Effects of dietary β-glucan and glycyrrhizin on non-specific immunity and disease resistance of white shrimp, Litopenaeus vannamei (Boone) challenged with Vibrio alginolyticus

The white shrimp Litopenaeus vannamei, fed immunostimulant-free, 0.2%β-glucan and 0.06% glycyrrhizin diets for 18 days, respectively, were challenged with Vibrio alginolyticus at 6.4 × 10 ⁴CFUshrimp ^-1. The total haemocyte count (THC), phenoloxidase (PO) activity, respiratory burst (RB) and superoxide dismutase (SOD) activity changes for a 120-h period were investigated, and shrimp mortality was also recorded. The results showed that PO activity, RB and SOD activity were significantly higher in shrimp fed the two immunostimulant diets after 18 days than those in shrimp fed immunostimulant-free diets. The THC and SOD activity decreased significantly from 0 to 24h post challenge, and then reverted to normal levels at 96 and 72h respectively. The values for PO activity and RB increased from 0 to 48h post challenge. Compared with those fed the control diets, shrimp fed immunostimulants had significantly higher PO activity and RB values at 120h post challenge. Mortalities after challenge with V. alginolyticus were significantly lower in shrimp fed with β-glucan or glycyrrhizin than in those fed with a diet without immunostimulants. It was concluded that dietary β-glucan and glycyrrhizin increased the shrimp immunity. Furthermore, β-glucan caused an increase in some immune parameters 12h earlier than glycyrrhizin after V. alginolyticus challenge.