Examination of entry portal and pathogenesis of Edwardsiella ictaluri infection in striped catfish, Pangasianodon hypophthalmus

Abstract

The host-bacterium interaction between striped pangasius catfish (Pangasianodon hypophthalmus) and Edwardsiella ictaluri was examined during experimentally induced bacillary necrosis of pangasius (BNP). After infection by immersion challenge, fish samples were taken over the course of the infection. Necrotic dermatitis, associated with vasculitis-perivasculitis changes, was the most notable gross and histopathological change observed, similar to that described during channel catfish enteric septicemia. Typical histopathological change associated with BNP were initially observed by 96h post-infection (hpi) and these lasted until 14days post infection (dpi) with no granulomatous formation evident. The presence of the bacterium was observed in various organs using immunohistochemistry (IHC), except for the brain, with positive staining observed within differing cell types, including phagocytic cells, myocardia, gill epithelia and capillary endothelia. No extracellular deposition of bacterial antigen was observed. Bacterial antigen persisted up to 1month in the independent phagocytic cells, necrotic-associated phagocytic cells and melano-macrophage centers, confirming the intracellular nature of E. ictaluri disseminated in the tissues during septicemia. The bacterium was evident in the gills early in the infection (6–12 hpi) and in necrotic skin at 72hpi. With the histopathological changes observed, this suggests that skin is another possible natural route for disseminating E. ictaluri in pangasius catfish during infection. Statement of relevanceThis information will improve our understanding of the pathogenesis of BNP, and establish more effective methods of control for this important disease, including methods for quantifying sub-lethal infections and assessing response against the invading pathogen.