Pathogenicity and systemic virulence of Aeromonas veronii in red-bellied pacu (Piaractus brachypomus): an experimental disease model

Abstract

Aeromonas veronii is an important etiological agent of motile aeromonas septicemia (MAS) in aquaculture; however, its pathogenicity in the emerging cultured species red-bellied pacu (Piaractus brachypomus) remains poorly understood. Pacu fingerlings (10±2 g; n = 180) were intraperitoneally challenged with A. veronii strain BLB-01 (GenBank accession: MF370515) at concentrations ranging from 10⁵ to 10⁹ CFU mL⁻¹, following OECD guideline 203. The median lethal dose (LD₅₀) was estimated using probit analysis. Clinical signs, haematological and biochemical parameters, and histopathological alterations were evaluated at 96 h post-LD₅₀ exposure. Probit analysis (Y = 0.591 + 2.185 log₁₀ concentration; R² = 0.984) estimated the LD₅₀ as 7.8×10⁶ CFU mL⁻¹ (95% CI: 5.2–11.4 ×10⁶ CFU mL⁻¹). Infected fish exhibited progressive MAS symptoms over 7 days. Haematological analysis revealed significant anaemia (decreased haemoglobin and total erythrocyte count), leukocytosis (increased total leukocyte count), and thrombocytopenia (reduced platelet count). Biochemical changes included stress-induced hyperglycaemia (elevated glucose), hepatocellular damage (increased ALP, AST, and ALT), and hypoalbuminemia (reduced albumin). Histopathological examination demonstrated severe lesions, including gill lamellar fusion (2.8±0.3), hepatic vacuolation (2.9±0.2), and renal tubular necrosis (2.8±0.2). Aeromonas veronii exhibits high pathogenicity in pacu, with an LD₅₀ of 7.8×10⁶ CFU mL⁻¹, providing baseline data for challenge studies and the development of vaccines and probiotics. A combination of haematological and biochemical markers particularly increased TLC, reduced platelet count, and elevated liver enzymes offers a practical tool for rapid field-level diagnosis and surveillance of MAS, supporting improved health management in Indian pacu aquaculture.