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Hematological and biochemistry parameters are essential for the evaluation of fish health status; it helps provide essential information to diagnose fish diseases. This experiment was conducted to investigate the affect of Streptococcus agalactiae infection on hematological and biochemical parameters in red hybrid tilapia (Oreochromis sp.). The fish were inoculated intraperitoneally with 1 x10 4 cfu/mL bacteria then blood samples were collected and tested during the experiment. After seven days The red blood cell (RBC) counts decreased from 2.31 x 10 6 to 1.68 x 10 6. The hemoglobin (Hb) and hematocrit (PCV) counts decreased to 7.17 g/dL and 19.2% in the infected fish compared to the control group (9.71 g/dL and 28.20%). In contrast, the white blood cell (WBC) counts increased from 4.20x10 3 in the control fish to 13.2x10 3. The biochemical results initially showed an increase in glucose levels of the infected fish compared to the control from 53.82 to 101.75 on the 72 nd hour, respectively. There were no significant differences in the levels of serum total protein (T. prot), albumin (ALB), and aspartate transminase (AST) between the infected and the control fish. The inoculated fish displayed marked clinical signs such as erratic swimming behavior, exophthalmia and lethargy during the experimental. The present study has established that the changes in hematological, biochemical and clinical signs of fish can be used as an early diagnosis of pathological and physiological status in red tilapia culture. 

Streptococcosis in fish refers to re-emerging diseases causing high mortality in a variety of freshwater and marine fish throughout the Asia-Pacific region. The growth of the pathogen Streptococcus agalactiae isolated from red hybrid tilapia (Oreochromis sp.) in a brain heart infusion broth was investigated under a variety of environmental conditions. The results revealed the optimal growth temperature (25 °C and 30 °C), salinity (5 and 10 ppt), and pH (7 and 7.5) values. The effects of different environmental conditions on the susceptibility of red hybrid tilapia (Oreochromis sp.) to S. agalactiae were also investigated. Oreochromis sp., which were kept under various environmental conditions, were intraperitoneally injected with 0.1 mL of 1.56 × 105 cfu/ml S. agalactiae. Two weeks after infection, the cumulative mortality of the red hybrid tilapia held at 33 °C was significantly higher (P < 0.05) than those of the fish held at 20 °C and 25 °C. In addition, the cumulative mortality of the fish in water at pH 6 was also higher than that of the fish maintained in more alkaline water. The mortality rates of tilapia at 0, 5, 10 ppt salinities were not significantly different (P > 0.05). These results indicate that environmental conditions at 33 °C, 15 ppt, and pH 6 increased the susceptibility of red hybrid tilapia to S. agalactiae and probably adversely affected the fish's immune system