Pathology of Edwardsiella tarda infection in African catfish, Clarias gariepinus (Burchell 1822), fingerlings

Edwardsiella tarda is one of the serious fish pathogens infecting both cultured and wild fish species. This study aimed to assess the phenotypic characterization and pathogenicity of E. tarda isolated from Clarias gariepinus (Burchell) with dropsy and histopathological alterations. The causative agent was identified with Vitek 2, and its pathogenicity was determined by intramuscular injection. The challenged catfish exhibited vertical hanging, frothing, excess mucus production, listing, swollen abdomen, anorexia, fin and tail rot, and reddish operculum. The LD₅₀ of E. tarda PBB and PBP strains was found to be 8.52 × 10⁶ and 1.68 × 10⁷ cells fish^-1, respectively. Histopathological observations on catfish infected naturally revealed lymphocyte infiltration in muscle and focal necrosis, hyperplasia, edema, and swelling of the gill lamellar epithelium. The kidney of diseased fish exhibited ischemic type tubulopathy, necrosis of nephritic tubules, hyperplastic hematopoietic tissue, rupture of the tubular basement membrane, hydropic dystrophy of nephritic cells, neutrophil infiltration, fibrinoid necrosis of nephretic tubules, hemosiderin deposition, and edema. The liver sections revealed lymphocyte infiltration, dilation of hepatic sinusoids, expansion of space between hepatic sinusoids, and focal necrosis. The inflammatory responses observed in kidney and liver in the present study were presumably suppuration and were attributed to the potential virulence factors of E. tarda.