THE EFFECT OF CUTANEOUS INJURY ON A REPRODUCIBLE IMMERSION CHALLENGE MODEL FOR FLAVOBACTERIUM COLUMNARE INFECTION IN CHANNEL CATFISH (ICTALURUS PUNCTATUS)

Authors: Bader, Joel; MOORE, SARAH - AUBURN UNIVERSITY STUDENT; NUSBAUM, KENNETH - AUBURN UNIVERSITY

Submitted to: Aquaculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/30/2005
Publication Date: 4/1/2009
Citation: Bader, J.A., Moore, S.A., Nusbaum, K.E. 2006. The effect of cutaneous injury on a reproducible immersion challenge model for Flavobacterium columnare infection in channel catfish (Ictalurus punctatus). Aquaculture 253: 1-9

Interpretive Summary: Columnaris disease is caused by the bacterium Flavobacterium columnare and is the second most important fish pathogen of commercial catfish aquaculture in the Southeastern United States. A reproducible immersion (dip) challenge model for columnaris disease in channel catfish is a prerequisite for testing of therapeutics to protect against columnaris disease in this species. A common model for infecting channel catfish with columnaris disease is to immerse the fish in a bath containing a high concentration of bacteria. Unfortunately, this method is not always effective in inducing disease. In this study, we report on the development of a more consistent infection model. Our model incorporates man-made injury to the catfish skin with the more typical bacterial immersion challenge. Analyzed in this study were four different methods of inducing physical injury; physical abrasion, thermal branding, both hot and cold, and chemical abrasion with a strong base. These methods were evaluated both with and without bacterial challenge. Only physical abrasion and hot branding caused enough change in the skin of the fish to induce a standardized bacterial infection. The lethal bacterial dose causing 50% (LD50) death in a population was determined for both of these methods. In a final study, we use this model to determine the LD50 for StresscoatTM, an artificial sliming agent (AS) and used AS to study the susceptibility of channel catfish to F. columnare. Our results suggest that the use of the AS can reduce columnaris disease outbreaks resulting from the handling and transport of catfish.

Technical Abstract: Columnaris is a disease of fish caused by the bacteria, Flavobacterium columnare. The disease continues to cause significant losses of channel catfish, Ictalurus punctatus in the Southeastern United States whenever temperatures are between 15 and 30' C. It can also cause disease in many other commercial fish following handling or stressful environmental conditions. Examination of four methods that produce cutaneous injury; physical abrasion, hot and cold branding, and chemical scalds indicated that only physical abrasion and hot branding produced lasting alterations of epithelial architecture. Immersion challenge of physically abraded (PA) or hot branded (HB) fish with 108 CFU/mL virulent F. columnare at 29 ' 2' C immediately after treatment resulted in death of most fish within the first 4 h. Fish challenged by immersion 4 h after disruption of the epithelium resulted in 100 % and 67% mortality for PA and HB, respectively. Immersion challenge 24 h after cutaneous injury resulted in 60 % mortality with PA and 2% in HB. Cutaneous injury 48 to 72 h before challenge did not result in significant mortality (' 4%). A significant reduction in mortality and delay in death were noted when abraded fish were treated with StresscoatTM , an artificial slime agent (AS) and immediately challenged. Lethal dose (96 h LD50) for abrasion (10 4.4 ' 0.3CFU/mL) was a log higher than for branded fish (10 5.0 ' 0.4 CFU/mL). The LD50 for HB plus AS was 10 5.8 ' 0.4 CFU/mL a log higher than for HB fish without AS, 10 5.0 ' 0.4 CFU/mL. Hot branding produces more consistent infection in terms of 96 h LD50s and has the additional advantages of being more controllable and reproducible then PA. Induction of experimental columnaris is dependent on the length of time between injury and exposure to pathogen (<4 hour), size of area disrupted on the epidermis of the fish and dose of bacteria. Disease can be minimized or prevented by protecting the wound with AS. A columnaris immersion challenge model for channel catfish needs to include a characterized virulent isolate, reproducible area of epithelial disruption, i.e hot branding, standard bacterial dose; based on LD50 data (106 CFU/ml), consistent immersion time (15 min), consistent stocking rates (no crowding), controlled temperature (29 ' 2' C), water quality, and well nourished fish. A reproducible immersion challenge model is an invaluable tool for any future testing of therapeutics to protect against columnaris disease and also will aid in understanding the pathogenesis of columnaris disease.