The two leading diseases affecting the catfish industry are the bacterial pathogens Edwardsiella ictaluri and Flavobacterium columnare (formerly Flexibacter columnaris). Treatment of these two pathogens has been difficult because only two antibiotics, Romet and oxytetracycline, have been approved by the Food and Drug Administration (FDA), and only one, Romet, can be effectively incorporated into feed. There is no vaccine commercially available for either bacterium, though at least one is undergoing tests for E. ictaluri (seepage17). Toour knowledge, there has been no successful vaccination protection against F. columnare.
Traditional vaccines for livestock have consisted of a killed organism, or component of an organism, suspended in an adjuvant (a substance to stimulate the immune system), which is injected into an animal. Depending on the vaccine, a booster may be required later. This approach has been successful in domesticated livestock and in Atlantic salmon, where the cost of an individual animal is large enough to justify the expense of handling and injecting each animal.
Unfortunately, injectable vaccines are not an option for channel catfish because of the low cost of an individual animal. The only economically feasible vaccines for channel catfish are immersion and oral vaccines. An immersion vaccine is one in which the fish is completely immersed in a high concentration of the vaccine for a short time, usually 30 seconds to 2 minutes. The fish is vaccinated when the vaccine is ingested or crosses the gills. Oral vaccines are added to feed. Both methods depend on large amounts of vaccine being ingested and crossing the gut wall and entering the bloodstream, or entering the bloodstream directly by crossing the gills. Previous attempts in vaccinating channel catfish using killed E. ictaluri have resulted in low levels of protection.
Another approach is to use alive attenuated vaccine, which is alive culture of a bacterium that has been altered by disrupting a gene so that it does not live long enough to reproduce in the host animal. Because it is live, the bacterium is able to enter the host on its own, but, being attenuated, it is no longer able to cause disease in the host. This approach allows sufficient numbers of the organism to enter the animal and be seen by the immune system.
Our goal is to develop a live-attenuated vaccine of F. columnare by genetically altering the chromosome (DNA) of the bacterium. Despite the economic impact of this organism in the catfish industry, surprisingly little is known about F. columnare. Though still in the early stages of the project, we are making significant progress in working with the bacterium. By identifying segments of DNA (genes) responsible for how the bacterium can survive and cause disease in the host, we can gain a better understanding of the disease process. Genes required for the disease process can be interrupted (their function disrupted, possibly resulting in attenuation) using techniques developed for other bacteria. By placing F. columnare containing an interrupted gene back into the catfish, we can determine whether or not a particular gene is involved in the disease process and if the bacteriumis attenuated. An attenuated bacterium is a good candidate for development as a vaccine.
The goal is to have the bacterium invade the fish and survive several days without causing disease or harm to the fish. The longer the attenuated bacterium survives, the longer the immune system has to establish protective mechanisms against the bacterium. The end goal is to have the fish develop a long lasting immunity against pathogenic (diseasecausing) F. columnare.
Development of a vaccine to F. columnare will provide fish farmers in Louisiana with another management tool that, if used correctly, will help lower the cost associated with producing fish. With the difficulty and expense in obtaining FDA approval on new antibiotics for food fish, the use of live-attenuated vaccines is an attractive alternative to increasing production and lowering production costs. Acknowledgments
This work has been made possible with assistance from Brandye Smith, Jan Lousteau and Eric Amy. Richard K. Cooper, Associate Professor, and Jacqueline McManus, Research Associate, Department of Veterinary Science, LSU Agricultural Center, Baton Rouge, La.
(This article was published in the fall 1999 issue of Louisiana Agriculture.)