An experimental challenge model for infecting African catfish (Clarias gariepinus) with Edwardsiella tarda.

Edwardsiella tarda is an important fish pathogen that causes outbreaks leading to significant losses in aquacultural species such as African catfish (Clarias gariepinus). In order to study host-pathogen interactions and design preventive strategies, it is essential to prepare a reliable experimental infection model. The aim of this study was to develop a reproducible immersion challenge model for E. tarda infection in C. gariepinus. The present study consisted of preliminary experiments and a main experiment evaluating the different routes of infection and the bacterial concentrations and exposure durations in an immersion bath. Preliminary experiments demonstrated that infection could be induced by either intraperitoneal injection or immersion bath, and that infection was more successful at higher concentrations. The main experiment involved two concentrations (108 and 2 × 108 CFU/ml) of immersion bath and two immersion durations (1 and 2 h), each repeated twice. The fish were monitored for clinical signs and mortality throughout the experiment and samples were collected on day 10 post-infection for haematological, immunological, and pathological assessment. High-dose immersion reliably induced infection, with disease progression depending on bacterial concentration and exposure duration. This established model is a practical and reproducible tool for future studies on the pathogenesis and immunity of C. gariepinus, as well as the use of immunostimulants and therapeutics.