Abstract
This study was designed to appraise the effect of licorice herbal supplement on the immune status of rainbow trout fingerlings. Accordingly, five diets were formulated with different levels of licorice extract (LE) including 0 (control), 0.5 g kg−1 (LE0.5), 1 g kg−1 (LE1), 2 g kg−1 (LE2), and 3 g kg−1 (LE3). The fingerlings (10.0 ± 0.1 g initial mean weight) received the diets in triplicates (30 fish in each replicate) for 56 days. The results showed that the white blood cells and their differential number (lymphocytes and monocytes) were remarkably increased by LE2 supplementation (P < 0.05). The oral administration of LE2 significantly increased the levels of serum immunoglobulin (Ig), lysozyme activity, and complement components (C3 and C4) compared with others. Meanwhile, the serum bactericidal activity against Yersinia ruckeri in LE2 and LE3 treatments was significantly higher than others except for LE1 (P < 0.05). In addition, serum alternative complement activity significantly improved in all treated groups except LE0.5 compared with the control group (P < 0.05). In terms of skin mucosal immunity, the fish fed with LE2 and LE3 diets exhibited notably higher lysozyme activity, alkaline phosphatase activity, and Ig value than other groups (P < 0.05). The highest skin mucus bactericidal activity against Y. ruckeri was obtained in LE2 treatment (P < 0.05). In addition, dietary LE2 significantly increased the relative expression of immune-associated genes including tumor necrosis factor-α, interleukin-1β, interleukin-8, and IgM and the former treatments showed higher values than the control group. The cumulative mortality of fish against Y. ruckeri infection was notably reduced from 53.6% in the control group to 29.0% in LE3 treatment. Overall, the dietary administration of LE at 2 g kg−1 had the best effects on immunocompetence in rainbow trout.
Highlights
Over the past decade, the global production of rainbow trout (Oncorhynchus mykiss) has grown significantly and nearly doubled
The White blood cells (WBCs) count in the groups fed with LE1 and LE2 diets was significantly higher than LE0.5 and the control groups (P < 0.05)
The application of feed supplements to improve the immune system of aquatic animals can reduce the economic losses due to infectious diseases outbreak in aquaculture sectors
Summary
The global production of rainbow trout (Oncorhynchus mykiss) has grown significantly and nearly doubled. A healthy balanced diet should be formulated based on the essential requirements of aquatic animals but should contain some ingredients to ameliorate the overall performance of fish in captivity [5,6,7]. In this regard, several functional feedstuffs such as acidifiers, probiotics, and medicine plants with diverse biological activities such as antioxidant and immuneboosting properties have entered into aquaculture nutrition [8,9,10,11]. Herbal supplements display immunostimulatory effects on the host due to several bioactive compounds that are unique for each medicinal plant species [12]
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