Comparison of static-bath and flowing-water Flavobacterium columnare challenge methods with juvenile Chinook Salmon

Flavobacterium columnare is a common pathogen of Chinook Salmon Oncorhynchus tshawytscha in the Klamath River. Elevated water temperatures invoke congregation behavior within thermal refugia and are associated with columnaris disease. A flowing-water F. columnare challenge system was compared with the standard static-bath challenge as an initial step in simulating a riverine exposure.

Juvenile Chinook Salmon were exposed to 103 CFU/mL F. columnare for 20 h either in an aerated static bath or within a recirculation swim chamber set at one body length per second. Fish were held at a constant 20°C or exposed to short-term temperature fluctuations to a maximum of 24°C prior to the challenge. Mucus and gill samples were collected at the end of the 20-h challenge and from fish held up to 96 h postchallenge. Samples were assayed for detection of F. columnare by quantitative PCR and conventional plate culture method.

In static-bath challenge groups, F. columnare was detected in asymptomatic (38%) and moribund Chinook Salmon (29%). In contrast, F. columnare was detected in only one asymptomatic (4%) and one moribund (4%) Chinook Salmon in the flowing-water challenge groups. Prechallenge temperature conditions had no effect on infection. Other yellow-pigmented bacteria were isolated from the Chinook Salmon (particularly static-bath challenge) but were not associated with morbidity or amplified in the F. columnare quantitative PCR.

Low transmission of F. columnare occurred among juvenile Chinook Salmon under flowing-water conditions simulating a thermal refugia during early summer (20°C, flow of one body length per second, 20-h exposure to 103 CFU/mL). The flowing-water system is sufficient to examine the environmental factors (velocity, temperature, host density, duration, and bacterial concentration of exposure) of riverine exposures on F. columnare transmission to juvenile Chinook Salmon.