Flavobacterium columnare genomovar influences mortality in channel catfish (Ictalurus punctatus)

Flavobacterium columnare, causal agent of columnaris disease, is pathogenic to many species of freshwater fish throughout the world. 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 min immersion exposure to ∼5 × 105 to 1 × 106 CFU/mL) were carried out to assess virulence of genomovar I and II isolates to channel catfish. The results demonstrated that genomovar II (n = 4) isolates were significantly (P < 0.05) more virulent to channel catfish fry (92–100% mortality) than genomovar I (n = 3) isolates (0–46% mortality). In vivo adhesion of the genetically characterized F. columnare also correlated (r2 = 0.73) to increased mortality in the challenged fry. In fingerling channel catfish, significantly higher mortality (P < 0.05) resulted with genomovar II isolates ALM-05-182 and ALG-00-530 as compared to all the genomovar I isolates (n = 3). Mortality of genomovar II isolate BGFS-27 with similar to genomovar II isolate (ALG-00-530) and two genomovar I isolates (ALM-05-53 and 140). The results suggest that although both genomovars are present in the aquatic environment, genomovar II appears to be more pathogenic for channel catfish.