Determination of florfenicol dose rate in feed for control of mortality in Nile tilapia infected with Streptococcus iniae.

A dose titration study was conducted to determine the dosage of florfenicol (FFC) in feed to control Streptococcus iniae-associated mortality in Nile tilapia Oreochromis niloticus. Six tanks were assigned to each of five treatments: (1) not challenged with S. iniae and fed unmedicated feed; (2) challenged with S. iniae by injection and fed unmedicated feed; (3) challenged with S. iniae and given FFC at 5 mg/kg of body weight (bw) in medicated feed; (4) challenged with S. iniae and given 10 mg FFC/kg bw; and (5) challenged with S. iniae and given 15 mg FFC/kg bw. Treatment was initiated the day after inoculation, and feed was administered for 10 d. Cumulative mortality was 0% in the unchallenged, untreated group; 35.8 +/- 4.4% (mean +/- SE) in the challenged, unmedicated group; 19.2 +/- 2.7% in the 5-mg/kg treated group, 12.5 +/- 3.8% in the 10-mg/kg group, and 2.5 +/- 1.1% in the 15-mg/kg group. The cumulative mortality was significantly less in each challenged, FFC-treated group than in the challenged, unmedicated controls (5 mg/ kg: P = 0.0156; 10 mg/kg: P = 0.0007; 15 mg/kg: P < 0.0001). The efficacy of the 10- and 15-mg/kg FFC dosages was studied in a separate dose confirmation study. Fish in all tanks were injected with S. iniae. At 4 h postinoculation, 10 tanks were assigned to each of three feed treatments: (1) unmedicated feed; (2) 10 mg FFC/kg bw; and (3) 15 mg FFC/kg bw. Cumulative mortality was 20.5 +/- 2.0% in the challenged, unmedicated group; 11.0 +/- 2.1% in the 10-mg/kg group; and 5.5 +/- 2.4% in the 15-mg/kg group. Mortality was significantly less in the medicated groups than in the challenged, unmedicated control group (10 mg/kg: P = 0.0270; 15 mg/kg: P = 0.0007). Fish in both studies were necropsied, cultured for bacteria, and examined for gross lesions. The minimum inhibitory concentration of FFC against S. iniae in both studies ranged from 0.5 to 1.0 microg/mL. Florfenicol was palatable, safe, and efficacious for control of Nile tilapia mortality due to S. iniae infection.

In vitro evaluation of the susceptibility of Edwardsiella ictaluri, etiological agent of enteric septicemia in channel catfish, Ictalurus punctatus (Rafinesque), to florfenicol.

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 Aquaculture 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 microg/ml. Edwardsiella ictaluri tested in these studies were highly sensitive to FFC in vitro.