Submitted to: Global Aquaculture Advocate
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/13/2003
Publication Date: N/A
Citation: N/A Interpretive Summary: Flavobacterium columnare is the causative agent of columnaris disease. This disease is a cause of great economic loses in cultured catfish in the United States and in a number of other cultured fish species world-wide. Annual loses attributed to columnaris disease are estimated at $50-60 million to the catfish industry. Presently, there are several vaccines and chemotherapeutic agents in development that have the potential to prevent columnaris disease in catfish. Prior to official approval and use of these vaccines and chemotherapeutic, a standardized challenge method or model needs to be developed to artificially infect channel catfish with this disease. This manuscript describes the early steps at the United States Department of Agriculture, Agriculture Research Service, Aquatic Animal Health Research Unit in Auburn, Alabama to develop such a model. It is a summary of three previously published manuscripts dealing with the subject and alludes to upcoming results to be published in the near future. Studies reviewed were 1) Early entry of F. columnare evaluated by Polymerase chain reaction (PCR)and enhancement of bacterial uptake by the use of abrasion prior in bacterial challenge abrasion, 2) The comparison of four models to increase catfish susceptibility to columnaris disease prior to bacterial immersion challenge: physical abrasion, acid burn, hot branding and cold branding and 3)The effect of a artificial sliming agent, Aloe-Vera, on catfish following physical abrasion and heat branding. Data from these studies suggest that physical disruption of the epithelium and underlying musculature, factors thought to influence columnaris and that mimic handling stressors, must be a prerequisite for reproducible columnaris disease challenges and need to be included in any future model. Heat branding, we show, is the best physical disruption method, because it was more reproducible in producing a measurable wound than physical abrasion. Finally, we show that Aloe-Vera and similar substances act to protect the fish from the early uptake of the pathogen and aid in wound protection. They can therefore be used to delay and prevent infection. Further, such agents demonstrate the effect of the bacterial challenge dose on columnaris infection. The fewer bacteria entering a wound, the less chance a fish has of becoming infected. Therefore, dose too needs to be considered in any future columnaris challenge model. All these finding offer significant promise in the control of columnaris disease following harvest or shipping of catfish and may lead to a standardized columnaris challenge model. Standardized challenge models are necessary to test future therapies to control columnaris disease.
Technical Abstract: Initially, the Polymerase Chain Reaction (PCR) was used to evaluate how early Flavobacterium columnare accumulates in channel catfish, Ictalurus punctatus (Rafinesque). Experimental fish were, prior to bath immersion infection, either physically abraded; which is supposed to mimics fish handing practices, or left unabraded. Next, various methods of abrasion were evaluated in the bacterial immersion model. And finally, an artificial sliming agent, Aloe-Vera, was used with the previously described abrasion model to help explain how the columnaris disease model functions. Our PCR results showed that the presence of F. columnare alone was not sufficient to cause clinical disease and that catfish that were abraded or unabraded, acquired F. columnare in some tissues; mucus, skin, and gill tissues, after 5 min post challenge and all other tissues; blood, liver and kidney within an hour after challenge at 29±2' C. Unabraded fish, however, had no evidence of clinical disease, whereas abraded fish exhibited the typical saddle-back lesions and died. These observations were explained by a series of experiments that compared models of disease causation. We found that physical abrasion, acid burn, hot branding and cold branding all predisposed fish to columnaris infection, but the best methods were either physical abrasion or heat brand, each of which disrupts the epithelium of the fish. Both methods showed regeneration after 2 days indicating that rapid epithelial disruption was more important than long term disruption or method of disruption. Disruption of the epithelium appears to be the major prerequisite for disease to occur and could be reversed, rather simply, by the application of Aloe-Vera. In work to be published, we observed that when comparing the four models of injury that might occur due to handling or deteriorating water conditions, one feature of infection was clear: the larger the wound, as in the abrasion, the more bacteria are able to enter the fish and cause disease. However, protection of the wound (artificial slime) or reduction in the number of F. columnare in the water (dose) significantly lowers columnaris incidence in fish. These finding offers significant promise in the control of columnaris following harvest or shipping of fish.