Anaesthesia of Wild Animals
If pharmacological restraint is required it should be given under veterinary supervision. Anaesthetics and analgesics suitable for the species and procedure should be used, which allow the animal to make a rapid return to consciousness. Animals should only be released when they have recovered sufficiently from the procedures to be able to cope with hazards in the environment. Since there may not be the opportunity to check on the animals post-operatively, surgical procedures must be performed to a particularly high standard, using strict aseptic techniques and good surgical and suturing methods. A wide range of smaller wildlife species can be successfully lightly anaesthetised for restraint using isoflurane, either alone or in combination with injectable agents. For larger species such as badgers, otters and foxes induction of anaesthetic is generally by injectable agents. These are administered through the mesh of the trapping cage, such as a crush cage (see Figure 15.2), or by means of a blowpipe or dart gun. Use of these is controlled under the UK Firearms Act (1968) and requires permits. Many commonly used injectable combinations include non-reversible agents with associated prolonged recovery times. This is generally undesirable in the field where there are unlikely to be facilities to maintain adequately warm temperatures; it is difficult for personnel to monitor the recovery of more than a few individuals and animals are prevented from resuming normal activities (for example, hunting for food or protecting young). Well-controlled anaesthesia is important for the operator: an animal suddenly recovering from anaesthesia is likely to bite and so present a risk of injury and transfer of zoonotic disease.
Although fieldwork presents particular challenges, many of the procedural refinements used in the laboratory or surgery can be transferred. A folding table (such as of the type used for pasting wallpaper), covered in thick polythene sheeting and disposable paper covers, functions well as an operating table which can readily be disinfected between animals (see Figure 15.3). Some of the problems encountered in the trapping and subsequent anaesthesia of small wild animals are due to their very small size and high metabolic rate. Their high ratio of surface area to body weight is such that they lose heat rapidly and hypothermia must be prevented. All animals should therefore be placed on bedding, over gel heat pads or hot-water bottles, during anaesthesia and recovery. They must be placed in sheltered positions, provided with food, and their holding cages covered with blankets or bubble wrap during recovery. Death rates in traps can be particularly high for shrews, possibly because of their very high metabolic rates and thermal requirements. Small birds also have a higher basal metabolic rate (BMR) than larger ones: a hummingbird weighing 3.5 g has a BMR of around 6720 kJ/kg per day, whereas a swan weighing 9 kg has a BMR of around 200 kJ/kg per day. This high metabolic rate renders birds prone to hypoglycaemia even if starved for only a few hours. The heart is relatively large and beats very fast, and birds will readily develop acute cardiovascular failure if stressed. Their body temperature is often 41°C or more, which allows the rapid metabolism required, but which also renders birds prone to hypothermia. Body temperature is regulated by physiological and behavioural means which varies between species depending on their natural habitat. Birds will fluff up their feathers, take shelter and shiver when it is cold, but they have limited capacity to lose heat if it is warm. They do not possess sweat glands, and therefore heat loss can only be effected by panting or gular fluttering, which is the rapid movement of the thin floor of the mouth and upper area of the throat. This must all be taken into account when they are anaesthetised.
Apparatus is available for inhalational anaesthesia in the field that is light enough to be portable in a simple backpack7. An isoflurane vaporizer, which weighs approximately 7 kg, is attached to a portable oxygen cylinder (see Figure 15.4). It is important to note even with a temperature-compensated model, prolonged exposure to temperatures below 10°C (for example, overnight storage in a vehicle in winter) will adversely affect the calibration for some time even after air temperatures have risen. This is due to the slow rate at which the vaporizer is designed to expand and contract. This problem can be overcome in the field by protecting the vaporizer from protracted exposure to low temperatures using an insulated box, if necessary supplied with hot-water bottles. The vaporizer is connected using standard circuitry to a portable 230 litre medical oxygen cylinder (weight 2.9 kg). The cylinder can be refilled from a standard-sized canister using a recharging adaptor. The cylinder should be fitted with a regulator permitting flow rates of 1–15 litre/min.
The protocol used for induction depends on the size of the animal. Small mammals caught in metal box Longworth traps can be gently tipped with their bedding into a transparent polythene bag. This bag then serves as an induction chamber, with the neck of the bag being held closed around the anaesthetic gas tube (see Figure 15.5). Larger species caught in wire mesh traps, such as squirrels, rats and rabbits, can be anaesthetised as above, except the whole trap is placed inside a polythene bag. Larger animals such as badgers and foxes are better given short-acting reversible injectable anaesthetic for induction (Figure 15.6). All species are then transferred to a face mask or intubated for maintenance (see Figures 15.7 and 15.8). Since fieldwork is usually conducted outside there may be no need for a scavenging system to be included in the circuit. The main advantage of this inhalation system is its safety for the animal, despite their unknown health status at induction. In addition, the animals recover very quickly from isoflurane: most are fit for release within 20 min of the end of anaesthesia and can be returned to the wild with less disruption to them and to their population than using injectable methods alone.