Oral vaccination of Grass carp (Ctenopharyngodon idella) against Aeromonas veronii as a sustainable strategy for disease control and biodiversity conservation

Ctenopharyngodon idella (C. idella), a freshwater species of significant ecological and economic value, plays a vital role in aquaculture and aquatic ecosystem management. However, bacterial pathogens such as Aeromonas veronii (A. veronii) represent a major threat to fish survival, biodiversity, and sustainable aquaculture production. The current research assessed the protective potential of a monovalent oral vaccine formulated against A. veronii in grass carp (C. idella). Experimental feeds were formulated by two delivery methods: spraying and incorporation with either fish oil or mineral oil, included as adjuvants (10%). Healthy fingerlings (20 ± 5 g) were randomly assigned to five dietary groups: spray vaccine + mineral oil (SV-MO), incorporated vaccine + mineral oil (IV-MO), spray vaccine + fish oil (SV-FO), incorporated vaccine + fish oil (IV-FO), and unvaccinated control (C). The feeding trial lasted 60 days. After 56 days of dietary vaccination, vaccinated fish demonstrated markedly (p < 0.05) improved growth, antioxidant capacity, serum biochemical profiles (including total protein, albumin, and globulin) and immunological responses (lysozyme activity and antibody agglutination) relative to the control group. The dietary group IV-FO achieved the highest growth performance (12.7 ± 0.15a g) and exhibited superior immune responses relative to other treatments. Challenge with live A. veronii demonstrated that IV-FO provided the greatest protection (87%), with only 13% mortality and a relative percent survival (RPS) of 85%. Protection rates were comparatively lower in SV-FO (67%, RPS 62%), SV-MO (67%, RPS 62%), and IV-MO (54%, RPS 70%), while the control group displayed minimal protection (14%) with 86% mortality. The findings indicate that oral delivery of vaccines, particularly via incorporation with fish oil, provides an effective strategy to enhance fish resilience against bacterial infection. Fish oil acted as an immunomodulatory adjuvant that strengthened antibody production, balanced oxidative stress, and improved mucosal defense, whereas mineral oil mainly stabilized the antigen and prolonged its immune-stimulating effect. These differences explain the superior vaccine performance observed in the incorporated fish-oil formulation (IV-FO). By improving fish health and reducing disease-induced mortality, this approach supports sustainable aquaculture practices while contributing to biodiversity conservation in freshwater ecosystems.