Studies On The Pathogen And Immune Intervention Of Fulminant Septicaemia Megalobrama Amblycephala

Fulminant septicemia is a widespread disease which harm to aquaculture fish. In this research, the diseased Megalobrama amblycephala were collected from aquaculture farm around Nanchang, the isolation and identification of pathogen, artificial infection and immunization intervention were carried on. The results are as follows:1. Two germs 07-A and 07-B were isolated from lesions of diseased fish and which could cause disease and death in healthy fish by artificial injection and immersion experiments. It was confirmed by recursive infection that the germs were the pathogens of the fulminant septicemia. Morphological observation revealed that the germs were gram-negative bacteria, which was identified as Aeromonas hydrophila by biochemical detection and analysis of 16SrDNA sequence.2. Antigen were prepared by two different methods, one was inactivating bacteria for 24h with 0.2% formalin under the condition of temperature, 25℃, and the other was inactivating for 24h with 0.5% formalin under the condition of temperature, 4℃. Experiment of immunization intervention to blunt snout bream showed that both antigen could stimulate the strong immunization reaction of fish by injecting them at the bases of pectoral fins to immune the fish.3. Effect of immunization was evaluated by detecting the agglutinating antibody titer in the serum, NBT-positive cells and the phagocytic percentage of leucocytes in the blood. After immunized by injection, the agglutinating antibody titer in the serum rose gradually, the average titer of two immunization groups reached the peak, the highest titer was 1:256, then started to go down. The amount of NBT-positive cells in two immunization groups is significantly higher than the control group (t-test P<0.05), and the amount of NBT-positive cells reached the peak at 21d. The phagocytic percentage of leucocytes in the blood of immunization groups also significantly enhanced(t-test P<0.05), the highest phagocytic percentage of immunization group 1 was over than 50%, and group 2 was over than 45% too, but the control group was under 35%.4. The result of challenge showed that the relative percent of survival (RPS) of two immunization groups were 76.9% and 61.6% respectively, it manifested that the two kinds of methods could make fish obtain some immunity. All the three immunization indexes and RPS detection showed that effects of the vaccine inactivated with 0.2% formalin was better than the vaccine inactivated with 0.5% formalin, but there was no significant difference between them.