Early-Stage Adhesion Dynamics of Viral Hemorrhagic Septicemia Virus in Olive Flounder, Paralichthys olivaceus.

Viral hemorrhagic septicemia virus (VHSV) is a major pathogen in olive flounder (Paralichthys olivaceus) aquaculture, leading to high mortality rates and significant economic losses. Recurrent outbreaks underscore the need for a deeper understanding of the early-stage adhesion and infection mechanisms of VHSV. To address this gap, this study investigated the adsorption and replication dynamics of VHSV in various olive flounder tissues under different temperature conditions (15°C, 20°C, and 25°C). Viral RNA levels were quantified using RT-PCR, and viral recovery from seawater was assessed using Centricon ultrafiltration (30 kDa). VHSV was detected in the gills and mucus within 1 h post-infection, with peak viral loads observed between 1 and 3 h at 15°C and 20°C, indicating that these external tissues serve as initial adsorption sites. A marked increase in viral load in the mucus at 12 h post-infection suggests that mucus not only contributes to early viral capture but may also facilitate re-release into the environment. In tanks containing both fish and virus, viral concentrations were initially lower than those in virus-only controls but eventually equalised, supporting the hypothesis that adsorbed viruses can be released back into the water. The ultrafiltration method demonstrated recovery efficiency, exceeding 90% at viral concentrations above 10 copies/200 mL, demonstrating its effectiveness for environmental monitoring of VHSV. Overall, VHSV initially adheres to the gill and mucus layers before spreading internally, with host and environmental factors potentially influencing its persistence and transmission in aquaculture systems.