ARS | Publication request: Flavobacterium columnare genomovar type influenced mortality in channel catfish Ictalurus punctatus

Submitted to: Annual Meeting World Aquaculture Society
Publication Type: Abstract Only
Publication Acceptance Date: November 6, 2007
Publication Date: February 9, 2008
Citation: Shoemaker, C.A., Olivares-Fuster, O., Arias, C.R., Klesius, P.H. 2008. Flavobacterium columnare genomovar type influenced mortality in channel catfish Ictalurus punctatus [abstract]. World Aquaculture Society. p. 358.

Technical Abstract: Genetic diversity has been documented and confirmed in Flavobacterium columnare, causal agent of columnaris disease. This aquatic bacterium is pathogenic to many species of freshwater fish throughout the world, and the United States channel catfish (Ictalurus punctatus) aquaculture industry is severely impacted by columnaris disease. The majority of the F. columnare isolates recovered from diseased channel catfish belonged to either genomovars I or II. The objective of the present study was to determine if differences existed in the ability of these genomovars to induce mortality in channel catfish. Single strand conformation polymorphism analysis (SSCP) was used to ascribe the isolates used in this study to the appropriate genomovar. Immersion challenge experiments (15 minute immersion exposure to ~ 5 X 10 to the fifth power to 1 X 10 to the sixth power CFU/ml) were carried out to assess virulence of genomovar I and II isolates to channel catfish. The results demonstrated genomovar II isolates were significantly (P < 0.05) more virulent in channel catfish fry (92-100% mortality) than genomovar I isolates (0-46% mortality). Genomovar effect in fingerling channel catfish was also significant (P < 0.05) with genomovar II resulting in greater mortality (74.7 % versus 30.2 %). In vivo adhesion of the genetically characterized F. columnare was also strongly correlated (r squared = 0.73) to increased mortality in the challenged catfish. The molecular typing method (ISR-SSCP) demonstrated that the isolates used to infect the channel catfish were the same genotype as the isolates recovered from the moribund fish, at least with respect to the isolates that caused mortality and were re-isolated in pure culture. This is particularly important when challenging fish with F. columnare since channel catfish can be asymptomatic carriers for columnaris disease and therefore might harbor additional strains of the pathogen. Interestingly, environmental isolates obtained from apparently healthy wild channel catfish (Mobile River basin) were virulent in laboratory studies. Studies have suggested that F. columnare from the normal aquatic bacterial community is probably the source of pathogenic F. columnare. Our results suggest that although both genomovars are present in the aquatic environment and routinely isolated from diseased channel catfish, genomovar II appears to be more pathogenic for channel catfish.