Pathogenesis and pathological analysis of Edwardsiella tarda from Dabry's sturgeon (Acipenser dabryanus) in China

Dabry's sturgeon (Acipenser dabryanus) is designated as critically endangered in the IUCN Red List and a first-class protected animal in China. Recently, A. dabryanus in a fishery institute in southwestern China suffered from a high rate of mortality (similar to 70%). This study confirmed the etiology of moribund A. dabryanus. Bacteria isolated from heart, liver, gill and intestine samples of infected fish were identified via biochemical characterization and molecular analysis. A phylogenetic tree generated from 16S rDNA sequences revealed that one isolate, designated strain EA, had the basic characteristics of Edwardsiella tarda, the causative agent of bacterial hemorrhagic septicemia in fish. Moreover, we detected six virulence genes (citC, fimA, gadB, katB, mukF and gyrB) in strain EA; however, none possessed the esrB gene. Virulence tests were conducted by injecting EA (1.0x10(6) CFU/g fish) into healthy fish, and the results demonstrated that EA was lethal to A. dabryanus, with a mortality rate of 88% (7/8). The challenged fish showed similar clinical symptoms to those that were naturally infected. To our knowledge, this is the first report of E. tarda infection associated with mass mortality in A. dabryanus. The results from this study elucidate the pathogenesis and severe pathogenicity that E. tarda in farmed A. dabryanus and highlight the need for appropriate disease control and prevention measures.