Dietary Ocimum gratissimum Leaf Extract Modulates Growth Performance, Haemato-Biochemical Responses, Immune...

The present study evaluated the effects of dietary Ocimum gratissimum leaf extract (OGE) on growth performance, haemato-biochemical responses, immune status, and tissue integrity in Labeo rohita (Hamilton, 1822). Among different solvent fractions, ethanol and acetone extracts exhibited superior antibacterial activity against major fish pathogenic bacteria. The ethanolic extract demonstrated strong bactericidal activity against Edwardsiella piscicida, Aeromonas veronii, A. sobria, Serratia nematodiphilia, and Pseudomonas putida, with minimum inhibitory concentrations ranging from 0.0078 to 0.0156 mg mL^-1. Time-kill kinetics further confirmed concentration- and time-dependent antibacterial effects, with ≥ 3× MIC producing marked reductions in viable bacterial counts. A 90-day dose standardisation feeding trial using graded dietary inclusion levels of OGE (1×-4× MIC) identified supplementation at 3× MIC for 60 days as the optimum regimen for sustained enhancement of innate immune and antioxidant responses in L. rohita. Fish fed the 3× MIC diet exhibited markedly higher respiratory burst activity, ceruloplasmin, antiprotease activity, myeloperoxidase, and ferric reducing antioxidant power (FRAP). Dietary OGE supplementation markedly improved growth performance, including final body weight, weight gain, specific growth rate, and feed conversion ratio. Haematological indices showed no adverse changes during the feeding phase but improved significantly following A. veronii challenge, with elevated RBC, WBC, haemoglobin, and packed cell volume in OGE-fed group. Serum biochemical analyses revealed transient elevations of glucose, cholesterol, total protein, ALT, AST, LDH, and CRP in OGE-fed fish, accompanied by faster recovery towards basal levels following infection. Dietary OGE supplementation also improved post-challenge survival against A. veronii infection, resulting in a relative percent survival (RPS) of 71.4%. Histopathological severity scoring further demonstrated significantly reduced renal and hepatic lesion scores in OGE-fed fish, whereas the positive control group developed progressive lesions characterised by tubular degeneration, glomerulonephritis, hepatocellular necrosis, vacuolation, and cellular hypertrophy. Overall, dietary supplementation with OGE at 3× MIC for 60 days effectively enhanced growth, immune competence, antioxidant defence, haemato-biochemical stability, and tissue integrity in L. rohita, highlighting its potential as a sustainable phytogenic feed additive in aquaculture.