Optimization of florfenicol dose against Piscirickettsia salmonis in Salmo salar through PK/PD studies

Abstract

Salmonid Rickettsial Septicemia (SRS) is the disease of greatest economic importance in the Chilean salmon farming industry, causing high mortality in fish during the final stage of their productive cycle at sea. Since current, commercially available vaccines have not demonstrated the expected efficacy levels, antimicrobials, most commonly florfenicol, are still the main resource for the treatment and control of this pathogen. The aim of this study was to determine the most appropriate single dose of florfenicol, administered through medicated feed, for the treatment of Piscirickettsia salmonis (P. salmonis), using pharmacokinetic/pharmacodynamic (PK/PD) models. Previously, Minimum Inhibitory Concentrations (MICs) of florfenicol were determined for 87 P. salmonis isolates in order to define the epidemiological cut-off point (COWT). The most commonly observed MIC was 0.125 μg mL-1 (83.7%). The COWT value was 0.25 μg mL-1 with a standard deviation of 0.47 log2 μg mL-1 and 0.36 log2 μg mL-1, for Normalized resistance interpretation (NRI) method and ECOFFinder method, respectively. A MIC of 1 μg mL-1 was considered the pharmacodynamic value (PD) to define PK/PD indices. Three doses of florfenicol were evaluated in fish farmed under controlled conditions. For each dose, 150 fish were used and blood plasma samples were collected at different time points (0-48 hours). PK parameters were obtained from curves representing plasma concentrations as a function of time. The results of Monte Carlo simulation indicate that at a dose of 20 mg/Kg l.w. of florfenicol, administered orally as medicated feed, there is 100% probability (PTA) of achieving the desired efficacy (AUC0-24h/MIC>125). According to these results, we suggest that at the indicated dose, the PK/PD cut-off point for florfenicol versus P. salmonis could be 2 μg mL-1 (PTA = 99%). In order to assess the indicated dose in Atlantic salmon, fish were inoculated with P. salmonis LF-89 strain and then treated with the optimized dose of florfenicol, 20 mg/Kg bw for 15 days.

Fig 1. Distribution of the Minimum Inhibitory Concentration values of 87 strains of Piscirickettsia salmonis versus florfenicol.

Distributions obtained through the ECOFFinder (a) and NRI (b) programs. Bars represent the gross count of the number of isolates in each concentration of antimicrobial; lines represent the curves of best fit for the distribution of WT strains.

Fig 2. Average plasma concentrations (μg/mL) by sampling time (hours) and dose: 10, 15 and 20 mg/Kg.

Different letters represent statistical differences between groups (p<0.05).

Fig 3. Cumulative mortality (%) in Atlantic salmon after P. salmonis challenge in dilutions of 1/10, 1/100, 1/1000 and 1/10000. TK: Tank.

Fig 4. Specific Feed Rate (%) by date of experiment.

Challenge group: Tanks 2 and 3; Control (+) (without medication): Tanks 5 and 6; Control (-) (without challenge): Tank 9. TK: Tank; SFR: Specific Feed Rate. For more information, see S4 Table.

Fig 5. Cumulative mortality (%) by date of experiment.

Challenge group: Tanks 2 and 3; Control (+) (without medication): Tanks 5 and 6; Control (-) (without challenge): Tank 9. For more information, see S5–S7 Tables.