A tolerance study was conducted to determine the palatability of florfenicol to channel catfish Ictalurus punctatus. Four tanks of fish (20 fish/tank) were assigned to each of five treatments distinguished by the amount of florfenicol given in feed per kilogram of body weight, namely, 0, 10, 20, 40, or 100 mg. Fish were fed at a rate of 2.5% of body weight per day for 10 consecutive days. On day 11, all surviving fish were euthanatized, counted, and weighed as a group. Florfenicolmedicated feed was palatable to fish at doses of 10, 20, 40, and 100 mg for 10 consecutive days.

In vitro studies were conducted to assess the sensitivity of Edwardsiella ictaluri, the etiological agent of enteric septicemia of catfish (ESC), to the antibacterial drug florfenicol (FFC). Twelve different E. ictaluri isolates from cases submitted between 1994 and 1997 to the Thad Cochran National Warmwater Aqua-culture Center fish diagnostic laboratory (Stoneville, MS) were used for testing. These isolates originated from channel catfish ( Ictalurus punctatus) infected with E. ictaluri through natural outbreaks of ESC in the commercial catfish ponds in Mississippi. Seven hundred sixty-seven additional cultures of E. ictaluri were obtained from channel catfish infected experimentally with E. ictaluri. In some of these experimental infections, FFC was used for treatment. These cultures of E. ictaluri were identified by morphological and biochemical tests. Kirby-Bauer zones of inhibition (in mm) for FFC against E. ictaluri were determined using standard methods. The minimum inhibitory concentration (MIC) of FFC was determined for the natural outbreak E. ictaluri isolates and arbitrarily selected experimental cultures. The zones of inhibition for FFC tested with E. ictaluri ranged from 31 to 51 mm. The MIC for FFC tested with E. ictaluri was consistently 0.25 μg/ml. Edwardsiella ictaluri tested in these studies were highly sensitive to FFC in vitro.

–A dose titration study was conducted to determine the appropriate dosage of florfenicol in feed to control mortality in channel catfish Ictalurus punctatus associated with enteric septicemia of catfish caused by Edwardsiella ictaluri. Six tanks (20 fish/ tank) were assigned to each of the following treatment: 1) not challenged with E. ictaluri and fed unmedicated feed; 2) challenged with E. ictaluri and fed unmedicated feed; 3) challenged and fed 5‐mg florfenicol/kg body weight (kg bw); 4) challenged and fed 10‐mg florfenicol/kg bw; or 5) challenged and fed 15‐mg florfenicol/kg bw. Treatment was initiated the day after inoculation, and feed was administered by hand at 2.5% body weight for 10 consecutive days. Feeding activity was scored for all groups and was noted to be significantly less than the challenged, unmedicated group. Cumulative mortality in the challenged untreated group was 60%. The mortality in the unchallenged untreated group was 0%, and in die 5‐, 10‐, 15‐mg florfenicol/kg bw group was 2.5%, 0.8%, and 2.5%, respectively. The mortality in each challenged, treated group and the non‐challenged control group was significantly less than the challenged, unmedicated controls (P < 0.0001 for each contrast). There were no pairwise statistically significant contrasts among the florfenicol treated groups and the non‐challenged control group. All 600 fish in the study were necropsied, cultured for bacteria, and examined by gross pathology. No specific lesions that could be associated with the antibiotic were observed. The efficacy of the 10 mg/kg dosage was confirmed in a separate dose confirmation study. In this study, fish in 30 tanks (20 fish/ tank) were infected with E. icraluri by immersion. Two days post‐inoculation, fish in 15 tanks were hand‐fed unmedicated feed, and 15 tanks were hand‐fed medicated feed at a dosage of 10‐mg florfenicol/kg bw at 2.5% body weight for 10 d. Feeding activity was scored and was noted to be significantly less than the challenged, unmedicated group. Cumulative mortality in the florfenicol group (14%) was significantly less than cumulative mortality in the untreated group (87.3%) (P < 0.0001). All 600 fish were submitted for bacterial culture, necropsied. and examined for gross pathology, and once again, no specific lesions that could be associated with the antibiotic were observed. The minimum inhibitory concentration of florfenicol against E. ictaluri in both studies was 0.25 ug/mL. Florfenicol was palatable, safe, and efficacious for control of mortality due to infection by E. ictaluri in catfish.