Responses of giant freshwater prawn (Macrobrachium rosenbergii) to challenge by two strains of Aeromonas spp.

The virulence of two Aeromonas strains (A. veronii and A. caviae) isolated from the hepatopancreas of apparently healthy giant freshwater prawns (Macr obrachium rosenbergii) was compared using a challenge by injections. For th e A. veronii strain, challenge with 3.7 x 10(5) cells/g of body weight led to 100% mortality; for the A caviae strain, 3.8 x 10(6) cells/g produced 10 0% mortality. The 50% lethal doses (LD50) were 2.0 x 10(3) cells/g for A. v eronii and 51.2 x 103 cells/g for A caviae. Use of different culture media (trypti-case soy broth vs prawn muscle extract) did not significantly affec t the virulence of A veronii. Injection of a sublethal dose (1 x 10(3) cell s/g) of A. veronii led to a significant decrease in the total hemocyte coun t (THC) between 4 and 24 h after injection. Saline injections also caused a similar though less decrease in THC. In the first 24 h after injection of A. veronii (1 x 10(3) cells/g), the change in the percentages of granulocyt es (both granular cells and semigranular cells) in the hemolymph was signif icantly different. After a significant initial increase, the percentage of hyaline cells fell by a factor of 4, from 9 to 2%. Phenoloxidase activity i ncreased fourfold immediately after injection and returned to preinjection levels at 24 h. (C) 2000 Academic Press.