Adjuvanted Edwardsiella anguillarum vaccine confers protection and cross-protection against E. piscicida in Japanese eel (Anguilla japonica).

Edwardsiellosis, caused by Edwardsiella species, is a major bacterial disease in eel aquaculture in East Asia. However, effective vaccines for Japanese eel (Anguilla japonica) remain unavailable. In this study, a formalin-killed Edwardsiella anguillarum vaccine (FKC) was evaluated with two adjuvants: squalene-aluminum hydroxide (Squ) and Montanide™ ISA 760 VG (ISA). Protective efficacy varied among formulations, with survival increasing from FKC (26.7%-65%) to FKC-Squ (55%-85%) and highest in FKC-ISA (80%-95%). FKC-Squ induced early protection, whereas FKC-ISA provided sustained protection up to 12 wpv. Cross-challenge with E. piscicida showed significant protection only in the FKC-ISA group (76.5%-90% survival). Agglutination titers were elevated only against the homologous antigen, appearing earlier in the FKC and FKC-Squ groups but later in the FKC-ISA group. In contrast, ELISA responses were higher in the FKC-Squ and FKC-ISA groups against both antigens. Western blot analysis revealed shared immunogenic proteins between the two species, with adjuvant-dependent banding patterns. Histology showed persistent residues and granulomatous inflammation in the FKC-ISA group, whereas FKC-Squ caused milder tissue reactions. Collectively, these findings demonstrate that the adjuvanted E. anguillarum vaccine provides effective and sustained protection in Japanese eel and confers moderate cross-protection against E. piscicida, likely due to shared immunogenic proteins between the two species, with the level of protection influenced by the adjuvant formulation.