Immune responses and seasonal red sea bream iridovirus dynamics following formalin-inactivated vaccination in farmed rock bream (Oplegnathus fasciatus).

Red sea bream iridovirus (RSIV) remains a major constraint on rock bream aquaculture, but vaccine-associated immune responses during natural field exposure are poorly understood. We monitored formalin-inactivated RSIV-vaccinated and unvaccinated rock bream from post-vaccination land-based rearing through a seasonal RSIV episode in commercial marine net pens. Splenic RSIV load and the mRNA expression of MyD88, IL-1β, IRF3, Mx, PKR, membrane-bound IgT (mIgT), and secretory IgT (sIgT) were assessed longitudinally. Full-length major capsid protein sequences obtained from RSIV-positive fish in both cohorts were identical and clustered within the broad RSIV-type lineage represented by the vaccine antigen. Vaccinated fish maintained lower splenic viral loads than unvaccinated fish throughout field sampling and showed a coordinated immune-gene mRNA response during natural exposure. MyD88, IRF3, Mx, and PKR remained elevated at the final sampling, although the broadest response, involving all seven genes, occurred earlier during the main field-response period. Duration-normalized area-under-the-curve analysis identified positive field-phase responses across the complete gene panel; principal component analysis and two-way hierarchical clustering identified coordinated vaccination- and phase-associated multigene profiles. These findings link formalin-inactivated vaccination with lower late-stage RSIV load and a temporally organized immune response under commercial net-pen conditions.