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

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Abstract This study was realized to investigate the lethal dose fifty (LD50) 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, 106, 107, 108, and 109 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 LD50 of Streptococcus pyogenes was 1×107 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 1×108 CFU/ml which showed high value led to 100% immediate death of the fish. On the other hand, the concentration 1×106 CFU/ml presented normal value with control which led to weak infection. The concentration 1×109 CFU/ml was violent and fish body can’t resist it

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