Phytobiotic-enriched diet enhances survival of European eels (Anguilla anguilla) infected with Vibrio vulnificus pathovar piscis

Intensive conditions experienced by aquaculture animals increase their susceptibility to bacterial diseases, which promotes antibiotic use and contributes to the emergence of more resilient and virulent strains. Therefore, finding complementary effective alternatives to antibiotic therapy is a priority in animal health. Among them, one promising approach is the use of functional diets. Previous results of our group have demonstrated the benefits of phytobiotics (formulated as microencapsulated essential oils) in improving the resistance of shrimp against Vibrio parahaemolyticus infection. Thus, the aim of this proof-of-concept study was to evaluate the effect of a phytobiotic-enriched diet on survival of eels against Vibrio vulnificus pathovar piscis. This pathogen is responsible for warm-water vibriosis, a major concern in eel farming, causing significant economic losses and posing a threat for public health. First, phytobiotics were screened for in vitro bactericidal activity against multiple V. vulnificus strains, and the most effective candidates were incorporated into diets (0.9 g/kg feed). After four or five weeks of feeding, innate immune parameters were assessed, and the UV-1 supplemented diet contributed to the increase of plasma lysozyme activity in eels fed for five weeks. Second, two concentrations of the UV-1 (0.9 or 1.8 g/kg) were evaluated in terms of disease resistance after immersion challenge with the pathogen. After a controlled feeding trial, UV-1-enriched diets reduced mortality and significantly delayed disease progression, increasing survival by 1.9 days compared to the control group. These findings reinforce the potential of the UV-1 phytobiotic for developing functional diets that improve fish health (protection against bacterial diseases) and reduce antibiotic dependence in relevant cultured aquatic species.