Effects of dietary Scote pine (Pinus sylvestris) essential oil on growth performance, digestive enzymes, immunological and antioxidant responses to Aeromonas hydrophila infection in rainbow trout (Oncorhynchus mykiss).

This study aimed to evaluate the effects of Scots pine (Pinus sylvestris) essential oil (EO) on growth performance, digestive enzyme activities, immunological and antioxidant parameters, and disease resistance in rainbow trout (Oncorhynchus mykiss). Over a 60-day period, fish were fed diets supplemented with graded levels of EO: 0 mg/kg (control; CTL), 100 mg/kg (100EO), 200 mg/kg (200EO), and 400 mg/kg (400EO), followed by an experimental infection with Aeromonas hydrophila. At the end of the rearing period, significant improvements were observed in growth performance, feed efficiency, digestive enzymes' activity, and skin mucus immunological parameters in the EO-treated groups compared to the CTL group. The highest specific growth rate (3.7-3.20 vs 2.85), gut amylase (17.0-19.1 vs. 15.5) and protease (5.8-6.0 vs. 5.1) activities, skin mucus lysozyme (23.5-26.2 vs. 15.3), peroxidase (11.4-12.0 vs. 10.4) and protease (17.9-20.3 vs. 13.3) activities, and total immunoglobulin (Ig) levels (0.20-0.24 vs. 0.18), post-challenge survival (60.0-66.7 vs. 43.3), and lowest feed conversion ratio (0.9-0.92 vs. 1.18) were observed in the 200EO and/or 400EO, compared to the CTL group. Also, these parameters showed quadratic relationships with dietary EO levels. Additionally, dietary EO supplementation significantly enhanced hematological parameters, including total leukocyte and neutrophil percentage, plasma albumin, globulin, lysozyme, alternative complement activity (ACH50), total Ig, gut lysozyme, and total Ig, as well as hepatic antioxidant parameters such as superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), total antioxidant capacity (TAC), and reduced glutathione (GSH), with the highest values generally observed in the 200EO and 400EO treatments. Conversely, blood lymphocyte percentages, plasma cortisol levels, and hepatic malondialdehyde (MDA) concentrations significantly decreased along with increasing dietary EO supplementation, particularly in the 200EO and 400EO groups. In conclusion, this study demonstrates that dietary EO serves as an effective feed additive for rainbow trout, enhancing growth rates, feed efficiency, humoral and mucosal immunity, and antioxidant status. Collectively, these benefits enhance resistance to aeromonad septicemia. Based on the results, a dietary supplementation of 200 mg/kg EO is recommended for rainbow trout farming.