The effects of dietary oregano essential oil on the growth performance, intestinal health, immune, and antioxidative responses of Nile tilapia under acute heat stress

Climatic changes are impacting the aquaculture industry and result in low productivity and high economic loss. Herein, we evaluated the long-term acute heat stress on the productivity and performances of Nile tilapia and the ameliorative role of oregano essential oil. Fish were raised in outdoor concrete tanks during the summer season under high water temperature (32 degrees C) and fed four test diets supplemented with oregano essential oil at 0, 0.25, 0.5, and 1 g/kg for eight weeks. The final weight, weight gain, and specific growth rate were markedly enhanced (P < 0.05) by 0.5 and 1 g oregano essential oil/kg diet. Nevertheless, the feed conversion ratio was reduced by oregano essential oil (P < 0.05). The foregut, midgut, and hindgut of the intestines of fish-fed oregano essential oil showed apparent branching of villi and increased villi length and width, with the highest being in fish treated with a 1 g/kg diet (P < 0.05). The activities of aspartate aminotransferase and alanine aminotransferase were markedly reduced (P < 0.05) in Nile tilapia fed 0.5 and 1 g oregano essential oil/kg. At the same time, the total protein, albumin, and globulin levels were higher (P < 0.05) in fish-fed oregano essential oil than in the control. The superoxide dismutase, catalase, phagocytic index, and lysozyme activity were meaningfully increased (P < 0.05) in Nile tilapia fed oregano essential oil and raised under acute heat stress. The concentration of malondialdehyde and relative expression of heat shock protein 70 (HSP70) were markedly decreased (P < 0.05) in fish treated with oregano essential oil, with the lowest level in fish fed 1 g/kg. In conclusion, dietary oregano essential oil relieved the impacts of heat stress on Nile tilapia by increasing the growth performance, regulating the blood biochemical traits, improving the immune and antioxidative responses.