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 (Macrobrachium rosenbergii) was compared using a challenge by injections. For the 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 100% mortality. The 50% lethal doses (LD50) were 2.0 x 10(3) cells/g for A. veronii and 51.2 x 10(3) cells/g for A. caviae. Use of different culture media (trypticase soy broth vs prawn muscle extract) did not significantly affect the virulence of A. veronii. Injection of a sublethal dose (1 x 10(3) cells/g) of A. veronii led to a significant decrease in the total hemocyte count (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 granulocytes (both granular cells and semigranular cells) in the hemolymph was significantly different. After a significant initial increase, the percentage of hyaline cells fell by a factor of 4, from 9 to 2%. Phenoloxidase activity increased fourfold immediately after injection and returned to preinjection levels at 24 h.