Comparison of the efficacy and safety of medetomidine-ketamine versus medetomidineazaperone-alfaxalone combination in free-ranging Rocky Mountain bighorn sheep (Ovis canadensis)

Chemical immobilization is a key aspect of wildlife management. To minimize dose-dependent adverse effects, immobilization protocols often include two or more synergistic agents, which allows for reductions in individual drug dosages. Free-ranging bighorn sheep (Ovis canadensis) in Canada (n = 74) were remotely injected with a combination of medetomidine (0.16 +/- 0.04 mg/kg) and ketamine (4.0 +/- 1.4 mg/kg) (MK), or combination of medetomidine (0.14 +/- 0.06 mg/kg), azaperone (0.21 +/- 0.11 mg/kg), and alfaxalone (0.45 +/- 0.21 mg/kg) (MAA). Once recumbency was achieved, arterial blood samples were collected and immediately analyzed for blood gas and acid-base status. Rectal temperature, heart rate, and respiratory rate were recorded upon recumbency and throughout anesthesia at 5-15 min intervals. At conclusion of the procedures, medetomidine was reversed by intramuscular atipamezole at five times the medetomidine dose. Induction times (mean +/- standard deviation) of animals that became immobilized with one dart (8.7 +/- 3.2 min, 7.3 +/- 3.9 min) and recovery times of all animals (3.4 +/- 1.5 min, 3.9 +/- 1.6 min) were not significantly different between MK and MAA groups, respectively. Both MK and MAA groups experienced severe hypoxemia (PaO2 42 +/- 9 mmHg, 40 +/- 10 mmHg, respectively). PaCO2 was significantly higher (P = 0.0248) in the MK group (median 54 mmHg) than the MAA group (median 48 mmHg) with a trend towards lower pH (7.40 vs 7.42, respectively, P = 0.07). Initially, MK animals had higher heart rates than MAA animals (median 49 vs 40 beats/min), which decreased over time. In bighorn sheep, both MK and MAA produced reliable, reversible immobilization with smooth inductions and recoveries. However, less respiratory depression was seen with MAA than MK.