Effects of feeding with red algae (Laurencia caspica) hydroalcoholic extract on antioxidant defense, immune responses, and immune gene expression of kidney in rainbow trout (Oncorhynchus mykiss) infected with Aeromonas hydrophila

In this study, the effects of hydroalcoholic extract of red algae (Laurencia caspica) at different levels were evaluated on antioxidant defense, innate immune responses, and expression of immune genes in rainbow trout (Oncorhynchus mykiss). A total of 750 rainbow trout (39.43 +/- 0.44 g) were randomly assigned to five treatments with three replicates (50 fish per replicate) in 15 ponds and reared for 8 weeks. Fish samples were fed a diet containing different levels of algal extract (0, 5, 10, 15, and 20 g extract per kg of basal diet). At the end of feeding period, the fish were challenged with Aeromonas hydrophila and fish mortality was recorded over a 10-day period. To evaluate the antioxidant and immune parameters, fish were sampled at day 0, end of weeks 4 and 8, and after bacterial challenge. Expression of immune genes (IL-1 beta, lysozyme II, Complement C3, and TNF alpha) was also examined after bacterial challenge. Results indicated that application of algal powder in fish diet improved lysozyme activity, complement system, and immunoglobulin levels of extract fed-fish compared to control group (p < .05). The activities of antioxidant enzymes (catalase, glutathione peroxidase, and superoxide dismutase) were affected by the algal extract, with increasing levels in algal extract treatments compared to control fish (p < .05). According to the results, relative expression of target genes showed significant increases in 1.5% treatment compared to the other and control groups (p < .05). At the end of 10-day fish exposure to A. hydrophila, fish survived significantly higher in treatments received the algal extract than the control group, with the highest survival rate recorded in 1.5% algal extract (p < .05). According to the effective results of L. caspica extract on stimulating the immune system and increasing fish resistance to A. hydrophila, it is recommended to use 1.5% of this algal extract in rainbow trout diets.