Time-dependent hematological and immunological responses of olive flounder (Paralichthys olivaceus) to experimental infection with Photobacterium damselae subsp. damselae.

Photobacterium damselae subsp. damselae (Pdd) is an opportunistic pathogen in marine aquaculture, but the temporal host responses to infection remain poorly understood. In this study, olive flounder (Paralichthys olivaceus) were challenged with Pdd by intraperitoneal injection, and hematological, biochemical, cellular, humoral, and immune gene expression parameters were analyzed at 1, 2, 3, 5, 7, and 10 days post-infection (dpi). Mortality began at 2 dpi and reached 80% by 6 dpi. Hematocrit levels declined markedly and reached their lowest values at 5 dpi, coinciding with elevated plasma aspartate aminotransferase activity and reduced total protein, triglyceride, and glucose levels, indicating severe systemic tissue damage and metabolic disturbance. Flow cytometric and blood smear analyses revealed early expansion of granulocyte/monocyte-like cells accompanied by marked lymphocyte depletion. Respiratory burst activity showed a transient increase in infected fish at 2 dpi, whereas plasma superoxide dismutase levels decreased at 3-5 dpi, suggesting disruption of oxidative homeostasis. Head kidney IL-1β, IL-6, and TNF-α expression was upregulated during the early stage of infection, with TNF-α showing a secondary increase at 10 dpi among surviving fish. Plasma IL-10 levels increased later, whereas head kidney IL-10 expression increased early and remained elevated at later time points among surviving fish, indicating tissue- and time-dependent immune regulation. Among surviving fish, total IgM and Pdd-specific IgM levels increased at 7-10 dpi, together with increased serum agglutination activity against Pdd and elevated head kidney IgM expression. Overall, Pdd infection induced coordinated responses characterized by early inflammatory activation, severe systemic disturbance during the middle stage of infection, and delayed IL-10- and IgM-associated responses in surviving fish.