The intraspecific variability of E. ictaluri isolates from different origins was investigated. Isolates were recovered from farm-raised catfish (Ictalurus punctatus) in Mississippi, USA, tilapia (Oreochromis niloticus) cultured in the Western Hemisphere and zebrafish (Danio rerio) propagated in Florida, USA. These isolates were phenotypically homologous and antimicrobial profiles were largely similar. Genetically, isolates possessed differences that could be exploited by repetitive-sequence-mediated PCR and gyrB sequence, which identified three distinct E. ictaluri genotypes: one associated with catfish, one from tilapia and a third from zebrafish. Plasmid profiles were also group specific and correlated with rep-PCR and gyrB sequences. The catfish isolates possessed profiles typical of those described for E. ictaluri isolates; however, plasmids from the zebrafish and tilapia isolates differed in both composition and arrangement. Furthermore, some zebrafish and tilapia isolates were PCR negative for several E. ictaluri virulence factors. Isolates were serologically heterogenous, as serum from a channel catfish exposed to a catfish isolate had reduced antibody activity to tilapia and zebrafish isolates. This work identifies three genetically distinct strains of E. ictaluri from different origins using rep-PCR, 16S, gyrB and plasmid sequencing, in addition to antimicrobial and serological profiling.

The 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.

Plasma distribution and elimination of florfenicol in channel catfish were investigated after a single dose (10 mg/kg) of intravenous (i.v.) or oral administration in freshwater at a mean water temperature of 25.4 °C. Florfenicol concentrations in plasma were analyzed by means of liquid chromatography with MS/MS detection. After i.v. florfenicol injection, the terminal half-life (t(1/2)), volume of distribution at steady state (V(ss)), and central volume of distribution (V(c)) were 8.25 h, 0.9 and 0.381 L/kg, respectively. After oral administration of florfenicol, the terminal t(1/2), C(max), T(max), and oral bioavailability (F) were 9.11 h, 7.6 μg/mL, 9.2 h, and 1.09, respectively. There was a lag absorption time of 1.67 h in oral dosing. Results from these studies support that 10 mg florfenicol/kg body weight in channel catfish is an efficacious dosage following oral administration.

The median lethal dose of botulinum serotype E in 5.3-g channel catfish Ictalurus punctatus fingerlings was determined. Five tanks (five fish/tank) were assigned to each of the following treatment groups: 70, 50, 35, 25, or 15 pg of purified botulinum serotype E. Fish were injected intracoelomically and observed for 96 h. Administration of the toxin resulted in initial hyperactivity followed by erratic swimming, paresis, and death. The cumulative mortality by treatment group was 100% at 70 pg, 96% at 50 pg, 100% at 35 pg, 88% at 25 pg, and 56% at 15 pg. The median lethal dose was calculated as 13.7 pg/fish (equivalent to a 0.81 median lethal dose for mice Mus musculus) using a logistic regression model. All fish were necropsied; lesions included exophthalmia, ascites, splenic congestion, intussusception of the intestines, congested spleen, and blanching of the intestinal tract. The resultant clinical signs and lesions were similar to those noted in the syndrome of visceral toxicosis of catfish. This study indicates that channel catfish are more sensitive to the effects of botulinum serotype E than laboratory mice, and the signs and lesions of visceral toxicosis of catfish were replicated by injecting catfish with the toxin.