Seasonal appearance of Edwardsiella tarda and its bacteriophages in the culture farms of Japanese flounder

Abstract

This study was conducted to investigate seasonal appearance of Edwardsiella tarda and E. tarda bacteriophages in cultured Japanese flounder Paralichthys olivaceus and the culture environment. The surveys were carried out twice a month at 4 commercial fish farms in Ehime Prefecture in 2002-2004. E. tarda was isolated from the environmental seawater and the organs (kidney and spleen) of apparently healthy fish from June to December when mortality due to edwardsiellosis was found among cultured populations. As expected, the outbreak of the disease preceded a rapid increase in the titers of serum agglutinins against E. tarda. In contrast, E. tarda phages were frequently identified in the environmental seawater at least one month before the disease outbreak and during the disease prevalence, but not detected after the outbreak terminated. The present study suggests that the appearance of E. tarda phages can be an indicator for existence of E. tarda in the culture environment.