Abstract

An epizootic characterized by progressive lethargy, anorexia, ocular discharge, edema, weakness, and paraparesis attributed to a new Mycoplasma organism, tentatively named Mycoplasma lacerti, has been documented in a captive herd of male American alligators (Alligator mississippiensis).^1,2 Minimum inhibitory concentrations (MIC) for nine antibacterial agents were determined for isolates obtained from symptomatic alligators.³ Based upon these results, enrofloxacin and oxytetracycline were chosen for pharmacokinetic evaluation. Preliminary kinetic parameters of both drugs administered intravenously in American alligators have been described.³ Further characterization of enrofloxacin and oxytetracycline disposition in the American alligator was performed.

The ten alligators used in this study were captive-reared, unknown gender, clinically healthy, and ranged in weight from 2.1–9.7 kg. Alligators were acclimated to 27°C for five days prior to the study and maintained at this temperature during the period of sample collection. Prior to drug administration, a time zero blood sample was obtained for baseline analysis and generation of a standard curve. All alligators were manually restrained, and blood samples collected from the supravertebral vein and placed into lithium heparinized tubes. Five alligators received oxytetracycline (Liquamycin LA-200, 200 mg/ml, Pfizer, New York, NY, USA) at 10 mg/kg as a single bolus administered in the left biceps brachii muscle. Blood samples for analysis were collected at 0.5, 1, 2, 4, 8, 12, 24, 48, 72, 96, 120, 144, 168, and 192 hours following drug injection. Five alligators received enrofloxacin (Baytril, 22.7 mg/ml, Bayer Co., Shawnee Mission, KS, USA) at 5 mg/kg as a single oral bolus, administered through a pre-measured polyurethane tube passed through the mouth into the stomach. Blood samples for analysis were collected at 1, 4, 8, 12, 24, 36, 48, 60, 72, and 96 hours following drug injection. All alligators were fasted prior to and during the study period. Plasma was separated by centrifugation, aliquoted into 1 ml plastic cryotubes and frozen at -100°C.

Sample analysis for both drugs was performed using high-performance liquid chromatography (North Carolina State University, Raleigh, NC, USA). Ciprofloxacin, a known metabolite of enrofloxacin, which exhibits a similar spectrum of activity, was also assayed. The limit of quantitation (LOQ) for enrofloxacin was 0.05 µg/ml and the limit of detection (LOD) 0.02 µg/ml. For oxytetracycline, the LOQ was 0.25 µg/ml and the LOD approximated 0.20 µg/ml. For each animal, a plasma drug concentration-versus-time curve was generated. Compartmental modeling was performed using computer software for polyexponential curve stripping, fitting and least squares parameter estimation of the data (RSTRIP II version 1.0, MicroMath, Salt Lake City, UT, USA).

A two-compartment model best described both enrofloxacin and oxytetracycline. Oxytetracycline exceeded the established MIC for Mycoplasma lacerti of 1.0 µg/ml, with a mean peak plasma concentration of 6.85 µg/ml occurring at 1-hour post-injection. Average plasma concentrations of oxytetracycline at 192 hours post-injection were four times the minimum inhibitory concentration. Plasma enrofloxacin concentrations remained below the target MIC of 1.0 µg/ml throughout the sampling period, with a mean peak plasma concentration of 0.45 µg/ml occurring at 24-hour post-administration. Mean plasma ciprofloxacin levels did not rise much above the LOD (0.02 µg/ml), with average concentrations ranging from 0.06–0.12 µg/ml throughout the sampling period.

Based upon the data evaluated here, enrofloxacin administered orally to fasted American alligators at 5 mg/kg is not expected to achieve minimum inhibitory concentrations established for Mycoplasma lacerti. Although present, the small concentration of ciprofloxacin detected should not significantly contribute to the in vivo efficacy of enrofloxacin. Long-acting oxytetracycline, administered intramuscularly to American alligators at 10 mg/kg, is expected to exceed minimum inhibitory concentrations established for Mycoplasma lacerti, with average plasma drug levels maintained above the target MIC at seven days post-administration.

Literature Cited

1.  Brown, D.R., T.L. Clippinger, K.E. Helmick, I.M. Schumacher, R.A. Bennett, C.M. Johnson, K.A. Vliet, E.R. Jacobson, and M.B. Brown. 1996. Mycoplasma isolation during a fatal epizootic of captive alligators (Alligator mississippiensis) in Florida. Inter. Org. Mycoplasmology Letters. 4: 42–43.

2.  Clippinger, T.L., R.A. Bennett, C.M. Johnson, K.A. Vliet, E.R. Jacobson, D.R. Brown, and M.R. Brown. 1996. Mycoplasma epizootic in a herd of bull alligators (Alligator mississippiensis). Proc. Amer. Assoc. Zoo Vet.; pp. 230–234.

3.  Helmick, K.E., M.G. Papich, K.A. Vliet, R.A. Bennett, M.B. Brown, and E.R. Jacobson. 1997. Preliminary kinetics of single-dose intravenously administered enrofloxacin and oxytetracycline in the American alligator (Alligator mississippiensis). Proc. Amer. Assoc. Zoo Vet; pp. 27–28.