Phytogenics enhance welfare and vaccine efficacy against Vibrio anguillarum in European seabass (Dicentrarchus labrax) juveniles

Disease prevention is pivotal in aquaculture, and while vaccines offer protective immunity, challenges such as cost and low efficacy persist. The present study investigated the potential of plant-derived compounds, known as phytogenics, to bolster the effectiveness of vaccines against vibriosis in European seabass. Two phytogenic blends, namely PHYTO1 (terpenes) and PHYTO2 (terpenes and flavonoids) were supplemented to a commercial diet to obtain three experimental diets: a non-supplemented control diet, PHYTO1 (a 200-ppm blend of garlic and Lamiaceae oils with 87.5 mg kg(-1) terpenes), and PHYTO2 (a 1000 ppm blend containing citrus fruits, Asteraceae and Lamiaceae oils with 57 mg kg(-1) terpenes and 55 mg kg(-1) flavonoids). Following vaccination by bath immersion, juvenile European seabass were divided into groups and fed one of the three diets for 30 days. After this feeding period, fish were anesthetized and boosted with a single dose of vaccine through intraperitoneal injection. They continued to be fed their respective diets for another 30 days. At day 60, after the priming vaccination, fish were challenged with Vibrio anguillarum via intraperitoneal injection. Various parameters were measured at different time points post each vaccination, including total weight, circulating plasma cortisol and glucose levels, serum immunoglobulin M (IgM) titers, antioxidant power of leucocytes, and the expression of several antioxidant and immune-related genes. The results showed that fish fed with phytogenic supplements did not differ in weight compared to the control group. However, they exhibited lower plasma cortisol and glucose levels, increased IgM titers, and enhanced antioxidant protection and antioxidant power of head kidney leucocytes. In addition, phytogenics upregulated several immune-related genes in the gills and head kidney immediately after each vaccination. Notably, PHYTO2, enriched with flavonoids and terpenes, exhibited an even more pronounced positive effect on boosted fish by reducing vaccine-associated stress while improving antioxidant protection and modulating the vaccine-induced immune response. This synergistic effect of vaccination combined with phytogenics introduces new pathways for enhancing fish health in aquaculture.