Therapeutic effect of florfenicol on experimentally induced pseudotuberculosis in yellowtail.

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Abstract
Young yellowtails experimentally infected with Pasteruella piscicida were given single oral doses of florfenicol (FF, several strengths), ampicillin (ABPC), oxolinic acid (OA) or sodium nifurstyrenate (NFS-Na). The therapeutic effects of continuous administration of different doses of florfenicol mixed into pelleted feed and fish mince with and without binder were also evaluated. The first doses were given 3 hours postinfection and the fish were observed for 9 to 17 days after infection. The mortality rates were calculated on the basis of the number of fish which died during the experimental periods. Treatment with single dose administration gave mortality rates of 0, 0, 10, 70, 60 and 80% for the 100, 50 and 25 mg/kg FF, 20 mg/kg ABPC, 30 mg/kg OA and 50 mg/kg NFS-Na treated groups, respectively, and 90% in the non-medicated group. The therapeutic effects of the continuous administration of FF varied with the administration methods. The highest effect was obtained by mixing FF into pelleted feed. No deaths were observed in the groups given 6.3 mg/kg, or more of FF, and the ED50 was 0.82 mg/kg. On the other hand, the therapeutic effect of FF administered by mixing into fish mince without binder, was the lowest. The mortality rates of the 25 and 12.5 mg/kg FF treated groups were 20 and 40%, respectively, and the ED50 was 10.5 mg/kg. Relatively good therapeutic effects were obtained by mixing FF into fish mince with binder. The mortality rates of the 50, 25, 12.5 and 6.3 mg/kg FF treated groups were 0, 10, 0 and 10%, respectively, and the ED50 was considered to be lower than 6.3 mg/kg but higher than 0.82 mg/kg.

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