Virulence of Flavobacterium columnare genomovars in rainbow trout Oncorhynchus mykiss

Authors: Evenhuis, Jason; Lafrentz, Benjamin

Submitted to: Diseases of Aquatic Organisms
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/31/2016
Publication Date: 8/9/2016
Citation: Evenhuis, J., Lafrentz, B.R. 2016. Virulence of Flavobacterium columnare genomovars in rainbow trout Oncorhynchus mykiss. Diseases of Aquatic Organisms. 120(3):217-224.

Interpretive Summary: Flavobacterium columnare causes columnaris disease in numerous fin fish species. F. columnare isolates come from diverse fish species and geographic regions. These isolates are catagorized into one of 5 different genomovars, based on an enzymatic digestion of the 16S rDNA gene. Currently, F. columnare isolates taken from rainbow trout are all classified as genomovar I strains. We tested the virulence of several isolates from 4 of the 5 genomovars toward rainbow trout in 3 independent challenges. Nine genomovar I isolates were able to able to induce an average of 49% mortalities, while 4 genomovar II isolates produced an average of 1%, 3 genomovar II-B at 5% and 5 genomovar III at 7%. Genomovar I isolates were the only strains that were able to produce mortalities that were significantly different than unchallenged control tanks. These results differed greatly to previous study where two genomovar II isolates were able to induce more than 90% motilities in rainbow trout. Two factors differed between this study and the previous one; rainbow trout came from two independent producers and water chemistries varied between the two facilities. Specifically the water hardness was much higher at our facility as compared the facility were the original study was conducted. The results presented here suggest that genomovar I strains of F. columnare are the most virulent toward rainbow trout and that multiple factors may influence host vs. pathogen interactions.

Technical Abstract: Flavobacterium columnare is the causative agent for columnaris disease and responsible for significant economic loss in the aquaculture industry. F. columnare is a gram negative bacteria with several genomovar classifications, based on the restriction fragment length polymorphism of the 16S rDNA gene. The genomovar II isolates were thought to be more virulent to multiple species of fish, including rainbow trout. Our objective was to confirm previous results and to determine the susceptibility of rainbow trout to other genomovar isolates. Three independent challenges using 8 genomovar I, 4 genomovar II, 3 genomovar II-B, 5 genomovar III isolates, originating from various sources, were assessed for virulence toward rainbow. A cumulative percent mortality (CPM) of ~49% for genomovar I strains, ~1% for genomovar II, ~5% for the II-B strains and ~7% for the III strains were recorded. The inability of genomovar II strains to produce mortalities in rainbow trout was unanticipated but may be due to a multiple factors. The source of the rainbow trout for this study and a previous rainbow trout study, from Auburn, came from two independent rainbow trout stocks. Water chemistries, specifically water hardness, at the 2 testing facilities varied, 298 mg CaCO3/l at NCCCWA and 60 mg CaCO3/l at Auburn. This research demonstrates that F. columnare genomovar I strains are virulent toward rainbow trout but strains from other genomovar classifications are not, including genomovar II strains. The source of fish and/or the presence of a sub-optimal environment may influence the susceptibility of rainbow to multiple F. columnare genomovars.