Butorphanol-Azaperone-Medetomidine Is as Safe and Effective as Nalbuphine-Azaperone-Medetomidine for Immobilization of Juvenile American Black Bears (Ursus americanus) Open Access

Free-ranging American black bears (Ursus americanus) are immobilized frequently in the field for management purposes, often without veterinary presence. Butorphanol-azaperone-medetomidine (BAM; butorphanol (27.3 mg/mL), azaperone (9.1 mg/mL), medetomidine (10.9 mg/mL; ZooPharm, Wedgewood Pharmacy, Laramie, Wyoming, USA) is a commonly used safe and effective immobilization agent combination for wildlife. Benefits of BAM include low volume and rapid recovery compared to older protocols such as ketamine-xylazine (Wolfe et al. 2008; Williamson et al. 2018). Nalbuphine-azaperone-medetomidine (nalbuphine (40 mg/mL), azaperone (10 mg/mL), medetomidine (10 mg/mL; ZooPharm) is a similar immobilization kit shown to be effective in black bears but is not commercially available (Wolfe et al. 2019). However, it may be useful for wildlife agencies in the US because it does not contain US Drug Enforcement Administration controlled substances. Butorphanol and nalbuphine have the same mechanism of action, being opioid mu receptor antagonists and kappa agonists, and are antagonized using naltrexone. Medetomidine is a potent sedative alpha-2 agonist and is antagonized using atipamezole, and azaperone is a butyrophenone tranquilizer (Lamont and Grimm 2014). Our blinded, randomized study of black bears directly compared induction, recovery, and physiologic parameters, including arterial blood gas analysis, between these two immobilization kits while mimicking a field setting without supplemental oxygen.

This study was performed in accordance with the regulations set forth by the Institutional Animal Care and Use Committee at the University of Tennessee College of Veterinary Medicine, City, Tennessee, USA (protocol no. 2822-0221). Healthy bears (four females, six males, all 10 mo old) were completing rehabilitation at Appalachian Bear Rescue (ABR) in Townsend, Tennessee, USA, and presenting for prerelease examination followed immediately by transport and release (6–7 December 2021). Rehabilitation facility and procedure information has been described previously (Sheldon et al. 2022). Immediately before immobilization, bears were moved into either indoor or chain link netting-covered enclosures. Each bear was immobilized via remote delivery of 1 mL of BAM (n=5) or nalbuphine-azaperone-medetomidine (n=5) intramuscularly in the shoulder using a 1.5 mL compressed-air plastic dart, 2 × 40 mm needle, and CO₂-powered pistol (DanInject, Kolding, Denmark). Dosing was selected based on published studies (Wolfe et al. 2008, 2019; Williamson et al. 2018). The drug administered to each bear was randomized via a random number generator, and the lead veterinarian (JDS) was blinded to which drug was loaded into the dart.

Induction parameters were recorded, including time of dart placement, first effects (initial head droop, slowed pace, ataxia, or combinations of these), recumbency (lateral or sternal), and approach by veterinary staff (placed blindfold and loaded into stretcher). Bears were briefly transported in a truck to an onsite clinic for examination, weighed, and then intubated using a 12 mm endotracheal tube to enable measurement of end-tidal CO₂ for accurate ventilation evaluation in conjunction with blood gas analysis. Bears were not provided supplemental oxygen or ventilation. Bears were instrumented with an electrocardiograph to measure heart rate (HR), pulse oximeter to measure blood oxygen saturation (SPO₂), capnograph to measure end-tidal CO2 (ETCO₂) and respiratory rate (RR), noninvasive blood pressure cuff placed on the hindlimb below the stifle to estimate oscillometric mean arterial blood pressure (MAP), and a rectal thermometer to measure body temperature. Readings from these monitors were recorded every 5 min. Anesthetic depth was monitored continuously via palpebral reflex, ear tactile stimulus, and jaw tone. Once instrumented, a femoral arterial sample (0.2–0.5 mL) was collected using a 1 mL plastic syringe and 2.5 cm 22-gauge needle, and immediately analyzed on a point-of-care blood gas machine similar to a prior study (iSTAT 1 Handheld Analyzer, CG4+ cartridges, Zoetis Services LLC, Parsippany, New Jersey, USA) to provide pH, partial pressure of carbon dioxide (PCO₂), partial pressure of oxygen (PO₂), base excess (BE), bicarbonate (HCO₃), total carbon dioxide (TCO₂), oxygen saturation (SO₂), and lactate (Williamson et al. 2018). Bears also received a physical examination, sample collection, morphometrics, ear-tag placement, and lip tattoo. Bears were loaded into transport crates, reversed with atipamezole (5 mg per 1 mg medetomidine) and naltrexone (0.06 mg/kg), and extubated. Recovery times were recorded including time of initial arousal (any voluntary movement), head control, and standing.

A Wilcoxon two-sample test was used to compare weight (kg), dosage (mg/kg) of medetomidine, azaperone, butorphanol or nalbuphine, and induction and recovery parameters between the two protocols and between sex. Two-way analyses of variance (ANOVAs) on rank were used to determine the effect of drug or sex and their interaction term in pH, pCO₂, PO₂, BE, HCO₃, TCO₂, SO₂, or lactate. A mixed effect ANOVA model was used to determine the effect of the drug and the time nested within drug (the effect of drug changing over time), or the effect of sex, and the time nested within sex in HR, SPO₂, and ETCO₂, and to determine the effect of the drug, time, and interaction term in RR and MAP. Least square means were computed and separated with a Tukey-Kramer test. P<0.05 was considered significant. All calculations were carried out using (SAS, Version 9.4, release TS1M7, SAS Software, Cary, North Carolina, USA).

Quantitative results are provided in Tables 1 and 2. All 10 darting events were successful, with 100% injection on first attempt in the shoulder region muscles. Inductions were rapid, smooth, and complete. One bear that received nalbuphine-azaperone-medetomidine required a supplemental dose of 10.0 mg ketamine intramuscularly (ketamine HCL 100 mg/mL, Zetamine, VetOne, Boise, Idaho, USA) during induction immediately before transport, due to voluntary movement. During induction, bears were sensitive to auditory stimuli but once blindfolded were generally unresponsive. All bears were easily intubated once on the examination table. Under both protocols, bears maintained a palpebral reflex and intermittent blink response but were unresponsive to stimuli including switching recumbency position, venipuncture, ear tagging, and lip tattoo placement. Mucus membranes were consistently pale pink. All bears recovered quickly and smoothly and were released within 2 h after anesthetic recovery.

Statistically, there were no differences between protocols or sex in body weight, dosage of azaperone or medetomidine, induction or recovery times, or any physiologic parameter (Tables 1 and 2). Nalbuphine dosage was higher than butorphanol dosage due to a higher concentration supplied in the kit (P=0.028). Bears were hypertensive via noninvasive oscillometric blood pressure and normoxemic via arterial blood gas analysis (pO₂>100 mmHg). However, measurements on the pulse oximeters suggested low oxygen saturation (<95%). Only three blood gas samples in each protocol were truly arterial, therefore only n=6 are reported in Table 2. Arterial oxygen concentrations were subjectively high considering that bears were not provided oxygen supplementation. Some of this could be from air contamination due to bubbles in the syringe, although this was minimized, or being in a plastic, not glass, syringe (Biswas et al. 1982). Another speculation is that ventilation was improved with intubation, as discussed below. Time of reversal was sooner in bears administered BAM compared to those given nalbuphine-azaperone-medetomidine (P=0.047). This was unlikely to be clinically relevant, because the time of reversal was reliant on time it took to collect the samples and perform identification procedures, and the induction times and recovery times post reversal did not differ between protocols.

Based on our study, albeit of small sample size, nalbuphine-azaperone-medetomidine appears to work as clinically safe and effective as BAM in juvenile American black bears in a mimicked field setting. Dosing of 1 mL per approximately 45 kg bear led to an adequate sedative plane for examination and sample collection. Medetomidine dosage of both protocols (approximately 0.2 mg/kg) is high compared to dosages reported in other free-ranging bear protocols (0.04–0.05 mg/kg), but side effects seen in bears in our study (peripheral vasoconstriction leading to pale mucus membranes and inaccurate pulse oximeter readings) were minimal, suggesting a ceiling effect of the alpha-2 agonist in which dose increases while magnitude of physiologic effects do not (Caulkett and Fahlman 2014; Grimsrud et al. 2015). Dose decreasing studies may reveal that lower doses still may provide adequate immobilization; field biologists and co-authors occasionally use lower dosages of BAM that have (anecdotally) resulted in adequate immobilization. However, a previous study referenced unpublished data reporting that bears immobilized with only 0.5 mL of nalbuphine-azaperone-medetomidine occasionally required supplementation (Wolfe et al. 2016). Free-range darting scenarios differ in each situation and often provide more stimulation than that of a trapped bear; therefore, more field testing may be required to fine tune the recommended dosage to minimize induction and capture time.

Bears in our study were intubated without being provided supplemental oxygen in order to measure end-tidal CO₂ while mimicking a field setting as much as possible. However, maintaining an open airway using an endotracheal tube may have affected or improved oxygenation by preventing partial obstructions from soft tissues around the glottis that could occur as a result of muscle relaxation in nonintubated bears. A study comparing intubated and nonintubated bears using the same drug protocol would be required to determine if intubation without supplementation significantly affects oxygenation.

Our study was performed on a small subset of similar bears in the same 2 d of December 2021, just before denning season. Due to the drastic seasonal changes in black bear metabolism, time of year may play a role in drug metabolism and anesthetic results. Bear metabolism significantly decreases during denning season, and, anecdotally, black bears in dens require longer induction times and accurate intramuscular dart placement for adequate immobilization. Some literature reports lower immobilization dosages required for hibernating brown bears (Evans et al. 2012). The safety and efficacy of BAM or nalbuphine-azaperone-medetomidine in denning black bears is beyond the scope of this study and should be investigated further. Biologists in the GSMNP anecdotally report markedly low respiratory rates when using BAM in denning bears and often choose to use ketamine and xylazine instead. In general, biologists and veterinarians working with black bears should be ready to adjust drug protocols as needed depending on health status, age, and time of year.

Our findings suggest that when using these protocols in young healthy bears in nondenning season, one should expect quick induction and recovery, in addition to hypertension, poor peripheral perfusion, and low pulse-oximeter readings despite normoxemia. Further research is needed to determine safe and effective dosage ranges outside of the recommended 1 mL per 45 kg bear; accuracy of noninvasive blood pressure measurements in this species; total duration of safe immobilization without reversal and physiologic parameters during longer immobilizations; and safety and efficacy of these protocols for immobilization of denned bears.

We thank Appalachian Bear Rescue staff, National Park Service and state wildlife agencies, and University of Tennessee veterinary students, house officers, and technicians for providing care to these bears. Authors also thank the Companion Animal Fund at the University of Tennessee College of Veterinary Medicine Department of Small Animal Clinical Sciences for funding this project.