Abstract

In September 1995 an epizootic characterized by progressive lethargy, anorexia, ocular discharge, edema, weakness, and generalized paraparesis resulted in the death or euthanasia of 74 captive American alligators (Alligator mississippiensis) from a male herd of 89 conspecifics located in St. Johns County, Florida.² Pathologic findings included a fibrinous tracheitis, pneumonia, polyarthritis, and fibrinous coelomitis. In addition to a variety of gram-negative bacteria isolated from multiple tissues, a new Mycoplasma, tentatively named Mycoplasma lacerti, was cultured from the lung, trachea, joints, and cerebrospinal fluid of symptomatic alligators.¹

Nine Mycoplasma isolates were obtained from six symptomatic alligators and the minimum inhibitory concentration (MIC) for nine antibacterial agents were determined by serial dilution in broth and plate culture of each mycoplasma isolate. The MIC obtained for doxycycline, enrofloxacin, sarafloxacin, oxytetracycline, tilmicosin, and tylosin (<1 µg/ml) were lower than that of clindamycin (1–8 µg/ml), chloramphenicol (8–16 µg/ml), and erythromycin (32–128 µg/ml). Based upon these results, enrofloxacin and oxytetracycline were chosen for pharmacokinetic evaluation.

The seven alligators used in this study were captive-reared, of unknown gender, clinically healthy, and ranged in weight from 2.85–4.70 kg. Alligators were acclimated at 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. Five alligators received enrofloxacin (Baytril, 22.7 mg/ml; Bayer Co., Shawnee Mission, KS) at 5 mg/kg and two alligators received oxytetracycline (Liquamycin LA-200, 200 mg/ml; Pfizer, New York, NY) at 10 mg/kg as a single intravenous bolus in the supravertebral vein. All alligators were manually restrained, and 2.0 ml of blood collected from the supravertebral vein into lithium heparinized tubes at 0.5, 1, 2, 4, 8, 12, 24, 36, 48, 60, 72, 84, and 96 hours following drug injection. Plasma was separated by centrifugation, aliquoted into 1 ml plastic cryotubes and frozen at -10°C.

Sample analysis for both drugs were performed using high-performance liquid chromatography (HPLC) (North Carolina State University, Raleigh, NC). 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).

Both enrofloxacin and oxytetracycline were best described by a two-compartment model. At the doses administered, both enrofloxacin and oxytetracycline exceeded the target MIC for Mycoplasma lacerti of 1.0 µg/ml. Enrofloxacin achieved a mean peak plasma concentration of 6.03 µg/ml and oxytetracycline a mean peak plasma concentration of 98 µg/ml at time zero. Plasma enrofloxacin concentrations were maintained above 1.0 µg/ml for an average of 31 hours, while the mean plasma oxytetracycline level at 96 hours was 6.52 µg/ml. In order to maintain plasma concentrations above the target MIC for two-thirds of the dosing interval, enrofloxacin at 5 mg/kg should be administered every 36 hours for Mycoplasma lacerti infections. Further sampling beyond 96 hours is necessary before an accurate dosing interval can be proposed for oxytetracycline.

Literature Cited

1.  Brown DR, Clippinger TL, Helmick KE, et al. Mycoplasma isolation during a fatal epizootic of captive alligators (Alligator mississippiensis) in Florida. Inter Org Mycoplasmology Letters. 1996;4:42–43.

2.  Clippinger TL, Bennett RA, Johnson CM, Vliet KA, Jacobson ER, Brown DR, Brown MR. Mycoplasma epizootic in a herd of bull alligators (Alligator mississippiensis). In: Proceedings from the American Association of Zoo Veterinarians. 1996:230–234.