Effect of copper sulfate on the immune response and susceptibility to Vibrio alginolyticus in the white shrimp Litopenaeus vannamei

White shrimp Litopenaeus vannamei (Boone) held in 35‰ seawater were challenged with Vibrio alginolyticus at a dose of 3 × 105 colony-forming units (cfu) shrimp−1, and then placed in water containing concentrations of Cu2+ at 0 (control), 1, 5, 10 and 20 mg l−1. Mortality of shrimp in 5, 10 and 20 mg l−1 Cu2+ was significantly higher than those in 1 mg l−1 Cu2+ and the control solution after 24–96 h. In another experiment, L. vannamei which had been exposed to control, 1, 5, 10 and 20 mg l−1 Cu2+ for 24, 48 and 96 h were examined for THC (total haemocyte count), phenoloxidase activity, respiratory burst (release of superoxide anion), phagocytic activity and clearance efficiency to V. alginolyticus. Copper concentrations at 1 mg l−1 or greater for 24 h resulted in decreased THC, phenoloxidase activity, phagocytic activity and clearance efficiency, whereas copper concentration at 20 mg l−1 caused significant increase in respiratory burst of L. vannamei. In conclusion, concentration of Cu2+ at 1 mg l−1 or greater increased the susceptibility of L. vannamei to V. alginolyticus infection by a depression in immune ability. The release of superoxide anion by L. vannamei exposed to 20 mg l−1 Cu2+ was considered to be cytotoxic to the host.