Recirculation versus Flow-through Rainbow Trout Laboratory Flavobacterium Columnare Challenge

Authors: Birkett, Clayton; Lipscomb, Ryan; Moreland, Travis; Leeds, Timothy - Tim; Evenhuis, Jason

Submitted to: Diseases of Aquatic Organisms
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/22/2020
Publication Date: 6/4/2020
Citation: Birkett, C., Lipscomb, R.S., Moreland, T.D., Leeds, T.D., Evenhuis, J. 2020. Recirculation versus Flow-through Rainbow Trout Laboratory Flavobacterium Columnare Challenge. Diseases of Aquatic Organisms. 139: 213–221. https://doi.org/10.3354/dao03487.
DOI: https://doi.org/10.3354/dao03487

Interpretive Summary: Columnaris disease, caused by the bacterial pathogen Flavobacterium columnare, causes significant mortality in rainbow trout aquaculture. A laboratory challenge model to replicate columnaris disease has been developed and is critical to inform effective control strategies, but effects of water chemistry and quality parameters can result in poor repeatability of this challenge model. This study compares mortality of 20 rainbow trout families in a recirculating laboratory disease challenge model, which requires 10-fold less water, to the standard flow-through challenge model. Despite similar pathogen exposure, mortality was significantly lower in the recirculating challenge model, and there was very little correlation of family mortality between the two systems. This study demonstrates the poor repeatability of the laboratory challenge model as a result of water chemistry and quality parameters. By using less water, the recirculating laboratory challenge model offers greater ability to test and control specific water chemistry and quality parameters, and thus may allow development of a more repeatable laboratory disease challenge model.

Technical Abstract: Flavobacterium columnare immersion challenges are affected by water-related environmental parameters and thus are difficult to reproduce. Whereas these challenges are typically conducted using flow-through systems, use of a recirculating challenge system to control environmental parameters may improve reproducibility. Herein we compare mortality, bacterial concentration, and environmental parameters between flow-through and recirculating immersion challenge systems under laboratory conditions using 20 rainbow trout families. Despite identical dose and duration of challenge and temperature, average mortality in the recirculating system (42%) was lower (P < 0.01) compared to the flow-through system (77%), and there was low correlation (r = 0.24) of family mortality. Mean days to death (3.25 vs. 2.99) and aquaria-to-aquaria variation (9.6 vs. 10.4%) in the recirculating and flow-through systems, respectively, did not differ (P > 0.30). Despite 10-fold lower water replacement rate in the recirculating (0.4 exchanges/hour) compared to flow-through system (4 exchanges/hour), differences in bacterial concentration between the two systems were modest (< 0.6 order of magnitude) and inconsistent throughout the 21-day challenge. Compared to flow-through, dissolved oxygen during the 1-hour exposure and pH were greater (P < 0.02) and calcium and hardness were lower (P < 0.03) in the recirculating system. Although this study was not designed to test effects of specific environmental parameters on mortality, it demonstrates that the cumulative effects of these parameters result in poor reproducibility. A recirculating immersion challenge model may be warranted to empirically identify and control environmental parameters affecting mortality, and thus may serve as a more repeatable laboratory challenge model.