Comparative Challenge Model Of Flavobacterium columnare Using Scarified And Unscarified Channel Catfish, Ictalurus punctatus (Rafinesque)     Joel A. Bader1, Kenneth E. Nusbaum2, and Craig A.Shoemaker1   1United States Department of Agriculture, Agricultural Research Service, Aquatic Animal Health Research Laboratory, PO Box 0952, Auburn, AL; 2Department of Pathobiology, College of Veterinary Medicine, Auburn University, Auburn, AL     The early entry of the fish pathogen Flavobacterium columnare and enhancement by scarification was studied in channel catfish, Ictalurus punctatus (Rafinesque), using the polymerase chain reaction (PCR) and a species specific primer set for a bacterial 16-S rRNA gene product.  Evaluations were conducted following a typical abrasion bacterial bath immersion challenge with F. columnare.  The newly developed Fvp2-Fvp3 primer set was useful in detecting the early entry of F. columnare. The pathogen could be detected in various tissues as early as 5 min after immersion challenge at 29+/-2 C. The earliest detection of the pathogen was made in mucus, skin, and gill tissues, occurring at 5 min post challenge. Liver and kidney tissues required as little as 60 min prior to detection. Detection times for F. columnare in blood varied between 11.6 and 60 min. Scarification or abrasion, a practice which has been historically used prior to bacterial challenge, had little or no effect on the timing of early entry of pathogen at 292 C, which calls into question the efficacy of the F. columnare challenge model for channel catfish. However, survival data following abrasion challenge with F. columnare at 29+/-2C showed that scarification does significantly affect the survival of the fish.