The giant freshwater prawn Macrobrachium rosenbergii (12-18 g) was examined for phenoloxidase activity, respiratory burst (release of superoxide anion), phagocytic activity and clearance efficiency for the pathogen Lactococcus garvieae at 28 degreesC. Phenoloxidase activity was higher at intermoult (C stage), whereas respiratory burst was the lowest at C stage among the moult stages of A, B, C, D-1/D-2 and D-3. Phagocytic activity and clearance efficiency of prawns to L. garvieae decreased significantly at A, D-1/D-2 and D-3 stages as compared with those at C stage. In another experiment, M. rosenbergii at different moult stages were injected with tryptic soy broth (TSB)-grown L. garvieae (3 x 10(5) cfti prawn(-1)) and then held in water. After 6 It, the mortality of L. garvieae-injected prawns at D-3 was significantly higher than that at C and A stages. After 72 h, the mortality of L. garvieae-injected prawns was 100%, 40% and 60% at D-3, C and A stages, respectively. It was concluded that M. rosenbergii had decreased resistance at A, D-1/D-2 and D-3 through a reduction of its phenoloxidase activity, phagocytic activity and clearance efficiency against L. garvieae. The fact that the prawns displayed higher respiratory burst, but lower phenoloxidase activity, and the fact that the phagocytic activity and clearance efficiency of prawns to L. garvieae decreased at premoult and postmoult stages are considered to be modulation of resistance factors. (C) 2003 Elsevier Science B.V. All rights reserved.
