Determination of Dose Rate of Florfenicol in Feed for Control of Mortality in Channel Catfish Ictalurus punctatus (Rafinesque) Infected with Edwardsiella ictaluri, Etiological Agent of Enteric Septicemia

Abstract

Abstract–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.