Pathogenicity of Streptococcus pyogenes and its correlation with hematological alterations in infected Nile tilapia (Oreochromis niloticus) fingerlings

This study was realized to investigate the lethal dose fifty (LD 50 ) of Streptococcus pyogenes on Nile tilapia and to determine the effect of every dose on hematological parameters as indicator of infection. In this study, a total number of 100 healthy Nile tilapia ( Oreochromis niloticus ) fingerlings (15 ± 0.5 g) were infected with Streptococcus pyogenes . These fish were randomly divided into five groups, each with 20 fingerlings. Five concentrations of Streptococcus pyogenes (0, 10 6 , 10 7 , 10 8 , and 10 9 CFU/ml) were used to realize the objective of this study. Group 1 was injected inter-perennial with 0.2 ml sterilized saline solution and it was considered as a control group. The other 4 groups were injected inter-perennial at a rate of 1ml per 10 g of inoculum. All experimental groups were examined for infection signs, mortality rate and hematological parameters. The results revealed that the LD 50 of Streptococcus pyogenes was 1×10 7 CFU/ml with ten days of tilapia infection. Also, it affected significantly on the hematological parameters and this concentration showed moderate value of infection and mortality rate compared with 1×10 8 CFU/ml which showed high value led to 100% immediate death of the fish. On the other hand, the concentration 1×10 6 CFU/ml presented normal value with control which led to weak infection. The concentration 1×10 9 CFU/ml was violent and fish body can’t resist it