Effects of infection modes of Streptococcus agalactiae on blood routine and antibody fluctuation of tilapia （Oreochromis niloticus）

【Objective】To clarify whether skin injury or inflammation caused by some reasons was an important inducement of tilapia Streptococcus agalactiae disease outbreak, and provide reference for tilapia aquaculture epidemic early warning and development of S. agalactiae vaccine.【Method】A total of 180 Gift Oreochromis niloticus with a body weight of about 40 g were randomly dividedt into three groups named heathy group, skin-injury group, and enteritis group.They were infectedS. agalactiae(1.5×10⁶ CFU/mL) by immersion. At the same time, three blank groups(not infected with S. agalactiae) were set up as a control. When at eight periods of time before tilapia infected with S. agalactiae and at 24, 48, 72, 96, 120, 144 and 168 h after infection, fifteen tilapias were randomly chosen in each group for blood sampling. The blood was smeared and stained with Giemsa for observing the cell morphology and counting the number of peripheral blood leukocytes, neutrophils, monocytes and lymphocytes. The number of dead tilapias was recorded at the same time. The samples of serums were subjected to ELISA to determine the IgM antibody titre against S. agalactiae once a week for 10 weeks.【Result】Compared with healthy group, the dead time of skin-injury group and enteritis group was brought forward, and the survival rates were significantly lower. The number of total leukocytes, neutrophils and lymphocytes in the peripheral blood of three infection modes all increased at the initial phase of infection. While the skin-injury group and enteritis group were extremely significantly higher than that in the healthy group (P<0.01, the same below), indicating that tilapia with skin-injury and enteritis after infected by S. agalactiae had stronger immune response than healthy tilapia. The number of monocytes in the skin-injury group after infection was extremely significantly higher than healthy group at 72-168 h, and that of enteritis group was extremely significantly higher than healthy group at 24 h after infection. The serum antibody level of tilapia in enteritis group increased earlier and lasted longer than that in healthy group and skin-injury group. The specific antibody appeared at similar time in healthy group and skin-injury group, but maintained high level for longer in skin-injury group.【Conclusion】Mechanical or immune pathological damage and inflammation in tilapia before and after S. agalactiae infection would lead to increased infection mortality. That is, the infection of S. agalactiae would aggravate the skin damage of tilapia due to transfer, transportation, parasite infection, or enteritis caused by feed mildew, which would lead to an increase in the mortality of tilapia. The variation of tilapia's blood routine and antibody levelscan reflect the infection status of S. agalactiae. Among them, changes in the number of peripheral blood leukocytes can be used as a reference index for pathological damage and immune response of tilapia infected with S. agalactiae.