Dietary administration of soybean isoflavones enhances the immunity of white shrimp Litopenaeus vannamei and its resistance against Vibrio alginolyticus

The growth response, total hemocyte count (THC), respiratory burst (release of superoxide anion), phenoloxidase (PO) activity, superoxide dismutase (SOD) activity, nitric oxide synthase (NOS) activity and resistance to the pathogen Vibrio alginolyticus were measured in white shrimp (Litopenaeus vannamei) which had been fed diets supplemented with soybean isoflavones (SI) at 0, 5, 10, 20, 40 mg kg^-1 feed for 8 weeks. Dietary SI had no significant influence on survival rate of the shrimp. Significantly greater SGR was observed in treatments with 10, 20 and 40 mg kg^-1 SI supplement than that in control (P < 0.05). Shrimp fed a diet with 20 mg kg^-1 SI showed significant increase in THC, PO activity, respiratory burst activity, SOD activity as well as NOS activity (P < 0.05). Increased THC, respiratory burst activity, SOD activity and NOS activity were also noticed in shrimp fed SI at 10 or 40 mg kg^-1 (P < 0.05). Significant lower cumulative mortality after challenge was observed in shrimp fed diets with SI supplement (10.0-13.3%) than that in control group (36.7%) (P < 0.05). It is therefore concluded that oral administration of SI at 20 mg kg^-1 for 8 weeks could enhance the immune ability of shrimp and improve production in shrimp farming.