Abstract
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 UL 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.
Highlights
Equid herpesvirus 1 (EHV-1) is an alphaherpesvirus, classified among the Herpesviridae family and the Varicellovirus genus [1]
In 2018, 82 horses located in a single site in France and kept in different stable blocks showed clinical signs of disease associated with EHV-1 infection over a 6-month period from March 2018 to November 2018
2018 (12 outbreaks between 1 January 2018 and 31 March 2018), and rapidly after the first clinical signs appeared in a first stable block (5 March), the 64 horses kept in this stable block were monitored for a set of clinical signs of respiratory disease
Summary
Equid herpesvirus 1 (EHV-1) is an alphaherpesvirus, classified among the Herpesviridae family and the Varicellovirus genus [1]. EHV-1 first infects and replicates in the epithelial cells of the upper respiratory tract [4,5]. At this stage of EHV-1 infection, horses may present a respiratory form of disease associated with nasal discharge, cough and pyrexia. Circulation of infected leucocytes during cell-associated viraemia disseminates EHV-1 through the organism to secondary sites such as the central nervous system or the reproductive tract. In these cases, EHV-1 can induce equine herpesvirus encephalomyelitis (EHM; from a mild limb ataxia to quadriplegia) or abortion, stillbirth and neonatal death [7,8,9,10]. Like all herpesviruses, EHV-1 has the ability to establish latency after primo-infection and to reactivate under environmental stress conditions, immune weakness or after corticoid-based treatments [11,12,13,14,15]
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