Location: Cool and Cold Water Aquaculture ResearchTitle: Virulence variations of flavobacterium columnare in rainbow trout (Oncorhynchus mykiss) eyed eggs and alevin
Submitted to: Journal of Fish Diseases
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/13/2021
Publication Date: 4/6/2021
Citation: Evenhuis, J., Lipscomb, R.S., Birkett, C. 2021. Virulence variations of flavobacterium columnare in rainbow trout (Oncorhynchus mykiss) eyed eggs and alevin. Journal of Fish Diseases. 44(5): 533-539. https://doi.org/10.1111/jfd.13343.
Interpretive Summary: Columnaris disease, caused by the bacteria Flavobacterium columnare, is an emerging problem in rainbow trout aquaculture. Scientists are using bacterial genetic manipulation techniques to develop effective, live-attenuated vaccines that will control this disease, but are lacking a high-throughput challenge model to rapidly screen bacterial mutants for loss of virulence. The use of early life-stage rainbow trout in a challenge model will allow more rapid screening of bacterial mutants, and will minimize welfare concerns, compared to the use of older juvenile fish. This study evaluated the virulence of two F. columare strains in early life-stage rainbow trout (eyed eggs and newly-hatched sac fry) from two populations. Important differences in virulence of the two bacterial strains were identified, but these differences were dependent upon the population and life stage of the trout used. Factors identified in this study that affect virulence in early life-stage rainbow trout will better inform development of a challenge model to rapidly screen bacterial mutants as candidates for live-attenuated vaccines.
Technical Abstract: Flavobacterium columnare (Fc) is the causative agent for columnaris disease (CD) and considered an emerging problem in the rainbow trout industry. Herein, we characterize the virulence phenotype of two Fc isolates, CSF-298-10 and MS-FC-4, against rainbow trout from two sources, NCCCWA and a commercial production stock (PS), at the eyed egg and alevin life stages. Immersion challenges of eyed eggs from the NCCCWA were susceptible to the Fc isolate MS-FC-4 (>97% mortality) but no increase in mortality was observed when PS eyed eggs were immersion challenged. The CSF-298-10 had little effect on eyed eggs from both sources and was not highly virulent to any alevin till day 6 post-hatch, up to 38% for the NCCCWA alevin and ~80% for the PS alevin. The MS-FC-4 stain produced 80% or greater mortality any day an immersion challenge was attempted post-hatch regardless of the source of the trout. Several log10 difference in CFU counts were recorded between the Fc strains when immersion challenges were performed 2 days post hatch. Counts for the NCCCWA stock of alevin were 4.4 x 103 CFU/ml-1 for the CSF-298-10 strain and 1.8 x 106 CFU/ml-1 for the MS-FC-4 strain and for the PS alevin CSF-298-10 measured 9.9 x 101 CFU/ml-1 and 3.8 x 105 CFU/ml-1 for MS-FC-4. These two Fc isolates present stark differences in virulence phenotypes to both eyed eggs and alevin and present an interesting model system for virulence kinetics and potentially alternative pathogenic pathways.