Unraveling the Systemic and Local Immune Response of Rainbow Trout () to the Viral Hemorrhagic Septicemic Virus.

Viral outbreaks have caused significant mortality and economic losses in aquaculture, highlighting the urgent need for effective therapies and a deeper understanding of antiviral and immune mechanisms in key species. This study investigates the constitutive and virus-induced antiviral responses in juvenile rainbow trout () following infection with viral hemorrhagic septicemia virus (VHSV). Trout (30 g) were infected by immersion with VHSV (TCID = 10 mL) for two hours. Samples were collected at 24, 72, and 120 h post-infection to assess hematology, innate immunity, viral load, and transcriptomic response. At 24 h post-infection, no immune response or increase in viral load was detected, suggesting the host had not yet recognized the virus and was still in the incubation phase. By 72 h, viral replication peaked, with high viral loads observed in mucosal tissues (skin and gills) and immune organs (kidney, spleen, liver), alongside strong up-regulation of antiviral genes, such as . This gene maintained high expression through the final sampling point, indicating its key role in the antiviral response. At this stage, reduced immune competence was observed, marked by elevated nitric oxide and circulating thrombocytes. At 120 h, modest increases in peripheral monocyte, plasma lysozyme, and peroxidase activity were detected; however, these responses were insufficient to reduce viral load, suggesting the resolution phase had not yet begun. In summary, while a limited immune response was observed by the end of the trial, the consistent antiviral activity of from peak infection to 120 h post-infection underscores its importance in the defence against VHSV in rainbow trout.