Evaluation of Urtica dioica and Phillyrea latifolia extracts as feed additives for enhancing innate immunity in gilthead seabream (Sparus aurata L.)

This study was aimed at determining the effects of ethanolic extracts of nettle (Urtica dioica) and green olive tree (Phillyrea latifolia), used as feed additives, on innate immune responses in gilthead seabream (Sparus aurata). Ethanolic extracts from leaves of both plants were incorporated into feeds in two separate experimental setups, and the fish received 0, 50, 100, 200, and 400 mg/kg nettle or green olive tree extracts for 60 days. Growth performance was evaluated on the 30th and 60th days, and hematological parameters as well as the immune responses were assessed on the 60th day. Moreover, the nettle group was challenged with Vibrio anguillarum at the end of the 60th day. Supplementation at different doses increased lysozyme and myeloperoxidase activities significantly in both trials (p < 0.05). Respiratory burst activity also significantly increased after nettle supplementation (p < 0.05). It was determined that nettle supplementation resulted in 96% survival rate after the challenge test. In both trials, expression levels of IL-1β, IL-6, IL-10, IL-18, and TNF-α genes were analyzed and evaluated alongside other parameters. Consequently, the findings indicated that nettle, particularly at 100 mg/kg feed, provided the strongest support for immunostimulation and disease resistance in gilthead seabream. In contrast, green olive tree extract showed preliminary immunomodulatory effects that require further confirmation by deeper phytochemical standardization/quantitative characterization and a valid pathogen challenge.