Addition of nitrite-N at 1.5 mg l -1 in tryptic soy broth (TSB) significantly (p < 0.05) decreased the growth rate of the bacterial pathogen Lactococcus garvieae and significantly (p < 0.05) reduced mortality compared to zero nitrite controls when injected into giant freshwater prawns Macrobrachium rosenbergii at 5 × 10 5 colony-forming units (CFU) per prawn. In other experiments, whereby prawns were injected with TSB-grown L. garvieae (5 × 10 5 CFU prawn -1 ) and then held in water containing

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... itrite-N, mortality at 72 h post-injection was significantly (p < 0.05) higher for prawns held in water containing 1.68 mg l -1 nitrite than at lower concentrations. Prawns exposed to different concentrations of nitrite-N were examined for THC (total hemocyte count), phenoloxidase activity, respiratory burst, phagocytic activity and bacterial clearance efficiency. No significant differences in THC and phenoloxidase activity were observed among treatments. With prawns exposed to nitrite-N for 168 h (7 d) at 1.59 mg l -1 , phagocytic activity and clearance efficiency decreased, while at 1.15 mg l -1 or more, respiratory burst increased, generating the superoxide anion at levels considered cytoxic to the host. We conclude that nitrite-N at 1.68 mg l -1 causes depression in the immune response and increased mortality in M. rosenbergii infected with L. garvieae. KEY WORDS: Macrobrachium rosenbergii · Lactococcus garvieae · Nitrite · Challenge · Virulence · Hemocyte count · Phenoloxidase activity · Superoxide anion Resale or republication not permitted without written consent of the publisher Editorial responsibility: Timothy Flegel,