Dietary oregano essential oil improved antioxidative status, immune-related genes, and resistance of common carp (Cyprinus carpio L.) to Aeromonas hydrophila infection.

The use of herbal essential oils in aquafeeds is an important approach to maintain the fish health status. The oregano essential oil (OEO), extracted from Origanum vulgare, was included at levels of 0, 5, 10, 15, and 20 g per kg diet to evaluate its beneficial effects on antioxidant, immunity status, and expression of immune-related genes of common carp, Cyprinus carpio L. After 8 weeks, dietary inclusion of OEO significantly increased the activities of hepatic superoxide dismutase and catalase (P < 0.05); meanwhile, levels of hepatic malonaldehyde significantly decreased (P < 0.05). The levels of serum lysozyme activity, phagocytic activity, and phagocytic index have been linearly and quadratically increased by dietary OEO in a dose-dependent regime showing their highest levels in 15-20 g OEO/kg diet. Moreover, dietary OEO significantly upregulated the relative expression of hepatic interleukin-1beta (IL-1β) and interleukin-10 (IL-10) genes (P < 0.05). Dietary OEO maintained the normal histomorphological criteria and density of melanomacrophage centers of spleens, and hematopoietic tissues of the anterior kidneys in all fish groups with respect to the control. After challenge with pathogenic bacterium Aeromonas hydrophila, the 10-day cumulative mortality significantly decreased (P < 0.05); meanwhile, the relative percent of survival of common carp significantly increased by OEO administration (P < 0.05) in a dose-dependent manner. The obtained results reflect the beneficial roles of dietary OEO to boost the antioxidative status and the immune responses in common carp with an optimum level of 15 g/kg diet.