This study was conducted for isolation and identification of Streptococcus iniae from Oreochromis niloticus fish farms using traditional and molecular PCR techniques. Forty-seven isolates of Streptococcus were recovered from 232 fish samples from successive 4 seasons during 2015. Summer showed the highest prevalence with 34.9%. S. iniae was identified as the causative agent for streptococcosisin 3 seasons by PCR using S. iniae specific-16S rRNA primers. Experimental infection was performed to determine the pathogenicity of S. iniae for O.nloticusand the best antibiotics to be used for treatment of streptococcosis. S. iniae caused 70% mortalities in experimentally infected fish. Several histopathological alterations were observed in the liver, spleen, kidney and gills of the infected fish. The most effective antibiotics for treatment were Ampicillin with no mortalities, followed by Penicillin and Tetracycline with 10% mortality for both.
S. iniae, Oreochromis niloticus, antibiotics, Egypt.