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Title: Reproducible challenge model to investigate the virulence of Flavobacterium columnare genomovars in rainbow trout Oncorhynchus mykiss

Author
item Lafrentz, Benjamin
item LAPATRA, SCOTT - Clear Springs Foods, Inc
item Shoemaker, Craig
item Klesius, Phillip

Submitted to: Diseases of Aquatic Organisms
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/21/2012
Publication Date: 11/8/2012
Citation: Lafrentz, B.R., Lapatra, S.E., Shoemaker, C.A., Klesius, P.H. 2012. Reproducible challenge model to investigate the virulence of Flavobacterium columnare genomovars in rainbow trout Oncorhynchus mykiss. Diseases of Aquatic Organisms. 101:115-122.

Interpretive Summary: Flavobacterium columnare is a Gram-negative bacterium that causes columnaris disease and has significant economic impacts on aquaculture production worldwide. There is genetic diversity among isolates of this pathogen and a review of the literature revealed that all isolates recovered from salmonids and analyzed using a common technique belonged to one genetic group (genomovar I). The objective of this study was to develop a challenge model for columnaris disease in rainbow trout and test the hypothesis that genomovar I F. columnare isolates were more virulent in rainbow trout than genomovar II isolates because of their apparent species specificity. A reproducible challenge model was developed and the results suggest that genomovar II isolates are more pathogenic (virulent) in rainbow trout than genomovar I isolates. This research provides a basis for further defining the molecular diversity and virulence associated with F. columnare genomovars in rainbow trout and other salmonid species.

Technical Abstract: Flavobacterium columnare is a Gram-negative bacterium that causes columnaris disease and has significant economic impacts on aquaculture production worldwide. Molecular analyses have demonstrated that there is genetic diversity among F. columnare isolates. A review of the published literature that used restriction fragment length polymorphism analysis of the 16S rRNA gene revealed that all isolates typed from salmonids were Genomovar I. Our objective was to develop a laboratory challenge model for F. columnare in rainbow trout Oncorhynchus mykiss (Walbaum) and use the model to determine the virulence of Genomovar I and II isolates. Six F. columnare isolates were obtained from rainbow trout experiencing losses due to columnaris disease and were determined to be Genomovar I. Three of these were chosen for a preliminary assessment of virulence, and isolate 051-10-S5 was chosen for additional experiments to determine the reproducibility of the waterborne challenge model. In 2 independent experiments, cumulative percent mortalities (CPM) were 49 ± 10% and 50 ± 19%. Challenge of rainbow trout with Genomovar I and II isolates demonstrated a difference in the CPM, with the Genomovar II isolates inducing significantly higher CPM. This reproducible waterborne challenge model for columnaris disease in rainbow trout will be useful to investigate host- pathogen interactions, vaccine development, and other potential control strategies. This research also provides a basis for further defining the molecular diversity and virulence associated with F. columnare genomovars in rainbow trout and other salmonid species.