Abstract

In 30 immobilizations, eastern grey kangaroos (Macropus giganteus) were induced with an IM mixture of medetomidine (40.2±4.7 µg/kg) and ketamine (4.0±0.5 mg/kg). Induction was smooth, time to lateral recumbency was 6.1±2.0 minutes, with the depth of anesthesia being heavy sedation in six cases and light anesthesia in 24 cases. In 24 cases, immobilization was reversed with atipamezole (0.20±0.02 mg/kg IM). Recovery to standing occurred in 30±11 minutes and full recovery occurred within 79±17 minutes. In six cases, immobilization was reversed with atipamezole (0.22±0.04 mg/kg IV). Recovery to standing occurred in 29±20 min. No fatalities and no anesthetic complications occurred in this study. It is concluded that immobilization with medetomidine (40 µg/kg IM) and ketamine (4 mg/kg IM) and reversal with atipamezole (0.2 mg/kg IM) is a safe and reliable method for the chemical restraint of eastern grey kangaroos. To ensure smooth recovery, atipamezole should be administered IM at least 30 minutes after the immobilization dose.

Introduction

Eastern grey kangaroos (Macropus giganteus) are common in the wild in Australia and in captive situations throughout the world. Chemical restraint is a necessary tool in the successful veterinary management of both captive and wild populations of macropods. Rapid and smooth induction and recovery are required to avoid trauma-related injuries and capture myopathy in macropods. A variety of drugs and mixtures of drugs have been used to chemically immobilize macropods, including thiopentone, pentobarbitone, phencyclidine, phencyclidine-acepromazine, etorphine, etorphine-acepromazine, etorphine-methotrimeprazine, etorphine-ketamine, droperidol, droperidol-fetanyl, azaperone, alphaxalone-alphadolone, alpha-chloralose, xylazine, ketamine, ketamine-xylazine, and tiletamine-zolazepam.^{2-11,13,14,16-25} Macropods are prone to self-injury during prolonged recovery periods following anesthesia. Recovery periods of between 2–4 hours have been observed following immobilization of macropods using ketamine-xylazine and tiletamine-zolazepam. The use of medetomidine or ketamine alone can be unreliable and immobilized animals can be unpredictable for handling. The purpose of this study was to develop a safe, reliable, and reversible immobilization protocol for eastern grey kangaroos using medetomidine-ketamine and atipamezole.

Medetomidine-ketamine and atipamezole have been used in a variety of nondomestic species at dose ranges of 13–140 µg/kg of medetomidine, 0.8–8.0 mg/kg of ketamine and atipamezole at an atipamezole: medetomidine ratio of approximately 2–5:1 (µg: µg).^1,12,15,26 When doses of ketamine at the higher end of the range are used, it is recommended that atipamezole doses at the lower range be used to avoid the problems of “residual ketamine effect.”¹² Medetomidine-ketamine has been administered IM in the red-necked wallaby (Macropus rufogriseus) at 100 µg/kg of medetomidine and 5 mg/kg of ketamine.¹² In general, a combination of medetomidine at 50–100 µg/kg and ketamine at 2–3 mg/kg IM has been recommended for use in Australian native fauna.⁴

Methods

From July 1996–May 1997 at Currumbin Sanctuary, Australia, medetomidine-ketamine was used to immobilize healthy eastern grey kangaroos during an anti-GnRH vaccine trial to facilitate translocation, clinical assessment, and pouch examination in 30 cases. Twenty-nine females were immobilized by hand injection and one male was immobilized via blow dart. Body weight ranged from 12–35 kg (average 21±5 kg) and age ranged from 18–46 months (average 30±8 months). Equal volumes of 1 mg/ml medetomidine (Domitor®, Novartis Animal Health, Wentworthville, NSW 2145, Australia) and 100 mg/ml ketamine (Ketamil®, Ilium Veterinary Products, Smithfield, NSW 2164, Australia) were given IM. Doses were calculated on body weight estimates or previous weights using a dosage rate of 0.04 ml/kg (1 ml/25 kg). Atipamezole 5 mg/ml (Antisedan®, Novartis Animal Health, Wentworthville, NSW 2145, Australia) was administered IV in six cases and IM in 24 cases at 0.2 mg/kg, a volume equal to the volume of medetomidine given. Time to lateral recumbency during induction and time to standing and full recovery following reversal were recorded. Animals in lateral recumbency, but aware of sound, movement and painful stimuli were considered under heavy sedation, while animals in lateral recumbency and unaware of all but deep pain were considered under light anesthesia. In eight cases, body temperature, heart rate and respiratory rate were recorded. Anesthetic records were collated on computer using MedARKS (ISIS, Apple Valley, MN 55124 USA).

Results

Dosage rates of 0.04 ml/kg on estimated or previous weights resulted in dose rates calculated on actual weights of 40.2±4.7 µg/kg of medetomidine (range: 31.8–50.0 µg/kg) and 4.1±0.5 mg/kg of ketamine (range: 3.2–5.0 mg/kg). Mean time to lateral recumbency was 6.1±2.0 minutes (range: 3–12 minutes). Induction was smooth and the depth of anesthesia produced was light anesthesia in 24 cases and heavy sedation in six cases.

Physiologic data were collected in eight cases, 20 minutes postimmobilization: body temperature was 36.0±1.6°C (range: 33.9–38.0°C), heart rate was 44±18 beats per min (range: 22–68), and respiratory rate was 34±20 respirations per min (range: 8–60). No fatalities and no anesthetic complications occurred during this study.

In 24 cases, immobilization was reversed with atipamezole at a dose of 0.20±0.02 mg/kg IM (range: 0.17–0.22 mg/kg). Atipamezole was administered 40±13 min after the immobilization drugs were given (range: 22–60 min). Times to standing were 30±11 min (range: 16–60 min) and times to full recovery were within 79±17 min (range: 57–98 min). All recoveries were smooth in cases where the antagonist was given >24 minutes postadministration of medetomidine-ketamine. In the three cases in which the antagonist was given ≤24 minutes of the immobilization drugs, recovery was characterized by hypersalivation, anxiety, and ataxia. It is believed that these reactions resulted from “residual ketamine effects.”

In six cases, recovery was effected with atipamezole at a dose of 0.22±0.04 mg/kg IV (range: 0.16–0.25 mg/kg). Atipamezole was administered 60 minutes following administration of the immobilizing drugs in all six cases and times to standing were 29±20 minutes (range: 2–48 minutes). Intravenous use of atipamezole appeared unreliable in duration of recovery period. Two recoveries were considered rough with excessive ataxia following very rapid recoveries (range: 2–5 minutes). It is unlikely that this was due to “residual ketamine effect.” Sudden awareness following immobilization may explain these reactions. Recovery in the other four cases was smooth (range: 37–48 minutes).

Discussion

Based on the results of 30 immobilizations, medetomidine at a dose of 40 µg/kg mixed with ketamine at a dose of 4 mg/kg administered IM produces a safe and reliable method of chemical restraint of eastern grey kangaroos. Atipamezole at a dose of 0.2 mg/kg produces reliable reversal of the medetomidine but should be administered IM at least 30 minutes postimmobilization to avoid “residual ketamine effect.”

Acknowledgments

Thanks to Wes Caton, Dr. Katie Reid, Heidi Hellingman, Nadia Valzacchi, and Sue Whyte for their assistance in this study.

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