Virulence of Flavobacterium columnare Genomovars in Rainbow trout (Oncorhynchus mykiss)  

Flavobacterium columnare is the causative agent of columnaris disease and is responsible for significant economic losses in aquaculture. F. columnare is a Gram-negative bacterium, and five genetic types or genomovars have been described based on restriction fragment length polymorphism of the 16S rRNA gene. To date is all isolates taken from disease outbreaks from rainbow trout are genomovar I. Previous research has suggested that genomovar II isolates are more virulent to multiple species of fish, including rainbow trout (Oncorhynchus mykiss). Our objective was to confirm previous results and to determine the susceptibility of rainbow trout to other genomovars.

The virulence of 8 genomovar I, 4 genomovar II, 3 genomovar II-B, and 5 genomovar III isolates, originating from various sources, was determined through three independent challenges in rainbow trout using an immersion challenge model. Mean cumulative percent

mortality (CPM) of ~49% for genomovar I isolates, ~1% for genomovar II, <10% for the genomovar II-B and III isolates was observed. The inability of genomovar II isolates to produce mortalities in rainbow trout was unanticipated based on previous studies, but may be due to a couple of factors including rainbow trout source and water chemistry. This research demonstrates that genomovar I isolates of F. columnare are virulent toward rainbow trout but isolates from other genomovars are not as virulent.  The source of fish and/or the presence of sub-optimal environment may influence the susceptibility of rainbow to different F. columnare genomovars.