Epidemiology and immunoprophylaxis of lactococcosis in European Sea bass (Dicentrarchus labrax): A comprehensive study from Egyptian Aquaculture.

Lactococcosis, caused by Lactococcus garvieae, is an emerging threat to the expanding European sea bass (Dicentrarchus labrax) aquaculture industry in Egypt. This study provides a comprehensive investigation into the pathogenicity, epidemiology, and immunoprophylaxis of L. garvieae from outbreaks in the Borg El-Arab and Mothalus El-Deeba provinces. We found that infected farms exhibited significantly deteriorated water quality, characterized by elevated ammonia, nitrite, pH, and reduced dissolved oxygen. Bacteriological and molecular analysis of 300 moribund fish revealed a high prevalence of L. garvieae (67.3 %), with significant geographical variation and a distinct organotropism for the liver and brain. The local isolates demonstrated a concerning antibiotic resistance profile, showing complete resistance to ciprofloxacin and tetracycline but full sensitivity to streptomycin and enrofloxacin. Experimental challenge established the median lethal dose (LD₅₀) at 2.31 × 10⁶ CFU/fish, confirming the strain's virulence. To develop a control strategy, we evaluated four inactivated whole-cell vaccine preparations (formalin-killed cells (FKC), chloroform-killed cells (CKC), phenol-killed cells (PKC), and heat-killed cells (HKC)) administered via intraperitoneal, immersion, or combined routes. The combined vaccination route using either FKC or CKC elicited the most robust immune protection, generating the highest specific antibody titers (up to 8.34 log₁₀), enhancing phagocytic activity (up to 39.21 %), and improving key hematological parameters. This regimen resulted in the highest relative percent survival (RPS) of 53.04 % and 45.41 % for FKC and CKC, respectively, and significant histopathological improvements in the kidney and spleen post-challenge. Our findings confirm L. garvieae as a primary pathogen in Egyptian sea bass aquaculture. They also demonstrate that a vaccination protocol using formalin- or chloroform-killed bacterins delivered through a combined prime-boost strategy offers a promising approach for disease management, offering partial protection that warrants further optimization for sustainable application.