Immune-Stimulatory Effects of Dietary Tea Tree Essential Oil on Growth Performance and Aeromonas hydrophila Resistance in Nile Tilapia Juveniles Fed on a Low-Fishmeal Diet
The tea tree (Melaleuca alternifolia) essential oil (TTEO) exhibited in vitro antioxidant activities with an inhibition concentration (IC50) of 29.02 ± 0.57 µg/mL. This is due to its content of bioactive compounds; practically α-pinene (1.52%), eucalyptol (9.05%), 2-bornanone (57.94%), 1-dotriacontanol (1.82%), 1-decanol, 2-hexyl- (1.99%), and 1-heptatriacotanol (4.90%). Therefore, this research aimed to determine changes in growth, antioxidant capacity, immune functions, and disease resistance in Nile tilapia (Oreochromis niloticus) when fed on a low-fishmeal (FM) diet enriched with TTEO. Nile tilapia juveniles (14.6–15.9 g) were fed on a low-FM diet containing 0.0 (the control), 0.1, 0.15, 0.2, and 0.25 g TTEO/kg feed up to apparent satiety three times a day for 70 days. Following the feeding trial, fish were intraperitoneally injected with Aeromonas hydrophila, and fish mortality was recorded for an additional 10 days. It is noted that feeding the fish on 0.2 g TTEO/kg feed increased the feed intake as well as the growth indices. Marked declines in FCR values were seen in fish fed on 0.15–0.25 g TTEO/kg feed. However, fish fed on a diet with 0.2 g TTEO/kg feed exhibited highest enhancements in intestinal villi width/length and absorption area compared with other TTEO levels. No statistical (p > 0.05) differences were observed in glucose and cortisol levels among TTEO-fed groups, indicating no stress effects on fish. Conversely, ALT and AST activities in Nile tilapia declined significantly (p < 0.05), especially at 0.20 and 0.25 g TTEO/kg feed, with no difference (p > 0.05) between the two treatments. It is noted that high TTEO levels (0.2–0.25 g/kg feed) increased hepatic superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx) activities as well as the mRNA expression of hepatic SOD, CAT, and GPx genes. Compared with other TTEO groups, feeding the fish with 0.2 g TTEO/kg feed showed higher alkaline phosphatase, lysozyme, complement C3, and ACH50 levels, along with marked increases in IL-8 and IL-10 genes expression and declines in the expression TNF-α, IL-1ß, HSP70, Caspase-3, and Caspase-9 genes. After A. hydrophila infection, fish fed on the control diet exhibited the highest mortality (76.7%), whereas fish fed on 0.2 and 0.25 g TTEO/kg feed were more resistant to bacterial infection, with mortality rates of 23.3% and 26.7%, respectively, with no difference (p > 0.05) between them. The results confirmed that dietary TTEO enhanced growth, antioxidant capacity, immunological biomarkers, and disease resistance in Nile tilapia when added to low-FM diets, with an optimum dose of 0.2 g TTEO/kg feed.