A Novel Flavobacterium Sp. Causes Bacterial Cold Water Disease-like Symptoms in Steelhead and Rainbow Trout

Flavobacterium psychrophilum is thought to be the primary causative agent of bacterial cold water disease (BCWD) in freshwater-reared salmonids. However, we recently discovered a novel Flavobacterium resulting in clinical signs similar to BCWD in steelhead. Approximately 37% of the population was lost in a recurring mortality event involving this novel Flavobacterium. Kirby-Bauer disc-diffusion assays indicated the bacteria were highly resistant to Florfenicol. A challenge trial with the novel Flavobacterium was conducted using an immersion dose of 1.68x10⁸ cfu/mL for one hour in three 25-fish replicates (n=75 total) using pathogen free rainbow trout. Cumulative percent mortality after 28 days was 29% (SD = 16%). PCR amplification with 16S primers followed by Sanger sequencing was performed to molecularly characterize the novel Flavobacterium species. Phylogenetic analyses indicate this species is closely related to F. plurextorum, a newly described isolate from diseased farmed rainbow trout in Spain. These findings highlight the importance of non-F. psychrophilum bacteria in causing morbidity and mortality associated with BCWD-like symptoms in fresh-water reared salmonids. In addition, the resistance of this novel Flavobacterium to Florfenicol, one of few antibiotics labeled for use in aquaculture, further highlights its potential to have severe impacts on fish health and production.