Nutritional modulation using Nigella sativa with or without feed restriction as a novel strategy to boost growth and immunity and combat edwardsiellosis in Oreochromis niloticus

Abstract Medicinal plants occupy a privileged place and are considered one of the most important natural phytogenic feed additives. Black cumin ( Nigella sativa , NS) and its derivatives have several beneficial therapeutic properties for aquaculture farms. This study investigated the effects of NS with or without feed restriction on growth performance, haemato-biochemical profile, immune and oxidant/antioxidant status, as well as the resistance of Oreochromis niloticus ( O. niloticus ) to Edwardsiella tarda ( E. tarda ) challenge. A total of 360 fish with initial body weight (mean ± SE) 26.68 ± 0.08 g were randomly distributed into four groups with nine replicates (10 fish each) as follows: ad libitum control (Ad C), ad libitum Nigella sativa (Ad NS), restricted control (Res C), and restricted Nigella sativa (Res NS). The O. niloticus in the first (Ad C) and third groups (Res C) were fed with the basal control diet without NS supplementation. The second (Ad NS) and fourth (Res NS) groups were fed on diets supplemented with 3% NS. The fish in the Ad C and Ad NS groups were fed ad libitum during the experimental period, while the Res C and Res NS groups were subjected to 2 days of feed restriction weekly (third and seventh days of the week). At the end of the feeding trial (2 months), the experimental groups were combated against edwardsiellosis by intraperitoneally injection of 0.1 mL (1 × 10 5  CFU) E. tarda. The survival of the fish was evaluated for a further 7 days. The results revealed that growth metrics of fish were substantially higher in the Ad NS group, followed by the Ad C and Res NS groups. The feed conversion ratio (FCR) was substantially lower in the feed-restricted groups compared to the ad libitum groups. Biochemical analysis showed a significant decrease in hepatic enzymes as well as glucose levels in all experimental groups compared to Ad C. Moreover, total cholesterol and LDL-c were reduced ( P < 0.05 ) in both Nigella groups. The growth hormone and leptin were higher in all groups, particularly Res NS group  compared to the Ad C group however, cortisol level significantly reduced in the Res NS group only compared to the control group. There were substantial elevations in lysozyme and IgM in all groups, particularly the Res NS group. Also, the oxidant/antioxidant status had been improved mostly in the Res NS group. Histopathological examination revealed that combining NS with feed restriction maintained normal histology in the intestine, liver, kidney, and spleen. Furthermore, the combined treatment significantly reduced mortality rates following challenges with E. tarda , highlighting a potential protective role. In conclusion, dietary intervention of 3% NS with or without feed restriction can enhance fish productivity, immunity, organ function, and disease resistance. Moreover, NS rectifies the detrimental impact of feed restriction on performance. Lastly, our study suggests combining NS with feed limitation as a novel approach to improve growth, immunity, and resistance of O. niloticus to edwardsiellosis.