The effects of nano-selenium and/or vitamin C on the growth performance, blood health, organ histology, molecular alterations, and disease resistance of Nile tilapia (Oreochromis niloticus) against Saprolegnia ferax

Nanomaterials such as selenium nanoparticles (SeNPs) are commonly used in everyday products and offer numerous health benefits to aquatic animals. Vitamin C (Vit C) is an essential micronutrient for fish because of its strong antioxidant properties. However, the combined effects of SeNPs and Vit C on enhancing disease resistance in Nile tilapia (Oreochromis niloticus) have not been thoroughly explored. This study investigates the impacts of SeNPs and/or Vit C on the performance, blood parameters, histology, gene expression, and resistance to Saprolegnia ferax in Nile tilapia (O. niloticus). The fish fingerlings (n = 240, averaging 32 ± 3.0 g) were divided equally into four experimental groups, with 60 fish in each group. The fish were fed a basal diet as a control group (CON group) or supplemented with 100 mg of Vit C (VC100 group), 1 mg of SeNPs/kg (SeNP group), or a combination of SeNPs and vitamin C (SeNPs + VC100 group) for 70 days. At the end of the study, a challenge trial with S. ferax was conducted. The growth and feed efficiency results indicated a significant improvement in the combined group (SeNPs + VC100 group) compared with the other groups (P < 0.05). Significant reduction in fat and moisture contents and the crude protein and ash significantly improved in the combined group (P < 0.05). Liver enzymes were reduced, with the lowest values observed in the SeNPs + VC group, and total protein and globulin contents were improved in all supplemented groups (P < 0.05). All supplements resulted in significant improvements in digestive enzymes (lipase, protease, and amylase) and immune response (lysozyme and phagocytic activities) compared to the control group (P < 0.05). The serum levels of catalase and superoxide dismutase significantly increased in fish of the SeNPs and SeNPs + VC100 groups, while lipid peroxidation (MDA) significantly reduced by the treatments with the best reduction in the combined group. The liver of fish in the supplemented groups showed improved structural integrity of hepatocytes, hepatic sinusoids, and pancreatic ducts, with reduced inflammatory cell presence and fibrocytes compared to the control fish. The intestinal structure of fish fed supplemented diets exhibited normal histomorphometry with an increase in villus length, height, and layer thickness. Diets fortified with SeNPs, VC100, or their mixture significantly boosted the growth (GH and IGF-1) and immune-antioxidant-related genes (IL-1β, IL-8, CAT, and SOD) compared with the untreated groups. Fish treated with a combination of nutrients exhibited higher survival rates and increased resistance to S. ferax. This study revealed that a dietary combination of SeNPs and Vit C synergistically enhanced tilapia fish’s antioxidant capacity, immune response, organ health, growth, and disease resistance. This approach offers a promising strategy to reduce production losses in tilapia aquaculture, particularly during disease outbreaks.