First Report of Mass Mortality in Post-Metamorphosis Magallana Gigas Associated with OsHV-1 in China: Epidemiological and Genomic Characterization.

During summer 2025, mass mortalities (20%-80%) affected juvenile triploid Pacific oysters (Magallana gigas) across major mariculture zones of Northern China. Epidemiological investigations of 941 individuals from 58 batches confirmed a strong association with high Ostreid herpesvirus 1 (OsHV-1) loads. Multivariable analysis revealed that each log₁₀ increase in viral load conferred a 55% increase in mortality odds (OR = 1.55, p < 0.001), with loads >106 GE/mg associated with 94.0% mortality. A significant viral load-temperature interaction (p = 0.027) demonstrated temperature modulates pathogenicity. After accounting for this interaction, host species showed no significant effect (p = 0.564), indicating apparent differences between M. gigas and M. angulata reflect thermal habitats rather than genetic resistance. Histopathological examination revealed characteristic tissue damage, including hemocyte infiltration and connective tissue lysis. This event marks the first documented transition of the pathogen to open-sea juvenile populations. Genomic reconstruction showed that tree topology was primarily shaped by host species. Within the oyster-associated clade, the 2025 sea-farm variants grouped with a 2023 northern hatchery variant (SD23L), separately from European μVars-a pattern consistent with hatchery-to-sea spillover. One southern variant clustered with European μVars, raising the possibility of international introduction. Collectively, these findings suggest that the outbreak associated with changes in aquaculture practices-particularly the "North-South Relay" and year-round seedling production-rather than with emergence of a novel variant. This study underscores the urgent need for enhanced biosecurity and viral screening in China's oyster industry.