Effects of Aeromonas hydrophila on nonspecific immunity of Anguilla japonica in a challenge experimentation

Eels(Anguilla japonica) were randomly divided into the infected and control groups.Fish in the infected group were intraperitoneally injected with Aeromonas hydrophila suspension in the concentration of 3106 CFUmL-1,while those in the control group the same amount of sterile saline solution(0.85% NaCl).The eels were sacrificed to determine their nonspecific immunity on parameters including haematocyte count(i.e.,erythrocyte,leucocyte,lymphocyte and granulocyte),leucocyte phagocytic activity and serum antibacterial activity.The results showed that(a) the A.hydrophila infection increased the numbers of leucocytes and lymphocytes in the peripheral blood of A.japonica,and significantly reduced the number of granulocytes;(b) the phagocytic index and phagocytic rate increased on the 1st day after injection,and were very significantly different(P0.01) or significantly different(P0.05) on the 5th day and 9th day,as compared with the control;(c) on the 1st day after injection,the antibacterial activity of the infected fish also increased,and on the 9th day and 13th day,it was very significantly different(P0.01) from the control.The results suggested that the intraperitoneal injection enhanced A.japonicas resistance to the invading A.hydrophila by means of lymphocyte,especially,the proliferation of leucocytes,the increase in leucocyte phagocytic activity and the excretion of antibacterial substances.