Morphometric and Cytological Characterisation of Enlarged Basophilic Cells in Rock Bream ( <i>Oplegnathus fasciatus</i> ) During Rock Bream Iridovirus ( <scp>RBIV</scp> ) Infection

Rock bream iridovirus (RBIV), a member of the Megalocytivirus genus, causes systemic infection and high mortality in rock bream (Oplegnathus fasciatus). Although enlarged basophilic cells are diagnostically relevant, their developmental trajectory and structural changes remain insufficiently characterised. This study provides the first quantitative morphometric and cytological characterisation of these cells, focusing on cytoplasmic and nuclear alterations, multinucleation and anucleation. Following oral administration of virus-containing tissue (200 mg; 6.12 × 10⁸ MCP copies), enlarged basophilic cells appeared from 11 dpi, only in fish with viral loads > 10⁶ copies/mg. Among 1664 cells analysed, mononucleate (A-1), multinucleated (two nuclei) (A-2), multinucleated (three nuclei) (A-3) and anucleate (B) types were identified. Mononucleate cells predominated at lower viral loads (over 83.1%), whereas anucleate cells comprised > 39.9% at higher loads. Exceptionally large cells with cytoplasmic axes of 50-60 μm were frequently observed. Cell and nuclear dimensions increased progressively with viral load, and multinucleate and anucleate forms predominated in fish with higher viral burdens. A schematic sequence of morphological progression from nuclear enlargement to nuclear loss is proposed as a working model, based solely on morphological evidence. Our study provides the first quantitative description of RBIV-induced enlarged basophilic cells and establishes a morphometric framework for future investigations into RBIV pathogenesis.