A Study on Bacterial Etiology and Histopathology Associated With Hemorrhagic Disease in American Shad Alosa sapidissima

American shad Alosa sapidissima is an important exotic farmed fish species in China. Recently, skin ulcers associated with significant morbidity and mortality have severely affected American shad culture in Spring. In this study, a bacterial strain C8 was isolated from diseased A. sapidissima and was identified as A. hydrophila based on traditional phenotypic typing and 16S rDNA sequence analyses. Drug susceptibility test revealed the isolate was sensitive to Quinolones and Amphenicols but resistance to Aminoglycosides, Sulfonamides, and Tetracyclines. Experimental infection with the isolate C8 showed the typical symptoms of acute sepsis caused by A. hydrophila infection and 100% mortality. Histopathological observation revealed that substantial inflammatory cell infiltration occurred in the skin lesion of diseased fish, and extensive degenerative and necrosis was observed in the parenchymal cells of the liver and kidney. This study is the first report on the histopathology of cultured A. sapidissima caused by A. hydrophila and provides useful information for health management.