St John's Wort ( Hypericum perforatum ) Oil on Immune Response, Growth Performance and Bacterial Resistance Against Lactococcus garvieae Pathogen in Rainbow Trout ( Oncorhynchus mykiss )

In this study, we investigated the immunomodulatory effects of St. John's Wort (Hypericum perforatum) oil on the growth performance, immune responses and resistance against Lactococcus garvieae in rainbow trout (Oncorhynchus mykiss). The plant oil was incorporated into feeds at concentrations of 1, 3, and 5 g kg⁻¹, designated as K1, K3, and K5, using a spray-coating method. A total of 12 tanks (100 L each) were used, with three replicates per group and 40 fish (10-11 g) stocked per tank. Trout fed with St. John's Wort oil exhibited significant improvements in immune parameters, particularly at the transcriptional level. Notably, after 40 and 60 days of feeding, the K3 group showed significant upregulation of immune-related genes, including IL-1β, IL-8, IFN-1, TNF-α, COX-2, TGF-β, and MHC-II in kidney and intestinal tissues. Following experimental challenge, survival rates reached 60% in K5, 50% in K3, and 35% in K1, compared with 30% in the control group. After 60 days, final body weights were 32.60 ± 0.34 g (K1), 34.83 ± 0.29 g (K3), 30.80 ± 1.46 g (K5), and 26.42 ± 0.36 g (control). The K3 group showed significantly higher final weight compared with all other groups (p < 0.05), whereas no significant differences were detected among the remaining treatments. These findings indicate that dietary inclusion of H. perforatum oil at 3 g kg⁻¹ enhances immune gene expression and disease resistance without compromising growth performance, suggesting its potential as an immunonutritional strategy in trout aquaculture.