Field efficacy of florfenicol for control of mortality in channel catfish, Ictalurus punctatus (Rafinesque), caused by infection with Edwardsiella ictaluri

A field study to assess the efficacy of florfenicol (FFC) against enteric septicemia of catfish (ESC) was conducted with pond-reared channel catfish fingerlings held in 0.1-acre earthen ponds. Fish were challenged with Edwardsiella ictaluri in a natural pond outbreak or by cohabitation with E. ictaluri-infected fish held in netpens. Fourteen ponds were assigned in equal number to two treatment groups, that is, either treated (with 10 mg FFC/kg body weight in medicated feed) or not treated (control) for 10 consecutive d. The threshold for enrollment into the study was 0.3% cumulative mortality attributed to ESC. Treatment was initiated on different dates for each pond because each pond was enrolled when 33 fish/pond were diagnosed with ESC based on clinical signs, lesions, or positive cultures. Mortality was monitored during the 10-d treatment period and during a 14-d posttreatment observation period. At the end of the 14-d posttreatment observation period, all fish were euthanized, and 20 fish from each pond were examined by gross necropsy and evaluated for the presence of E. ictaluri by bacterial culture. The odds of a mortality in the control group were 2.20 times the odds of a mortality in the FFC-treated group. Significantly fewer (P <= 0.05) FFC-medicated catfish died in comparison to unmedicated catfish. The minimum inhibitory concentration of FFC for this strain of E. ictaluri was 0.25 mu g/mL in all fish that were assayed. The mean zone of inhibition (Kirby Bauer) was 36.8 mm from E. ictaluri isolates of test fish. There were no FFC treatment-related lesions seen on gross pathology. FFC was efficacious and safe for control of mortality from E. ictaluri infection in catfish.