Immunological responses and disease resistance of rainbow trout (Oncorhynchus mykiss) juveniles following dietary administration of stinging nettle (Urtica dioica).

The present study investigated the effects of dietary supplementation of stinging nettle (Urtica dioica) on growth performance, skin mucus, immune response and disease resistance of rainbow trout (Oncorhynchus mykiss) fed with diets supplemented with U. dioica at 0, 1, 2 and 3%. After 8 weeks of feeding, the addition of U. dioica at 3% level resulted in improved weight gain, specific growth rate and feed conversion ratio significantly when compared to the other groups (P < 0.05). Hematological responses including: hematocrit (Htc), hemoglobin (Hb), lymphocyte and neutrophil populations enhanced significantly in fish fed 3% of stinging nettle when measured after 4 weeks; while, total red blood cells, white blood, Htc, Hb, lymphocyte and neutrophil populations significantly increased after 8 weeks in the same group (P < 0.05). Total serum protein and glucose contents increased significantly in fish fed stinging nettle at 3% when compared to the other groups after 8 weeks; however, triglycerides decreased significantly in the same group on the 4th and 8th week (P < 0.05). Additionally, several immune parameters, namely, IgM, lysozyme, complement components C3 and C4, and respiratory burst of blood leukocytes significantly increased in the 3% fed group on the 4th week; while, after 8 weeks the immune responses enhanced in fish fed 2 and 3% diets (P < 0.05). At the end of the feeding trial, mucus samples obtained from the fish fed stinging nettle supplementation exhibited improved antagonistic activities against several bacterial pathogens (Streptococcus iniae, Yersinia ruckeri, Vibrio anguillarum and Lactococcus garviae), skin mucus enzymes activities (alkaline phosphatase, lysozyme, protease and esterase) and protein levels in 2 and 3% groups with the highest being in case of 3% group when compared to the other groups (P < 0.05). The cumulative mortality of rainbow trout subjected to Y. ruckeri infectious exhibited relatively low mortality levels in all supplemented groups with the lowest being in fish fed 3% stinging nettle. The present findings demonstrated that dietary administration of U. dioica enhanced growth and stimulated fish immunity; thus, enabling the fish to be more resistant against bacterial infections.