Influence of in-feed oxolinic acid therapy on the brain histoarchitecture and virulence factors of Streptococcus agalactiae infecting Nile tilapia Oreochromis niloticus

Streptococcosis and its management are major constraints to tilapia aquaculture. The present study assessed the effect of oral administration of 12 mg oxolinic acid (OA)/kg fish/day for 7 uninterrupted days against Streptococcus agalactiae LCR1 (Sa) infection and its influence on histopathological anomalies in the forebrain, optic tectum region of the mid-brain and granular cell layer of the cerebellum of Oreochromis niloticus. Besides, attempts were made to understand how the OA impacts the glycosyltransferases and CAMP factor of S. agalactiae through molecular docking. The LD₅₀ of Sa was 1.26 × 10⁸ cells/fish. Sa infection was apparent in OA-treated and untreated groups, and the brain tissues exhibited the progression and reversal of meningitis. The forebrain exhibited thickening of the meninx primitiva, vacuolation, and degeneration in the brain parenchyma and meninx primitiva. Inflammatory changes such as meningitis and mononuclear cell infiltration were documented. The midbrain had edematous optic tectum, loosening of connective tissue, and leukocyte infiltration. In the granular cell layer, cerebellum changes such as spongiform encephalopathy and necrosis indicate region-specific damage. However, the OA effectively reduced the severity of brain tissue damage, possibly by its binding affinities and upsetting the activities of glycosyltransferases and CAMP factor, as confirmed by molecular docking. These results confirmed that OA can cross the blood-brain barrier and interact with virulence proteins to reduce the Sa infection in the brain. Furthermore, the results underscore its responsible use in aquaculture, as OA is a critically important human medicine and ought to be used as a secondary treatment.