Phylogenic study of Equine Herpesvirus 1 after a major outbreak in France in 2018

International audienceBackground : Equine herpesviruses (EHVs) are enveloped DNA viruses and members of the Herpesviridae family. Nine equine herpesviruses have been described and isolated among the equid family. They can cause different diseases and have the distinctive characteristic to establish latency in their host after infection and to reactivate under stress conditions. In this study, we will focus on EHV-1 and 4, which are of great interest as they represent an important threat for horse health and the equine industry. EHV-1 can cause respiratory distress, abortion or still-birth and neurological syndromes. EHV-4 essentially causes respiratory signs of disease and has recently been associated with sporadic cases of infectious abortion. In the beginning of 2018, a major viral epizooty was reported in France, associated with a very large number of EHV-1 and EHV-4 infections. To obtain a better understanding of EHV-1 strain circulation and diversity in France, a phylogenetic study of EHV-1 strains isolated from 2009 to this outbreak was conducted. The aim of the study was to consolidate our knowledge of these EHV-1 field strains. ORF30 was therefore sequenced and the controversial “neuropathogenic” genetic marker located on the DNA polymerase gene (ORF30 position 2254) was also characterised.Materials and Methods: According to the disease induced, different samples (nasal swabs, organs or total blood samples) were collected from 41 outbreaks (from January to May 2018). Nucleic acids were extracted from these samples and first tested by PCR for EHV-1 detection (Diallo et al. 2006). 79 EHV-1 strains isolated since 2009 were also analysed. Samples were typed by PCR for the neuropathogenic marker (Allen et al. 2007). A selection was sequenced for ORF30 using Sanger method.Results and discussion: 37 EHV-1 strains associated with the 2018 epizooty could be characterised and 2 strains were isolated on cell lines. Among respiratory outbreaks, 9 strains were typed with an Arginine on ORF30 2254 position, which correspond to the “non neuropathogenic” genotype (A2254), whereas 11 strains were typed with a Guanine, which corresponds to the “neuropathogenic” profile (G2254). Among neurological outbreaks, 2 strains were A2254 and two others were G2254. For abortions cases, 13 strains were A2254. Complete sequences were obtained for ORF30 for 6 strains from this 2018 epizootic and compared to sequences from 10 strains collected in France since 2009. Two phylogenetic groups could be observed according to the ORF30 sequences, which is in agreement with previous studies conducted in the United Kingdom (Nugent et al. 2006, Bryant et al. 2018). It also appeared that some strains seemed to be phylogenetically very close even though they had been collected all over the years. Conclusion: To conclude, it seems that both A2254 and G2254 strains are circulating in France and are not necessarily associated with the respiratory or neurological diseases. Although sequenced strains show 99% to 100% homology, they present some level of genetic diversity when compared together and could be classified in two subgroups according to ORF30 sequences, which could suggest a divergence of the virus considering the ORF30 region