Aeromonas hydrophila dose and post-immunostimulation period as immunomodulatory effect indicator on Nile tilapia (Oreochromis niloticus)

In fish challenge tests, there is no standardization in the concentration and analysis period of parameters. This study investigates the immunomodulatory and cytotoxic effects of A. hydrophila dose and postimmunostimulation periods on Nile tilapia. Four level of bacterial doses (½, ¼, ⅛, and 0 LD50) were administered, and immune parameters, hematological data, and erythrocyte abnormalities were analyzed at 3, 7, 10, and 14 days post-infection (dpi). The highest respiratory burst, total globulins, and lysozyme activity were detected at 7 dpi. Considering the bacteria doses, the lysozyme activity was higher in the ⅛ LD50 and ¼ LD50 of A. hydrophila. Erythrocytes, hematocrit, hemoglobin, leukocytes, lymphocytes, neutrophils, and monocytes counts remained unchanged. Ten days post-injection, the control group exhibited reduced abnormalities, showing a notable distinction from the ⅛ LD50 and ¼ LD50 treatments, particularly in hooked and notched nuclei abnormalities. Mortality rates were higher in infected groups, peaking at different times with the highest in ½ LD50. For evaluating Nile tilapia immunity post A. hydrophila infection, it is recommended ¼ LD50 dose and blood collection at 7 and 10 dpi for immunological parameters and abnormalities in Nile tilapia erythrocytes, respectively.