Early life stage rainbow trout (Oncorhynchus mykiss) mortalities due to Flavobacterium columnare in Idaho, USA

Flavobacterium columnare is the etiologic agent of columnaris disease, a pervasive disease of fresh water finfish. During the past 4 years, losses that ranged from 5 to 50% in rainbow trout (Oncorhynchus mykiss) fry being reared at a constant 14.5 °C (mean weight, 0.2 g; ~ 400°days post-fertilization), have been occurring in Hagerman Valley, Idaho, USA. A total of 70 different F. columnare isolates were obtained from diseased fish and the water they were reared in. All of the isolates were confirmed to be genomovar I by 16S rRNA restriction fragment length polymorphism. Sequencing of the 16S rRNA, 16S-23S rDNA spacer region and the gyrase B subunit genes from these 70 strains revealed no sequence differences among these isolates. Whole-cell protein profiling by SDS-PAGE also indicated low variation between isolates. Virulence was assessed for a representative isolate and demonstrated a high degree of pathogenicity against rainbow trout fry at 15 °C. These results suggest the emergence of a highly successful F. columnare strain that can affect very early life stages of fish being reared at a constant 14.5 °C at a commercial rainbow trout farm in Idaho.