Effects of Streptococcus agalactiae infection and oral florfenicol administration on the hemato-biochemistry, erythrocyte morphology and histopathology of Oreochromis niloticus

Streptococcosis is considered one of the most important diseases affecting farmed tilapia, causing severe economic losses. Antimicrobial therapy is the principal control measure applied during outbreaks. This research assessed the efficacy of florfenicol (FFC) when fed at 15 mg kg biomass–1 day–1 for 10 days against Streptococcus agalactiae infection in Oreochromis niloticus in terms of survival, changes in hemato-biochemistry, erythro-morphology, and histoarchitecture of the vital organs. Streptococcus agalactiae was moderately virulent to O. niloticus with a lethal dose (LD50) of 1.26×108 cells fish–1 and sensitive to FFC with a minimal inhibitory concentration of 0.78 μg mL–1. It caused systemic infection in tilapia. FFC therapy reduced bacterium-induced mortality and physiological stress. The histopathological findings indicated significant improvement in the kidney and liver tissues of fish. The erythrocyte morphological alterations noted in the challenged fish were irregularly shaped, elongated, crenated, and teardrop cells, hypochromic erythrocytes, ragged cytoplasmic membranes, hypertrophied nuclei, eccentric nuclei, and vacuolation, all of which were mitigated with FFC therapy. Eventually, FFC therapy improved wound healing, normalized plasma biochemistry, and aided recovery from bacterial infection. This study revealed that the therapeutic dose of FFC was effective against S. agalactiae infection in O. niloticus and lessened the physiological stress.