Effects of β-glucan derivatives on the immunity of white shrimp Litopenaeus vannamei and its resistance against white spot syndrome virus infection

Yeast β-glucan is widely used as an immunostimulant in aquaculture. However, the insolubility of β-glucan limited its immunoenhancing effects. In order to increase the solubility, in the present study, carboxymethylglucans (CMGs) and sulfoethylglucans (SEGs) were made. Both CMG and SEG had four derivatives with different degrees of substitution (DSs). Then, β-glucan and its eight derivatives were added to the diets at the contents of 0.1% or 0.2%, respectively, to prepare 18 experimental diets. Shrimps (initial average weight 0.65 g) were fed with experimental diets for 35 days, and then were sampled for immune analyses and WSSV challenging test. The results showed that the types of derivatives and their DS as well as contents in diet significantly influenced the immunity and resistance against WSSV infection of Litopenaeus vannamei. For the shrimps fed with CMGs, the total hemocyte count (THC), phenoloxidase (PO) activity, respiratory burst (RB), superoxide dismutase (SOD) activity and resistance against WSSV challenge significantly decreased with the increasing of DS. The DS of SEGs significantly influenced the RB, SOD activity and WSSV resistance. Diets with 0.1% β-glucan derivative resulted in higher immunity and resistance than those with 0.2% β-glucan derivative, regardless of the derivative type. Moreover, the shrimps fed with 0.1% of dietary CMG with DS 0.325 showed the highest immunity and WSSV resistance. This CMG could be a better immunostimulant for L. vannamei.