Outbreaks of edwardsiellosis caused by Edwardsiella piscicida and Edwardsiella tarda in farmed barramundi (Lates calcarifer)

The continued growth of the aquaculture industry and surging demand for barramundi (Lates calcarifer) has led to increased interest in farming this fish species in the USA. Regrettably, little is known about the fish-pathogenic microbes affecting barramundi, which limits aquaculture productivity outside its home range. Herein, we report on multiple outbreaks of edwardsiellosis associated with elevated mortality in US farmed barramundi stocks. Phenotypic characterization, gyrB sequencing and phylogenetic analyses, real-time multiplex PCR, and repetitive extragenic palindromic PCR (rep-PCR) fingerprinting identified the etiological agents as Edwardsiella tarda and E. piscicida. Plasmid profiling identified four unique E. tarda-associated plasmids encoding genes with putative functions relating to virulence (e.g., antibiotic resistance, cell killing, toxin delivery), plasmid replication/mobilization/integration, and a multitude of hypothetical proteins with unknown function. The external clinical signs in Edwardsiella tarda-infected barramundi included erythema and/or hemorrhage on the ventrum, gill pallor, and fin/oral cavity congestion, as well as internal hepatic pallor/mottling, erythema of the gastrointestinal tract, and evidence of previous hemorrhage within the swim bladder wall at necropsy. Edwardsiella piscicida-infected barramundi had similar external disease signs, but internally presented splenomegaly, renomegaly, and small whitish miliary nodules in the liver, spleen, and kidney. Histopathology on E. piscicida-infected tissues revealed granulomatous splenitis, hepatitis, and nephritis, with intrahistiocytic intracytoplasmic bacteria. Results from this study exemplify the damage E. tarda and E. piscicida can cause in farmed barramundi and highlight the need for appropriate disease control and prevention measures.