Evaluation of two different doses of butorphanol-medetomidine-midazolam for anaesthesia in free-ranging versus captive black-footed cats (Felis nigripes)

Abstract

The black-footed cat (Felis nigripes) is the rarest, smallest wild felid species of southern Africa and is categorized as vulnerable on the IUCN Red List 2012 (Sliwa 2008). These cats are solitary, nocturnally active predators with a high metabolic rate. In order to fuel this metabolism they hunt all night, catching and consuming approximately one fifth of their body weight. In order to facilitate research studies and sample collection a reliable, safe, fully reversible anaesthetic drug combination for use in the field is imperative to enable the cats to resume their hunting as quickly as possible post anaesthesia. Both the captive and the free-ranging populations face many survival challenges, including renal amyloidosis, and the use of an anaesthetic drug combination that does not compromise renal function would be of major benefit. To date, there is no published data on the physiological effects of any anaesthetic protocol in this species. The butorphanol-medetomidine-midazolam combination has been safely and successfully used in a variety of domestic and other wild animal species, and recently it has been used in black-footed cats in the field and zoo setting. We used the butorphanol-medetomidine-midazolam drug combination to anaesthetise blackfooted cats, both in captivity and in the wild. As the drug doses currently used in wild freeranging animals are approximately twice as high as those required in captive animals, concerns have been raised regarding the physiological safety of using a higher dose of this anaesthetic combination in these animals. My aim was to document, evaluate and compare the anaesthetic and physiological effects induced by the use of two different doses of the butorphanol-medetomidine-midazolam combination to effectively and safely anaesthetise captive and free-ranging black-footed cats. Due to the differing physiological stressors associated with the capture methods in the freeranging black-footed cats these animals were divided into two groups, namely those that were dug out of burrows, and those that had been caught after a short intense chase. Once captured, all the cats were hand injected into a hind limb muscle group with the anaesthetic drug combination. Injections for different mass categories had been prepared prior to capture and the doses that were given were based on quick visual estimates of an animal s mass. In total, 23 black-footed cats, nine captive and 14 free-ranging, were anaesthetised. Times to induction, quality of anaesthesia and times to recovery after reversal of the effects of the anaesthetic drugs were recorded. Physiological variables recorded every 10 minutes during anaesthesia were heart rate, respiratory rate, SpO2, ETCO2, rectal temperature, systolic, mean and diastolic non-invasive blood pressures. Three central venous blood samples, taken at 20- minute intervals during anaesthesia, were analyzed for pH, lactate and glucose concentrations and cTCO2 and base excess were determined. Anaesthetic induction and recoveries were quick, smooth and uneventful with mean times of 5.9 minutes and 4.5 minutes respectively. We determined that even higher doses than our original target doses were required for induction and maintenance of a 45 minute anaesthesia. The quality and depth of anaesthesia were optimal for the minor sampling procedures performed, with the majority of cats maintaining a deep level of anaesthesia (Score 5) for 45 minutes before the antagonist combination was administered. Cardiac variables remained within normal physiological limits throughout the anaesthesia. Mild hyperventilation, moderate hypoxaemia and a mild to moderate lactic acidosis were initially recorded in the chased freeranging animals. The respiratory and acid-base variables in the other two groups of cats were within clinically acceptable limits. A gradual decrease in body temperature, despite the animals being placed on hot water bottles, was recorded in all the cats and thus a better means to mitigate this heat loss is recommended. Hyperglycaemia was recorded in all black-footed cats, and the fear, anxiety and exertion of the chase would have contributed to the hyperglycaemia induced by the use of the ?2 adrenoreceptor-agonist medetomidine in the anaesthetic drug combination. Intramuscular injection of butorphanol, medetomidine and midazolam in black-footed cats induces rapid and smooth induction and is a safe and effective anaesthesia in both captive and free-ranging animals, despite higher doses used in free-ranging animals. The method used to capture free-ranging animals had a greater influence on the physiological stability of the anaesthetised animals than did the dose of drugs used. If black-footed cats exert themselves during a chase, before they receive this anaesthetic drug combination, they develop mild to moderate hypoxaemia, tissue hypoxia and metabolic lactic acidosis during the anaesthesia. Therefore, oxygen supplementation during anaesthesia is recommended in black-footed cats if they are chased directly before induction.