Columnaris disease, caused by Flexibacter columnare, is considered to be the second most common disease for channel catfish on farms, second only to enteric septicemia caused by Edwardsiell ictaluri. Columnaris disease is extremely common in freshwater fishes and is generally considered to be ubiquitous… found everywhere. Although the causative bacterial disease agent is found in most freshwaters, its presence does not automatically lead to columnaris disease. Environmental factors and physiological stress are important mediators in the columnaris disease processes.
The most commonly recognized environmental factor related to columnaris disease is water temperature. Columnaris is a warm water disease, generally occurring when water temperatures exceed 25o C. Any condition that causes physiological stress for the fish seems to facilitate an outbreak of columnaris. However, there seem to be “other factors” related to disease outbreaks. Casual observations by producers, as well as, research scientists, who noted unusual problems in research studies, have indicated that water quality factors are also involved in columnaris disease. One set of such observations involved research studies in Arkansas and Mississippi.
U.S. Department of Agriculture (USDA) research laboratories in Stuttgart, AR and Stoneville, MS could not produce similar results in columnaris challenge studies even though they used exactly the same model/protocols. The columnaris challenge model used at the Stuttgart National Aquaculture Research Center (SNARC) has worked extremely well and produced excellent results in research conducted there. However, the SNARC challenge model did not work at all in studies conducted in Stoneville, at the Warmwater Aquaculture Research Unit (WARU), formally known as the National Warmwater Aquaculture Research Center. The two laboratories are located approximately 125 miles (~200 km) from each other and have similar facilities,
The studies in question used the same Norris-strain channel catfish, matched experimental methods, and environmental conditions exactly, except for water quality. The labs have quite different water supplies, so it was impossible to have exactly the same water chemistry. The waters were known to differ in two major characteristics: WARU water contained considerably more dissolved organic matter (DOM ) and had very low concentrations of dissolved calcium and magnesium. Initial speculation focused on the potential role of tannin in the water at WARU, which might act as an inhibitor of columnaris disease. To test the possible effects of water quality factors, tank trucks hauled WARU water to the laboratories at SNARC and the challenge protocols were repeated in matched experiments.
In the first experiment, fish were exposed to a bacterial challenge of F. columnare in aquaria supplied with either SNARC or WARU water. Results differed greatly. Fish mortality after 4 days was 100% in SNARC water, while no fish died in WARU water. Observations of bacterial adhesion to gill surfaces also showed dramatic differences between fish in SNARC water compared to those held in WARU water. Adhesion was approximately 1900-times higher for fish challenged in SNARC water than for fish challenged in WARU water (more than 800,000 colony forming units versus less than 450 colony forming units).
In a second experiment, the waters from each laboratory were altered to remove the major differences in dissolved organic matter (DOM) and hardness (divalent cation concentration), so as to determine which factor was responsible for the differences in mortality and adhesion. Experiment 2 also was conducted with a lower bacterial challenge, to facilitate comparisons of results for factors other than mortality.
Results showed that complete removal of DOM from WARU water by carbon filtration did not affect bacterial adhesion to gills, nor cause any change in mortality in the challenged fish. Consequently, the researchers concluded that DOM did not contribute to the differences in mortality and adhesion seen in Experiment 1. In contrast, removal of most calcium and magnesium from SNARC water by ion-exchange filtration resulted in substantially lower bacterial adhesion to gills and decreased mortality rates compared to un-altered SNARC water.
The conclusion derived from these experiments is that the concentration of divalent cations (hardness) in the waters used to produce catfish, affects the pathogensis (development of disease) of columnaris disease in channel catfish. In the absence of relatively high levels of calcium and magnesium, columnaris bacteria cannot bind to the tissues of the fish and development of disease does not occur. Catfish farms supplied with relatively soft waters should expect fewer problems with columnaris disease.
— John Nickum
Full reference: David L. Straus, Bradley D. Farmer, Benjamin H. Beck, Brian G. Bosworth, Eugene L. Torrans, Craig S. Tucker. Water hardness influences Flavobacterium columnare pathogenesis in channel catfish. Aquaculture Volume 435, 1 January 2015, Pages 252–256.