Bacterial pathogens associated with red sore disease of American eels ( Anguilla rostrata) in Chesapeake Bay aquaculture

Recently, American eel (Anguilla rostrata) aquaculture operations in the Chesapeake Bay region experienced significant disease-associated mortality from red sore disease (RSD), typically a bacterial infection characterized by severe skin lesions. However, the microbial pathogens associated with this disease event are unknown, and studies of RSD in American eels are limited. We collected 128 eels from two aquaculture facilities located on the Chesapeake Bay, from 2018 to 2019. We developed and implemented a scale to record the clinical severity of RSD, ranging from 0 (healthy) to 3 (severe disease). Using standard bacteriological and molecular methods (16S rRNA gene sequencing), we also identified the genera of bacteria isolated from external lesions and internal organs. We found that the presence of culturable bacteria in eels increased the odds of clinical disease by 326 times. Vibrio spp. and Aeromonas spp. were the dominant genera identified, with Vibrio spp. having the greatest association with the likelihood of severe disease. The relative abundance of the dominant bacterial genera varied by the month of collection, which appeared to be driven primarily by temperature recorded in the aquaculture operation. The odds of isolating Vibrio spp. increased with temperature while they decreased for Aeromonas and other bacterial genera. Other environmental variables such as pH and DO were found to have limited associations with the presence of disease and bacteria. Our findings are similar to those reported in European and Japanese eel aquaculture for an RSD-like disease. These results provide insights into effective RSD mitigation and management of eel losses in North American aquaculture. Without an effective strategy to reduce the diseaseassociated mortality from RSD in eels, development of the emerging eel aquaculture industry is likely to continue to be hindered. Further characterization of the environmental drivers of the disease would bolster disease management.