Evidence based antimicrobial use in camelids (Proceedings)

Bacterial infections requiring extended antimicrobial therapy (such as pneumonia, peritonitis, sepsis, uterine and dental infection) are a significant cause of morbidity and mortality in camelids. However, to date only seven antimicrobials have been evaluated in the peer-reviewed literature and subsequently proven to have favorable pharmacokinetics for therapeutic use .

For example, bacterial isolates obtained from both llamas and alpacas have demonstrated favorable sensitivity patterns to the antimicrobial florfenicol . Isolates from tooth root abscesses commonly include Actinomyces spp and Actinobacillus spp , which are considered susceptible to the antibiotic florfenicol, based on breakpoints used for cattle . Florfenicol is a broad-spectrum antibiotic with penetration into most body tissues including internal organs, skeletal muscle, milk, synovial fluid and to a lesser extent aqueous humor and spinal fluid. However, recent data suggests that the subcutaneous dosing of 40mg/kg florfenicol in the form of Nuflor® in alpacas may be inadequate for some bacterial pathogens unless multiple doses are administered.

On the other hand, there is evidence of adverse effects caused by Nuflor® after repeated 40mg/kg SC doses. Holmes et al investigated the pharmacokinetics of florfenicol (Nuflor®; 300 mg/mL; Schering-Plough Corp., Kenilworth, NJ, USA) in eight healthy adult alpacas, following single dose injection of 20mg/kg IM (intramuscular) and 40mg/kg SC (subcutaneous) using a randomized, cross-over design and 28-day washout period. Subsequently, 40mg/kg Nuflor® (florfenicol) was administered SC every other day for 10 doses to evaluate long-term effects.

Maximum plasma florfenicol concentrations (Cmax, measured via high-performance-liquid-chromatography) were achieved rapidly, leading to a higher Cmax of 4.31+/-3.03 µg/ml following administration of 20mg/kg IM than 40mg/kg SC (Cmax: 1.95+/-0.94 µg/ml). Multiple SC dosing at 48hr intervals achieved a Cmax of 4.48+/-1.28 µg/ml at steady state. The area under the curve and terminal elimination half-lives were 51.83+/-11.72μg/ml.h and 17.59+/-11.69 hours after single 20mg/kg IM administration, as well as 99.78+/-23.58μg/ml.h and 99.67+/-59.89 hours following 40mg/kg injection of Nuflor® SC, respectively.

Florfenicol decreased the following hematological parameters after repeated administration between weeks 0 and 3: total protein (6.38 vs. 5.61 g/dL, P<0.0001), globulin (2.76 vs. 2.16 g/dL, P<0.0003), albumin (3.61 vs. 3.48 g/dL, P=0.0038), white blood cell count (11.89 vs. 9.66 x10^3/µL, P<0.044), and hematocrit (27.25 vs. 24.88%, P<0.0349). Significant clinical illness was observed in one alpaca.

The florfenicol product Nuflor Gold® (300 mg/mL; Schering-Plough) was recently introduced to the US market, in addition to standard Nufor®. This new formulation is claimed to show better absorption and leads to higher plasma drug concentrations in cattle following single 40mg/kg SC dosing (Nuflor Gold® Cmax: 5.93 mcg/mL; Nuflor® Cmax: 4.69 mcg/mL).11

Similarly, a 2011 study in camelids suggests that the maximum plasma florfenicol concentration is 3-4 times higher in alpacas receiving a single dose of 40mg/kg Nuflor Gold® SC vs. 40mg/kg Nuflor® SC. In alpacas, plasma levels remained above 1 mcg/mL for 24 hours (mean concentration of 1.94 +/- 1.5 mcg/mL at 18 hrs) following the administration of Nuflor Gold®, thus showing superior pharmacokinetics for use in alpacas in comparison to conventional Nuflor®.

The disposition of five therapeutic antimicrobial agents was studied in llamas following intravenous bolus administration by Christensen et al. Six llamas were each given ampicillin, tobramycin, trimethoprim / sulfamethoxazole, enrofloxacin and ceftiofur at a dose of 12 mg/kg, 1 mg/kg, 3 mg/kg / 15 mg/kg, 5 mg/kg, and 2.2 mg/kg of body weight, respectively, with a wash out period of at least 3 days between treatments. Plasma concentrations of these antimicrobial agents were determined by reverse phase HPLC over 12 hours following IV bolus dosing.

Based on the disposition parameters determined, and assumptions of likely effective minimum inhibitory concentrations (MIC), the dose and dosing interval for the following antimicrobial agents was suggested as follows: 6 mg/kg every 12 h for ampicillin; 4 mg/kg once a day or 0.75 mg/kg every 8 h for tobramycin; 5 mg/kg every 12 h for enrofloxacin; and 2.2 mg/kg every 12 h for ceftiofur sodium for llamas.

The pharmacokinetics and bioavailability of trimethoprim-sulfamethoxazole (TMP-SMX) were also studied in six healthy male-castrate alpacas after intravenous (IV) or oral (PO) drug administration of 15 mg/kg TMP-SMX using a crossover design with a 2-week washout period. After 90 days one group (n = 3) was given a PO dose of 30 mg/kg TMP-SMX and the other group (n = 3) was given a PO dose of 60 mg/kg TMP-SMX. After IV administration of 15 mg/kg of TMP-SMX the mean initial plasma concentration (C0) was 10.75 +/- 2.12 microg/mL for trimethoprim (TMP) and 158.3 +/- 189.3 microg/mL for sulfamethoxazole (SMX).

Elimination half-lives were 0.74 +/- 0.1 h for TMP and 2.2 +/- 0.6 h for SMX. The mean residence times were 1.45 +/- 0.72 h for TMP and 2.8 +/- 0.6 h for SMX. The areas under the respective concentration vs. time curves (AUC) were 2.49 +/- 1.62 microg h/mL for TMP and 124 +/- 60 microg h/mL for SMX. Total clearance (Clt) for TMP was 21.63 +/- 9.85 and 1.90 +/- 0.77 mL/min kg for SMX. The volume of distribution at steady state was 2.32 +/- 1.15 L/kg for TMP and 0.35 +/- 0.09 L/kg for SMX.

After intragastric administration of 15, 30 and 60 mg/kg the peak concentration (Cmax) of SMX were 1.9 +/- 0.8, 2.6 +/- 0.4 and 2.8 +/- 0.7 microg/mL, respectively. The AUC was 9.1 +/- 5, 25.9 +/- 3.3 and 39.1 +/- 4.1 microg h/mL, respectively. Based upon these AUC values and correcting for dose, the respective bioavailabilities were 7.7, 10.5 and 7.94%. Trimethoprim was not detected in plasma after intragastric administration. These data demonstrate that therapeutic concentrations of TMP-SMX are not achieved after PO administration to alpacas.

The disposition of ceftiofur sodium was studied in llamas following intramuscular (IM) administration and in alpacas following intravenous and IM administration. Eleven adult llamas were given ceftiofur sodium by IM injection. Each animal received either a standard dose of 2.2 mg/kg or an allometrically scaled dose ranging from 2.62 to 2.99 mg/kg in a crossover design. Ten adult alpacas were given ceftiofur sodium by IV and IM injections. Each animal received a standard dosage of 1 mg/kg or an allometrically scaled dose ranging from 1.27 to 1.44 mg/kg IV, and 1.31-1.51 mg/kg IM.

Blood samples were collected at 0, 0.25, 0.5, 1, 2, 4, 8, 12, 24, 36, 48, and 72 h after administration of ceftiofur. Pharmacokinetic parameters of ceftiofur in llamas and alpacas were similar following IM dosing at both dose levels. Overall, ceftiofur pharmacokinetics in llamas and alpacas were similar, and comparable to reported parameters for sheep and goats.

Both single and multiple dose trials to determine the pharmacokinetics of gentamicin sulfate have been performed in clinically normal, adult male llamas. Nineteen llamas were given gentamicin (5 mg/kg of body weight, IV) as a single bolus, and serum gentamicin concentration was monitored over the next 48 hours. Two months later, 10 llamas were given gentamicin (2.5 mg/kg) IV for the first day, then IM every 8 hours for 7 days.

The 5 mg/kg IV kinetic study revealed t1/2 alpha of 14.5 +/- 5.06 minutes and t1/2 beta of 166 +/- 20.5 minutes. Similarly, the 2.5 mg/kg IV kinetic study revealed t1/2 alpha of 17.7 +/- 6.59 minutes and t1/2 beta of 165 +/- 40.3 minutes. Peak serum gentamicin concentration averaged 10.1 micrograms/ml in the multiple-dose trial, and trough concentration averaged 1.5 micrograms/ml. Dose effects were not observed for gentamicin clearance, volume of distribution, or half-lives.

However, it has been shown that once daily dosing of a higher aminoglycoside concentration is associated with less renal toxicity than multiple lower daily doses, based on studies in humans and horses. In this context, it was identified that renal tubular uptake (and thus toxicity) of aminoglycoside is most dependent of the duration of drug exposure. Overall, gentamicin pharmacokinetic variables in llamas resemble those in other ruminant species.

Gandorf et al determined the plasma concentrations of enrofloxacin and the active metabolite ciprofloxacin after PO, SC and IV administration in alpacas. A crossover design was used for administration of 3 single-dose treatments of enrofloxacin to alpacas, which was followed by an observational 14-day multiple-dose regimen. Single-dose treatments consisted of IV and SC administration of injectable enrofloxacin (5 mg/kg) and PO administration of enrofloxacin tablets (10 mg/kg) dissolved in grain to form a slurry.

Plasma enrofloxacin concentrations were measured by use of high-performance liquid chromatography. The multiple-dose regimen consisted of feeding a mixture of crushed and moistened enrofloxacin tablets mixed with grain. Behavior, appetite, and fecal quality were monitored throughout the 14-day treatment regimen and for 71 additional days following treatment. Mean half-life following IV, SC and PO administration was 11.2, 8.7, and 16.1 hours, respectively.

For SC and PO administration, mean total systemic availability was 90.18% and 29.31%, respectively; mean maximum plasma concentration was 3.79 and 1.81 microg/mL, respectively; and area under the curve (AUC) was 50.05 and 33.97 (microg x h)/mL, respectively. The SC or PO administration of a single dose of enrofloxacin yielded a ratio for AUC to minimum inhibitory concentration >100 for many grampositive and gram-negative bacterial pathogens common to camelids. The administration of enrofloxacin (5 mg/kg, SC or 10 mg/kg, PO) may be thus be appropriate for antimicrobial treatment of adult alpacas.

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