Seasonal modulation of immune, hemato-biochemical, and antioxidant responses in Nile tilapia during ectoparasitic infection: insights into host-parasite-environment interactions.

Limited information is available on how seasonal changes affect host immunity, hemato-biochemical status, antioxidant responses, and parasite infection in cultured fish. Therefore, this study evaluated the associations among seasonal changes, parasitic infections (Cichlidogyrus tilapiae and Centrocestus formosanus), and hemato-biochemical, antioxidant, and immune responses in Nile tilapia (Oreochromis niloticus). Fish (n = 593) and water samples were collected seasonally from Abbasa Fish Farm, El-Sharkia Governorate, Egypt. Parasite prevalence was highest in summer and lowest in winter. Infected fish showed marked hemato-biochemical changes, including reduced RBCs, hemoglobin (Hb), total protein, albumin, and globulin, whereas WBCs increased in spring, summer, and autumn but declined in winter; uric acid, glucose, ALT, and AST activities increased, particularly in winter. Antioxidant enzymes in gills and liver (SOD, CAT, and GPx) increased in spring, declined in summer, partially recovered in autumn, and reached their lowest values in winter, while malondialdehyde (MDA) levels increased in summer and winter. Immune responses, including lysozyme activity, respiratory burst activity, phagocytic activity, complement C3, and IgM, were enhanced in summer and reduced in winter. Seasonal changes also modulated the expression of C3, LZ, IL-1β, IL-8, IL-10, COX-2, TNF-α, and IFN-γ genes. Marked seasonal histological alterations were observed in the gills, liver, and spleen of infected fish, especially during winter. PCA indicated seasonal separation associated with water quality and immune variables. Generally, season-specific health management strategies are essential to reduce parasite-related losses and improve the sustainability of tilapia aquaculture.