Field Prevalence and Pathological Features of Edwardsiella tarda Infection in Farmed American Bullfrogs (Aquarana catesbeiana)

Edwardsiella tarda is a zoonotic facultative intracellular bacterium whose impact on farm-raised amphibians is still poorly defined. We recovered seven strains from American bullfrogs (Aquarana catesbeiana) on four farms in Guangdong, China, and combined field surveillance with molecular and pathological investigations. Phylogenetic analysis of 16S rRNA and rpoB sequences confirmed species identity. Quantitative PCR of 192 apparently healthy frogs revealed intestinal carriage at every farm, with prevalence ranging from 39 to 77 percent and bacterial loads of 105–106 CFU/mL, indicating widespread subclinical colonisation. Virulence profiling demonstrated a conserved core gene set (gadB, mukF, citC, fimA, ompA) and accessory variation confined to the flagellar gene fliC. The strains resisted trimethoprim, ampicillin, and tetracyclines, yet remained susceptible to third generation cephalosporins, carbapenems, and most aminoglycosides. Infection trials showed that although very high inocula caused acute fatalities, an inoculum of 108 CFU/mL was sufficient to induce persistent enteritis characterised by suppressed tight junction proteins, elevated cytokine expression, and marked intestinal damage. These findings demonstrate that E. tarda circulates silently in bullfrog culture, carries an amphibian adapted virulence profile and still responds to key antimicrobials, providing a baseline for risk assessment, surveillance, and targeted control in amphibian aquaculture.