Epidemiology of columnaris disease affecting fishes within the same watershed

In the southeastern USA, columnaris disease (caused by Flavobacterium columnare) typically affects catfish raised in earthen ponds from early spring until late summer. Recently, unusually severe outbreaks of columnaris disease occurred at the E. W. Shell Fisheries Center located in Auburn, AL, USA. During these outbreaks, catfish and other aquaculture and sport fish species that were in ponds located within the same watershed were affected. Our objective was to investigate the genetic diversity among F. columnare isolates recovered from different sites, sources, and dates to clarify the origin of these outbreaks and, ultimately, to better understand the epidemiology of columnaris disease. A total of 102 F. columnare isolates were recovered from catfishes (channel catfish Ictalurus puntactus, blue catfish I. furcatus, and their hybrid), bluegill Lepomis microchirus, Nile tilapia Oreochromis niloticus, largemouth bass Micropterus salmoides, egg masses, and water during columnaris outbreaks (from spring 2010 to summer 2012). Putative F. columnare colonies were identified following standard protocols. All isolates were ascribed to Genomovar II following restriction fragment length polymorphism analysis of the 16S rRNA gene. Genetic variability among the isolates was revealed by amplified fragment length polymorphism. Date of isolation explained most of the variability among our isolates, while host was the least influential parameter, denoting a lack of host specificity within Genomovar II isolates. The susceptibility of each of the isolates against commonly used antibiotics was tested by antibiogram. Our data showed that 19.6 and 12.7% of the isolates were resistant to oxytetracycline and kanamycin, respectively.