Efficacy of Orally Administered Florfenicol in the Treatment of Furunculosis in Atlantic Salmon

This study was performed to determine the efficacy of orally administered florfenicol to treat experimentally induced furunculosis in Atlantic salmon Salmo salar held in seawater. A strain of the causative bacterium. Aeromonas salmonicida subsp. salmonicida, sensitive to florfenicol was used. In two trials, cohabitation challenges were performed by introducing six (trial 1) or eight (trial 2) fish-challenged in advance by an intraperitoneal injection of 2.2 x 10(4) colony-forming units-to eight aquaria, each containing 40 healthy fish. The treatment groups in both trials consisted of three groups receiving medication at 10, 13, and 16 d, respectively, postchallenge and one control group. An unchallenged, unmedicated group was used to determine the natural mortality in the population. The recommended therapeutic dose of 10 mg florfenicol/kg fish daily for 10 d was used. A higher mortality rate was observed in both trials for challenged unmedicated control fish than for unchallenged fish. In both trials, the cumulative mortality of fish treated with florfenicol was strikingly lower than the mortality of fish in the control groups. The mortality for the two untreated control groups was quite different; the lower mortality rate occurred in the group with the lower number of cohabitants. The mortality rates of the treated groups in trial 1 differed only slightly from each ether, whereas in trial 2. the group starting medication at 10 d had a substantially lower mortality rate than the group starting medication at 16 d postchallenge.