Effect of hypoxia on the immune response of giant freshwater prawn Macrobrachium rosenbergii and its susceptibility to pathogen Enterococcus

Giant freshwater prawns Macrobrachium rosenbergii (14–19 g) were challenged withEnterococcus (3×105 cfu prawn−1) previously incubated in TSB medium for 24 h, then placed in water having concentrations of dissolved oxygen (DO) at 7·75, 4·75, 2·75 and 1·75 mg l−1. Onset of mortality occurred after 6 h exposure to 1·75 mg l−1 DO, and after 12 h exposure to 2·75 mg l−1 DO. Cumulative mortality of prawns at 1·75 mg l−1 DO was significantly higher than that at 4·75 and 2·75 mg l−1 DO, and cumulative mortality of prawns at 4·75 and 2·75 mg l−1 DO was significantly higher than that at 7·75 mg l−1 DO after 96 h. The prawns (20–30 g) which had been placed in water for 0 to 120 h at 7·75, 4·75, 2·75 and 1·75 mg l−1 DO were examined for the THC (total haemocyte counts), DHC (differential haemocyte counts), phenoloxidase activity, respiratory burst, percentage phagocytosis and clearance efficiency. No significant difference in semi-granular cells and granular cells of prawns was observed among four treatments. The prawns following 120 h exposure to 2·75 mg l−1 DO decreased significantly the hyaline cells and THC by 39% and 36%, respectively. Phenoloxidase activity and respiratory burst decreased significantly by 33% and 11% when the prawns were exposed to 2·75 mg l−1 DO after 24 h, respectively. Percentage phagocytosis and clearance efficiency to Enterococcus decreased significantly by 44% and 54% for the prawns following 12 h exposure to 2·75 mg l−1 DO, respectively. It is concluded that DO as low as 2·75 mg l−1 and 4·75 mg l−1 causes depression in immune system ofM. rosenbergii , and increases its susceptibility to Enterococcus infection.