Whole genome characterisation of Australian and New Zealand OsHV-1 'μVar' like variants over time.

Ostreid herpesvirus 1 (OsHV-1) is a major pathogen of Pacific oysters (Magallana gigas), linked to mass mortality events worldwide and presents a substantial threat to global aquaculture. Since 2010, mortality events have been occurring in Pacific oysters within Australia with the cause being an OsHV-1 microvariant highly similar to OsHV-1 μVar first detected in France, 2008. In this study, fifteen OsHV-1 whole genomes were assembled from Australian and New Zealand mortality events, including twelve newly sequenced genomes and three assembled from public datasets. Whole-genome phylogenetic analyses indicate that Australian and New Zealand OsHV-1 isolates form a distinct clade, separate from European and East Asian genomes, and with moderate regional and temporal divergence across the past 14 years. Across the dataset, multiple biologically relevant mutations were observed in genes of known function, including DNA polymerase, ribonucleotide reductase subunits, and apoptosis-related proteins, though most mutations occurred in uncharacterised genes. These findings highlight the limitations of single-gene typing and support the need for whole-genome approaches in understanding OsHV-1 evolution. Critically, the lack of recent whole-genome data, particularly from East and Southeast Asia, restricts the ability to trace viral origins and detect novel variants, underscoring the need for expanded global OsHV-1 genomic surveillance.