LD50 determination The mortalities of Nile tilapia in the first 2 weeks after intraperitoneal injection of different dilutions of S.agalactiae are shown in Table 3. S. agalactiae was isolated from both the livers and kidneys of all dead fish. Based on the mortality, the calculated LD50 of S. agalactiae for Nile tilapia was 10−3.20, which is equivalent to 3.79×105 CFU/mL.