Effect of nitrite on immune response of Taiwan abalone Haliotis diversicolor supertexta and its susceptibility to Vibrio parahaemolyticus

Taiwan abalones Haliotis diversicolor supertexta held in 30parts per thousand. seawater and 26degreesC were injected with tryptic soy broth (TSB)-grown Vibrio parahaemolyticus (1.6 x 10(5) CFU [colony-forming units] abalone(-1)), and then placed in water containing different concentrations of nitrite-N (nitrite as nitrogen): 0.01 mg 1(-1) (control), 1.05, 3.04, 5.10 and 10.06 mg 1(-1). Mortality of the abalones increased in direct parallel to ambient nitrite-N concentration. Over 12 to 48 h, the mortality of V parahaemolyticus-injected abalones held in 3.04 mg 1(-1) nitrite-N was significantly higher than that of abalones in the control solution. Abalones that had been exposed to control, 0.96, 2.95, 5.03 and 10.16 mg 1(-1) nitrite-N for 24, 72 and 120 h were examined for THC (total hemocyte count), phenoloxidase activity, respiratory bursts (release of superoxide anion), phagocytic activity, and clearance efficiency of V parahaemolyticus. The THC increased in abalone after 72 h exposure to 0.96 and 2.95 mg 1(-1) nitrite-N, but decreased in abalones after 24 h exposure to 5.03 and 10.16 mg 1(-1) nitrite-N. Phenoloxidase activity and respiratory bursts increased, while phagocytic activity and clearance efficiency decreased in abalones exposed to greater than or equal to 0.96 mg 1(-1) nitrite-N for 24 h. It is concluded that nitrite-N in water at concentrations as low as 0.96 mg 1(-1) weakens the immune response and increases mortality of H. diversicolor supertexta infected with V. parahaemolyticus.