Administration of the hot-water extract of Spirulina platensis enhanced the immune response of white shrimp Litopenaeus vannamei and its resistance against Vibrio alginolyticus

White shrimp Litopenaeus vannamei which had been injected with the hot-water extract of Spirulina platensis at 6, 10, and 20 μg g−1, or immersed in aerated seawater containing extract at 200, 400, and 600 mg L−1 were challenged with Vibrio alginolyticus at 1.5 × 106 or 1.4 × 106 colony-forming units (cfu) shrimp−1, and then placed in seawater. Survival rates of shrimp that received the extract of S. platensis at 6–20 μg g−1, and those of shrimp immersed in seawater containing the extract at 400 and 600 mg L−1 were significantly higher than those of control shrimp after 24–96 and 48–96 h, respectively. In a separate experiment, the hyaline cell (HC) count, granular cell (GC, including semi-granular cell) count, total haemocyte count (THC), phenoloxidase (PO) activity, respiratory burst (RB), superoxide dismutase (SOD) activity, glutathione peroxidase (GPx) activity, and lysozyme activity were measured when shrimp were injected with the extract at 6, 10, and 20 μg g−1, and immersed in seawater containing the extract at 200, 400, and 600 mg L−1. These parameters directly increased with the concentration, and significantly increased when shrimp were immersed in the seawater containing the extract at 0.5–4 h. L. vannamei that received all doses of the extract via injection or via immersion all had increased phagocytic activity and clearance efficiency to V. alginolyticus at 12–72 h and 3–4 h, respectively. It was concluded that L. vannamei that received the hot-water extract of S. platensis had enhanced innate immunity and increased resistance against V. alginolyticus infection.