Efficacy of Florfenicol for Control of Mortality Caused by Flavobacterium columnare Infection in Channel Catfish

The efficacy of florfenicol against infection by the bacterium Flavobacterium columnare was studied in channel catfish Ictalurus punctatus fingerlings held in 80‐L aquaria. Nonabraded fish were challenged by immersion on day 0. Thirty 80‐L tanks were randomly assigned in equal numbers to two treatment groups, one in which fish were fed a commercial diet without florfenicol (unmedicated feed) and one in which they were fed a diet containing 10 mg of florfenicol/kg of body weight (medicated feed) for ten consecutive days. Mortality was monitored during the treatment period and a 14‐d posttreatment observation period. At the end of the posttreatment period, all fish were euthanized, examined for gross lesions, and cultured for F. columnare. Significantly fewer fish fed the medicated diet died (8.0%) than fish fed the unmedicated diet (54.2%). Flavobacterium columnare was cultured from 15.0% of the medicated fish, compared with 68.9% of the unmedicated fish. The gross lesions in the fish were consistent with columnaris disease, and F. columnare was cultured from 99.5% of the dead fish. No differences were observed in weight gain and appetence between the medicated and unmedicated groups. For the F. columnare strain used in this study, the minimal inhibitory concentration of florfenicol ranged from 0.5 to 1.0 mg/mL in the 30 bacterial cultures obtained from infected fish, and the mean disk diffusion zone of inhibition was 40 mm. There were no adverse effects among the medicated fish. Administration of florfenicol at a dosage of 10 mg/kg body weight for 10 d was efficacious and safe for the control of mortality from F. columnare infection in channel catfish.