To evaluate the protective effect of baicalin against a Vibrio parahaemolyticus challenge, immune parameters were analyzed and RNA interference (RNAi) was performed in the present study. The results from challenge tests showed that the cumulative mortalities in the 200 and 400 mg/kg baicalin groups after challenge with V. parahaemolyticus were significantly lower than they were in the 100 mg/kg baicalin or control groups (p < .05); the relative percent of survival reached 71.4%. Macrobrachium rosenbergii injected with V. parahaemolyticus exhibited a significant increase in lysozyme (LZM) and inducible nitric oxide synthase levels as well as phagocytic activity because of the immune response, while after treatment with baicalin, these immune parameters were further enhanced. The gene expression levels of anti-lipopolysaccharide factors (ALFs), LZM, relish, and crustin were increased after treatment with baicalin or challenge with V. parahaemolyticus. The results of RNAi experiments demonstrated that the expression of crustins (Cru2, Cru4, Cru5, and Cru8), ALFs (ALF1, ALF3, ALF4, and ALF5), lysozymes (Lys1 and Lys2), and relish were significantly elevated after V. parahaemolyticus challenge. After MrToll4 was knocked down by siRNA, the expression levels of antimicrobial peptides (AMPs), except relish and Cru4, were inhibited. However, after oral administration of 400 mg/kg baicalin, the expression levels of all these AMPs, except Cru2, Cru4, ALF3, and relish, were upregulated.
