Chemical Sedation of Wildlife

The capture of free-ranging animals is an important foundation of wildlife research and preservation. Minimizing harm to wildlife, and especially mortality, is a vital ethical concern.

Veterinarians and animal care specialists who are familiar with a given species can often capture these animals just by manually restraining them, without the need for chemical immobilization. There are a variety of specialty tools that can be used to assist in the capture of some species, such as nets and safe-capture poles. Small birds, some carnivores and neonates who evade hand-capture can be caught using a net and then manually restrained. In the case of most wildlife species, particularly larger animals and carnivores, more involved capture techniques and chemical restraint are usually necessary.

The chief goals of chemical restraint and anesthesia are:

• To render the animal unconscious or deeply sedated, with a minimum amount of stress and no injury to the animal
• To provide adequate intra-operative and post-operative pain control when painful procedures are performed, and
• To ensure safe and rapid recovery.¹

Wild animals are frequently anesthetized in hazardous environments. Capture equipment has the potential to induce severe injury to the target animal or personnel working with the animal. Drugs used for wildlife anesthesia have the potential to produce adverse effects in the animal and in members of the capture team. The risk of significant morbidity and mortality is often high.¹

Every possible step should be taken to decrease these risks. The following sections outline guidelines that should be followed to decrease stress and morbidity or mortality during chemical restraint and anesthesia.

Agents Used in Chemical Immobilization

The restraint and immobilization using chemical agents is an essential part of a veterinarian's role in managing wildlife. Drugs facilitate procedures that otherwise would be impossible to perform. Although many of the commonly used agents have only been widely used during the past 20 years, restraint drugs are not new, and, in fact, have been around since antiquity.²

In selecting an appropriate immobilizing agent, there are many areas to consider. An ideal restraint drug should:

• Have a high therapeutic index
• Be compatible with other drugs
• Have a short induction period
• Have a reversal agent
• Be stable in solution
• Be effective in small volumes
• Be non-irritating
• Be economical
• Can be administered intramuscularly.²

Drugs commonly used in the chemical immobilization of wildlife include: ketamine, zolazepam and tiletamine, medetomidine, xylazine, midazolam, and diazepam. Opioids such as butorphanol have occasionally been used in combination with other drugs to immobilize native carnivores; carfentanil has been used by some zoo and wildlife veterinarians to chemically restrain larger species.

Terminology

The following represent some of the most common terms associated with states of sedation and anesthesia.

• Anesthesia: A state of total loss of sensation in a body, induced by a drug that depresses activity of nervous tissue peripherally (local and regional anesthesia) or centrally (general anesthesia).
• Analgesia: The loss of sensibility to pain (relief pain) without loss of consciousness.
• Narcosis: A state of sleep accompanied by analgesia.
• Hypnosis: Artificially-induced sleep like state from which the animal can be aroused by stimuli.
• Sedation: Calming due to mild degree of depression of central nervous system, most sedative cause drowsiness.
• Tranquilization: A state of behavioral changes in which the animal is relaxed and unconcerned by its surroundings.
• Local analgesia (anesthesia): It is a loss of sensation in a defined area of the body.
• Regional analgesia: Loss of sensation in a larger but limited body area.
• Balanced anesthesia: General anesthesia produced by a combination of two or more anesthetic drugs or techniques to achieve optimum hypnosis, analgesia and muscular relaxation.
• General anesthesia: Complete unconsciousness produced by a process of controlled, reversible intoxication of central nervous system in which there is muscle relaxation and diminished response to external stimuli.³

Drug Classes Used in Wildlife Sedation

A variety of drug classes are used in the chemical restraint of wildlife. As such, there is no perfect drug or an anesthetic that will suit to variety of animal species. There are classes of drugs, such as opioids, that are used in veterinary medicine, but less frequently with wildlife in the field. The following represents a list of the drug classes most commonly used in the sedation and chemical immobilization of wildlife.

Neuromuscular blocking drugs - Neuromuscular blocking drugs act at the neuromuscular junction and paralyze muscle from functioning. These are some of the first drugs used for chemical immobilization of wildlife. There are three classes of neuromuscular blocking drugs: depolarizing, competitive (nondepolarizing) and ganglionic. Due to narrow safety margin and the risk of respiratory failure due to paralysis of diaphragmatic muscles, these drugs are not suitable in wild animals.

Central Nervous System (CNS) Depressants - These drugs have an effect predominantly on the CNS. The effects range from calmness (tranquilization), depression (sedation), loss of pain (analgesia) to a complete loss of consciousness (anesthesia). In this category of drugs, some drugs (for example diazepam) which act as tranquilizer at a lower dose may work as an anesthetic, although it is not recommended.

Tranquilizers/Sedatives - Tranquilizers produce calmness, loss of aggression and loss of alertness which is generally required during transportation. Animals do not get immobilized fully with tranquilizers and can be aroused by slight disturbances. Therefore, they are used primarily as adjuncts to dissociative anesthetics for hastening smoother induction and to reduce the quantity of anesthetic for achieving more effective immobilization. This combined synergistic effect of tranquilizer and anesthetic is far greater than the individual effect of either of the two drugs with respect to smooth induction, good muscle relaxation and smoother recovery.

Dissociative Anesthetics - These are also known as cyclohexanes. Anesthetics are used when an animal needs to be unconscious and unaware for an extended period of time, such as for surgery or performing assisted reproductive techniques. When used singly, they usually cause rough inductions and recoveries and convulsions may be experienced. Therefore, they are usually used in combination with sedatives or tranquilizer that produces synergistic effect to yield good induction and smooth recovery.³