Shrimp consumption has increased steadily around the world, as has the emergence of bacterial pathogens, which cause massive economic losses. The incorporation of plant extracts in aquatic animal feeds has been suggested to improve growth performance and resistance against bacterial pathogens. Therefore, the aim of this study was to evaluate the effects of Cajeput Melaleuca cajuputi leaf extract (MCLE) on the growth performance, physiological responses and resistance of Giant freshwater prawn Macrobrachium rosenbergii against Aeromonas hydrophila. The phytochemical composition of the MCLE was analysed using gas chromatography–mass spectrometry (GC‐MS). Thereafter, the MCLE was included in diets (0.0, 5.0, 10.0 and 15.0 g/kg) fed to the M. rosenbergii. The growth was observed after 45 days of the feeding trial, whereas physiological responses and clearance efficiency (%) against A. hydrophila were observed on the 60th day. 2‐Isopropyl‐10‐methylphenanthrene, phenanthrene, 1‐methyl‐7‐(1‐methyl ethyl) and 4H‐1‐Benzopyran‐4‐one, 3‐acetyl‐5, 7‐dihydroxy‐2‐methyl‐ compounds were detected as the major compounds in the MCLE. A significant improvement (p < 0.05) of growth and survival of M. rosenbergii fed with MCLE supplementation at 15.0 g/kg was observed. The total haemocyte count, hyaline cells and granular cells were also increased (p < 0.05) in the M. rosenbergii fed with diets included with MCLE. Upon challenging the primed M. rosenbergii with sub‐lethal doses of A. hydrophila, the highest clearance efficiency (73%) was achieved in the prawn‐fed 15.0 g/kg MCLE based diet (p < 0.05) after 96 h post injection. These findings indicate the potential of MCLE supplementation to improve growth, immune responses of M. rosenbergii and their resistance against A. hydrophila.

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