Hematological and Immunological plasma assays for grass carp (Ctenopharyngodon idella) infected with Aeromonas hydrophila as an immune model in carp aquaculture

• We measured hematological parameters (RBC, WBC, HCT, and MCV) in Ctenopharyngodon idella infected with Aeromonas Hydrophila, found only the WBC significantly increased at 7–21 DPI compared to the control group.
• The levels of monocytes, neutrophils were significantly increased at first stage of infection, thrombocytes, and lymphocytes were significantly decrease at last stage of infection.
• We discovered rupture of RBCs inside the fish blood caused by Aeromanas hydrophila.
• The levels of Lysozyme and IgM were significantly difference between the treatment group and the control group at the mid-phase of infection.

Aeromonas hydrophila is the causative agent of bacterial septicemia, a common disease observed in grass carp, Ctenopharyngodon idella. In our study, C. idella specimens were infected with A. hydrophila, and parameters of Hematological and Immunological plasma parameters were monitored. At blood cell level, levels of red blood cells (RBCs), hematocrit (HCT), and mean corpuscular volume (MCV) showed no differences between the treatment and control groups, but levels of white blood cells (WBCs) increased. The monocyte and neutrophil varied significant according to stimulation by A. hydrophila at 1 DPI, the thrombocyte and lymphocyte at 14 and 21 DPI. At serum level, total protein, lysozyme, and IgM increased at the early infection phase and then decreased at other time points; however, peroxidase levels were significantly lower in the treatment group than that in the control group during the early infection phase. ACH50 was significantly higher in the treatment group than that in the control group during the late infection phase. On the basis of the results, we suggest that innate and adaptive immune mechanisms of C. idella are able to neutralize the virulence factors secreted by A. hydrophila. Our findings would help in understanding the mechanisms underlying resistance to infection by A. hydrophila.