DAO 67:73-79 (2005)  -  doi:10.3354/dao067073

Experimental bath infection with Flavobacterium psychrophilum, inducing typical signs of rainbow trout Oncorhynchus mykiss fry syndrome

Masatoshi Aoki1, Motoki Kondo1, Kenji Kawai1, Syun-ichirou Oshima2,*

1Fish Disease Laboratory, Department of Aquaculture, Kochi University, Nankoku, Kochi 783-8502, Japan
2Laboratory of Cell Structure and Function, Division of Marine Bioresource Science, Graduate School of Kuroshio Science, Kochi University, Nankoku, Kochi 783-8502, Japan
*Corresponding author. Email:

ABSTRACT: Flavobacterium psychrophilum infection in salmonid fish, known as rainbow trout fry syndrome (RTFS) or bacterial coldwater disease (BCWD), is widespread in fish farms and natural waters. Despite many studies in which attempts at infection were made, an adequate method of infection has not yet been established. In this study, we evaluated a bath infection method in which we used bacteria at different stages of growth in the infection of rainbow trout Oncorhynchus mykiss. Rainbow trout with a mean body weight of 1.3 or 5.6 g, respectively, were infected by immersion in a bacterial suspension at different stages of growth (18 to 66 h shaking culture at 15°C). The fish immersed in a logarithmic phase culture showed higher mortality than those in other culture phases. Indeed, 1.3 and 5.6 g fish showed typical clinical signs including ulcerative tissue of the trunk and lack of caudal fin edge. F. psychrophilum was detected by immunohistochemistry (IHC) in these tissue samples. These results indicate that experimental bath infection using a logarithmic phase bacterial solution is the most appropriate method for studies of infectious mechanisms.

KEY WORDS: Flavobacterium psychrophilum · Logarithmic phase · Bath challenge · Virulence · Bacterial coldwater disease · Rainbow trout fry syndrome

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