Isolation and characterization of a highly virulent novel grass carp reovirus genotype II (GCRV-XT256) for vaccine development.

Grass carp hemorrhagic disease, caused by genotype II grass carp reovirus (GCRV-II), is a devastating disease that leads to high mortality in farmed grass carp. However, the absence of a standardized, high-virulence challenge strain that reliably recapitulates severe disease has hindered pathogenesis and vaccine research. Here, we report the isolation and characterization of a novel GCRV-II strain, GCRV-XT256. This strain induces consistent hemorrhagic pathology and high mortality, enabling the establishment of a robust infection model. We successfully purified GCRV-II virions, confirming their classic reoviral morphology, with a diameter of approximately 75 nm. Genomic analysis revealed high similarity to epidemic GCRV-II strains. The model using this strain elicited a potent immune response, marked by upregulation of genes such as nf-κb1, irf3, irf7, and il1β. Moreover, an inactivated vaccine prepared from GCRV-XT256 conferred up to ∼80% relative immune protection in grass carp. The GCRV-XT256 strain and the associated infection model thus serve as a foundational resource for future studies on GCRV-II virology and vaccine development.