Pathophysiological and Molecular Characterization of Streptococcus agalactiae in Farmed Nile Tilapia

This study investigated an outbreak of Streptococcus agalactiae infection in Nile tilapia (Oreochromis niloticus) farmed in Egypt, with a focus on pathogen identification and the resulting pathophysiological impacts. The investigation was carried out at a private aquaculture facility in the Manzala region, Dakahlia Governorate, during the summer of 2021. Clinically, affected fish showed signs of exophthalmia, skin haemorrhages, and marked congestion of internal organs. Bacteriological and molecular characterization using 16S rRNA gene sequencing confirmed S. agalactiae as the causative agent, with an overall prevalence of 15% among examined specimens. Experimental infection trials demonstrated significant oxidative stress responses, evidenced by increased levels of nitric oxide and malondialdehyde, and decreased antioxidant biomarkers including catalase (CAT), superoxide dismutase (SOD), glutathione (GSH), and glutathione peroxidase (GPx). Histopathological lesions were observed in multiple organs, including hepatic degeneration, renal tubular necrosis, splenic congestion, and meningoencephalitis. These findings highlight the systemic nature of S. agalactiae infection and underscore the urgent need for preventive strategies, early diagnostic protocols, and improved biosecurity measures to mitigate fish losses and safeguard aquaculture sustainability.