Cell-Type Specific Localization and Mitochondrial Remodelling during Nervous Necrosis Virus Infection in the Gills of Sevenband Grouper.

The gills of teleost fish represent a critical interface with the aquatic environment, but their role in the early stages of viral infection remains poorly understood. Nervous necrosis virus (NNV), a neurotropic pathogen causing viral nervous necrosis in marine fish, has been detected in the gill tissues of infected sevenband grouper without apparent histopathological damage. This study investigates the early cellular tropism and acute mitochondrial alterations associated with initial NNV infection in the gills. Using a combination of quantitative PCR, immunofluorescence microscopy, and transmission electron microscopy, we identified mitochondrion-rich (MR) cells and neuroepithelial (NE) cells as specific early-stage cellular targets for NNV localization. Notably, NNV infected MR cells exhibited altered mitochondrial morphology, specifically acute mitochondrial fragmentation. Furthermore, viral localization within NE cells suggests a potential physiological conduit for neuroinvasion. These findings demonstrate that NNV exploits specific gill cell types and induces early mitochondrial remodelling. Based on these structural alterations, we hypothesize that this pathogenesis may contribute to localized immune evasion and subsequent rapid dissemination to the central nervous system, rather than utilizing the gills as a primary site of robust, sustained viral replication. This is the first study to delineate the acute subcellular pathogenesis and entry dynamics of NNV in fish gill tissue.