared with A₂₂₅₄ and G₂₂₅₄ EHV-1 strains. An experimental infection was carried out on four Welsh mountain ponies to confirm the infectious nature of the C₂₂₅₄ strain. A rapid onset of marked respiratory disease lasting at least 2 weeks, with significant virus shedding and cell-associated viraemia, was observed. Finally, an in vitro antiviral assay using impedance measurement and viral load quantification was performed with three antiviral molecules (ganciclovir (GCV), aciclovir (ACV) and aphidicolin (APD)) on the newly isolated C₂₂₅₄ strain and two other A/G₂₂₅₄ field strains. The three strains showed similar sensitivity to ganciclovir and aphidicolin but both C₂₂₅₄ and A₂₂₅₄ strains were more sensitive to aciclovir than the G₂₂₅₄ strain, based on viral load measurement.

Equine herpesvirus 1 (EHV-1) is an Alphaherpesvirus infecting not only horses but also other equid and non-equid mammals. It can cause respiratory distress, stillbirth and neonatal death, abortion, and neurological disease. The different forms of disease induced by EHV-1 infection can have dramatic consequences on the equine industry, and thus the virus represents a great challenge for the equine and scientific community. This report describes the progress of a major EHV-1 outbreak that took place in Normandy in 2009, during which the three forms of disease were observed. A collection of EHV-1 strains isolated in France and Belgium from 2012 to 2018 were subsequently genetically analysed in order to characterise EHV-1 strain circulation. The open reading frame 30 (ORF30) non-neuropathogenic associated mutation A₂₂₅₄ was the most represented among 148 samples analysed in this study. ORF30 was also sequenced for 14 strains and compared to previously published sequences. Finally, a more global phylogenetic approach was performed based on a recently described Multilocus Sequence Typing (MLST) method. French and Belgian strains were clustered with known strains isolated in United Kingdom and Ireland, with no correlation between the phylogeny and the time of collection or location. This new MLST approach could be a tool to help understand epidemics in stud farms.

Equid alphaherpesvirus-1 (EHV-1) is one of the main pathogens in horses, responsible for respiratory diseases, ocular diseases, abortions, neonatal foal death and neurological complications such as equine herpesvirus myeloencephalopathy (EHM). Current vaccines reduce the excretion and dissemination of the virus and, therefore, the extent of an epizooty. While their efficacy against EHV-1-induced abortion in pregnant mares and the decreased occurrence of an abortion storm in the field have been reported, their potential efficacy against the neurological form of disease remains undocumented. No antiviral treatment against EHV-1 is marketed and recommended to date. This study aimed to measure the protection induced by valganciclovir (VGCV), the prodrug of ganciclovir, in Welsh mountain ponies experimentally infected with an EHV-1 ORF30-C₂₂₅₄ strain. Four ponies were administered VGCV immediately prior to experimental EHV-1 infection, while another four ponies received a placebo. The treatment consisted in 6.5 mg/kg body weight of valganciclovir administered orally three times the first day and twice daily for 13 days. Clinical signs of disease, virus shedding and viraemia were measured for up to 3 weeks. The severity of the cumulative clinical score was significantly reduced in the treated group when compared with the control group. Shedding of infectious EHV-1 was significantly reduced in the treated group when compared with the control group between Day + 1 (D + 1) and D + 12. Viraemia was significantly reduced in the treated group when compared with the control group. Seroconversion was measured in all the ponies included in the study, irrespective of the treatment received. Oral administration of valganciclovir induced no noticeable side effect but reduced clinical signs of disease, infectious virus shedding and viraemia in ponies experimentally infected with the EHV-1 C₂₂₅₄ variant.

L’herpèsvirus équin 1 (EHV-1), classé parmi les Herpesviridae, est l’un des pathogènes les plus fréquemment rencontrés au sein de la population équine. Ce virus peut être à l’origine de formes de maladie respiratoire, abortive ou néonatale, oculaire ou nerveuse pouvant conduire à l’euthanasie de l’animal. Les mesures de prévention incluant la vaccination constituent la première barrière dans la lutte contre ce virus. En cas d’épizootie, le suivi et l’identification des souches peuvent constituer une aide précieuse pour la compréhension des modes d’infection. Ce travail de thèse a porté sur le suivi moléculaire par différentes approches d’une population de souches d’EHV-1 isolées en France entre 2009 et 2018. Le typage de 137 souches d’EHV-1 pour le polymorphisme en position 2254 du gène de l’ADN polymérase (ORF30) associé à la pathogénicité des souches a montré une association significative des souches de type A2254 avec les cas d’avortements. Quatorze souches d’intérêt ont ensuite été analysées par séquençage de l’ORF30 et Multi Locus Sequence Typing (MLST). Le séquençage complet de l’ORF30 s’est montré peu discriminant mais a permis de mettre en évidence un troisième génotype (C2254 (H752)) en position 2254 de l’ORF30. L’analyse par MLST a permis de classer ces souches parmi 7 des 10 clades UL décrits par l’outil à ce jour. Deux autres souches d’herpèsvirus d’équidés, une d’EHV-8 et une d’EHV-9, ont pu être mises en évidence grâce au séquençage de l’ORF30. Ces différentes souches d’alphaherpèsvirus des équidés ont été caractérisées, comparées et leur sensibilité vis-à-vis de sept molécules antivirales a été évaluée in vitro. Leur proximité génétique a pu être visualisée par la construction d’un network phylogénétique basé sur les séquences de l’ORF30. Une seconde partie du travail a porté sur le suivi du niveau de protection conféré par les anticorps neutralisant l’EHV-1. Une technique innovante de quantification a été mise au point grâce à la technologie xCELLigence® sur cellules E. Derm et a permis de déterminer les titres induisant 50% de la neutralisation d’une concentration de virus (NT50) à partir d’une population d’échantillons qualifiés provenant d’une infection expérimentale avec la souche d’EHV-1 C2254. Les valeurs de NT50 obtenues ont montré une bonne corrélation avec les valeurs de titres obtenues avec la méthode de référence sur cellules RK13. Les réponses de chevaux à la vaccination ont ensuite pu être étudiées selon le protocole de vaccination reçu et selon l’historique des chevaux.