Virulence and molecular variation of Flavobacterium columnare affecting rainbow trout in Idaho, USA

Columnaris disease, caused by Flavobacterium columnare, is an emerging problem in the rainbow trout (Oncorhynchus mykiss) aquaculture industry of Idaho. All F. columnare isolates taken from disease outbreaks in the rainbow trout producing region of southern Idaho, and for rainbow trout, are all genomovar I. Virulence phenotyping of 11 genomovar I and 1 genomovar I/II isolates, taken from 6 different farms, suggests significant variation in virulence toward rainbow trout with the most virulent strains having an LD50 of 1 × 106 CFU/ml− 1 when using a waterborne challenge model. The least virulent strain required 8 × 106 CFU/ml− 1 to reach the LD50. AFLP fingerprinting of these strains and the resulting phylogenetic tree show that all of the strains tested had a percent similarity of 75% or higher, save one, the MS-FC-4 strain had < 60% identity to all of the other strains tested. These results suggest strains of F. columnare isolated from disease outbreaks in rainbow trout production have a higher degree of diversity than previously thought.