Abstract

Florfenicol is a synthetic, broad-spectrum antibiotic chemically similar to chloramphenicol. It is primarily bacteriostatic, inhibiting bacterial protein synthesis by binding to the 50S ribosomal subunit and inhibiting peptidyl transferase activity. It was approved for use in cattle for bovine respiratory disease (BRD) complex involving Pasteurella haemolytica, Pasteurella multocida, and Haemophilus somnus in May of 1996. There are currently no known interactions between florfenicol and other commonly used antibacterials. In cattle, peak serum levels of 1.43 to 5.60 micrograms per milliliter (µg/ml) are reached between 0.75 to 8 hours after an intramuscular injection (IM) at 20 milligrams per kilogram (mg/kg). Minimum inhibitory concentrations (MIC) for P. multocida and H. somnus of 0.5 µg/ml (1.0 µg/ml for P. haemolytica) are maintained for 48 hours.

Serum levels of florfenicol were obtained using high-performance liquid chromatography in two beluga whales, Delphinapterus leucas, and two bottlenose dolphins, Tursiops truncatus. Serum was collected from the beluga whales at 0.5 1, 2, 4, 6, 12, and 24 hours to determine the half-life (Figure 1). Florfenicol concentrations in samples collected at 24 hours from all four animals in this study were 3.82, 3.83, 3.12, and 2.84 µg/ml. Elimination half-live in the beluga whales was 12.5 hrs (n=2). Concentrations at 48 hours when the second dose is administered were projected to be 0.5 µg/ml (Figure 2). Continued dosing every 48 hours, should result in a potentially beneficial bioaccumulation (Dr. Richard Sams, Ohio State University, Analytical Toxicology Laboratory, personal communication).

The rate of elimination of this antibiotic does not appear to be affected by the metabolic rate of the animal (Dr. Sams, personal communication). This was consistent with our data that demonstrated similar 24 hr concentrations between bottlenose dolphins and beluga whales. We have achieved good clinical results using florfenicol at 20 mg/kg in two beluga whales. It was administered by deep IM injection with no more than 20 ml per injection site. Treatment was continued for four injections 48 hours apart. The only adverse reactions noted were mild elevations in serum aspartate aminotransferase (AST) and lactic dehydrogenase (LDH). These elevations were attributed to muscle trauma at the injection sites and returned to normal within two weeks after the last injection.

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Figure 1. Elimination half-life of florfenicol after a single IM injection at 20 mg/kg in two beluga whales.

Figure 2. Mean elimination curve of florfenicol in beluga whales (n=2).

Mean elimination half life was 12.5 hr. Estimated 36 and 48 hour concentrations are illustrated by dotted line and open circles. MIC ranges from 0.5 to 1.0 ug/ml for Pasteurella sp. and Haemophilis sp.