Abigail Armwood

In the mid‐2010s, Edwardsiella tarda was reaffiliated into three discrete taxa ( E. anguillarum , E. piscicida , and E. tarda ), obscuring previous descriptions of E. tarda ‐induced pathology in fish. To clarify ambiguity regarding the pathology of E. tarda , E. piscicida , and E. anguillarum infections in US farm‐raised catfish, channel catfish ( Ictalurus punctatus ), blue catfish ( I. furcatus ), and channel × blue catfish hybrids were challenged with comparable doses of each bacterium. The most severe pathology and mortality occurred in fish challenged with E. piscicida , supporting previous reports of increased pathogenicity in commercially important ictalurids, while E. anguillarum and E. tarda warrant only minimal concern. Acute pathologic lesions among bacterial species were predominantly necrotizing and characteristic of gram‐negative sepsis but became progressively granulomatous over time. After 100 days, survivors were exposed to the approximate median lethal doses of E. piscicida and E. ictaluri , revealing some cross‐protective effects among E. piscicida , E. anguillarum , and E. ictaluri . In contrast, no fish that survived E. tarda challenge demonstrated any protection against E. piscicida or E. ictaluri . This work supports reports of increased susceptibility of channel, blue, and hybrid catfish to E. piscicida , while highlighting potential cross‐protective affects among fish associated Edwardsiella spp.