Chemical Immobilization of Wild Animals

The chemical immobilization of wild animals is a form of veterinary anesthesia conducted under difficult circumstances.1Historically, chemical immobilization can be traced back to certain tribes from South America who used curare-coated arrows in their quest for food. Though this method was effective and curare derivatives were used for many years, an effort began in the late 1950s by wildlife managers in South Africa to develop new and more efficacious methods to immobilize animals for research.2 Since that time, a great deal of progress has been made in developing new drugs and techniques for delivery.

Due to constant changes in restraint techniques, equipment and even immobilization drugs, wildlife veterinary practices have improved over the past decades. This progress coupled with the experience of practitioners has contributed towards a safer practice for both the animals and people involved in procedures.

Wildlife Immobilization: Practical Concerns

Wildlife practice is a very specialized area of veterinary medicine. Restraining an individual animal often requires the use of anesthetic drugs to allow a safe and efficient immobilization before the practitioner proceeds with any procedure. Anesthesia plays an important role in wildlife veterinary medicine, since the majority of the procedures carried out involve chemical restraint techniques. Preventive medicine, surgery, imaging and anesthesia itself are the key interventions carried out on wildlife. Each time an individual animal is immobilized, any number of procedures may performed on the animal to take advantage of the immobilization, depending on need and circumstance. Wild animals may be immobilized for re-location purposes, bio-measured, ear-tagged, implanted with a microchip, vaccinated, radio collared or even de-wormed. In certain instances, blood and hair collection may also be performed for DNA testing.¹

Veterinarians in wildlife medicine must administer anesthetic drugs effectively, using different types of delivery systems. There are different approaches to administering drugs in chemical restraint procedures: oral, hand-held injection, pole syringe and darts. In cooperative animals, hand-held injections or the pole-syringed administration are usually the delivery routes of choice.³ This is more often the case in a zoo setting or with smaller animals in the wild. If an animal is large, dangerous or uncooperative, remote delivery systems using blow darts, gunpowder explosive darts or compressed gas projectors are the most suitable choices.

Many different classes of compounds, including anesthetics, analgesics, sedatives and tranquilizers have been used to immobilize wild animals. Only a few of these compounds can be used on a regular basis within the U.S. however, due to federal regulations. One of these regulations, the FDA's Controlled Substances Act of 1970, restricts the use and distribution of some of the more potent drugs, such as opioids.³ Fortunately, there are other drugs which are just as effective, safer and more accessible.

Typically, the veterinarian will select the best drug or drug formulation for each situation according to the physiology and behavior of the animal, as well as the practitioner’s experience and professional preference. Procedures chosen by the veterinarian for zoo animals in a controlled environment logically differs from procedures chosen for free-ranging, unpredictable animals. Chemical restraint is used mainly for large animals (and carnivores in particular) when physical restraint is impractical. Chemical methods are not usually appropriate for immobilizing large numbers of animals, but it is the primary choice for individuals.⁴

Drug Delivery Systems

Many different types of darting systems are available, with most incorporating a projectile dart that is shot from a specially-designed firearm. On impact, the dart injects the prepared dose of drugs via gas pressure or a gunpowder charge.¹ More often than not, these will consist of a rifle with gunpowder fired and gunpowder charged metal darts, a CO²-powered pistol with plastic air charged darts or a blowpipe with plastic air-charged darts. Another useful implement for closer range delivery of immobilizing drugs, antibiotics or vaccines is the spring-loaded pole syringe; however, its use requires the animal to be within about six feet of the person administering the drugs.²

Each circumstance requiring the use of chemical immobilization should be considered in light of what technique will provide the safest and most effective delivery of the drugs (i.e., most hoofstock species require darting at long range; thus, a rifle is most appropriate. For smaller-bodied animals, the blow dart is often the best technique, since it causes less physical trauma. These species are often conditioned to frequent smaller areas, so they often do not require the long range capability of the dart gun. Some species also tolerate the pole syringe well if trapped first, so when used appropriately, this is more practical than the blowpipe.³ CO₂ pistols are usually used smaller enclosures where ranges are shorter (e.g., certain hoofstock species that will allow teams to approach more closely than others). Additionally, the CO₂ pistol accommodates very large darts, needed for the large drug volume used for some larger hoofstock species.⁴

Cautionary Notes

No chemical immobilization should be taken lightly due to the possibility of complications that could include physical injury during the induction or recovery period, and given the possibility of physiological complications from drug formulations. For example, a veterinarian may decide to monitor lameness in a captive animal for a day or two instead of immobilizing it right away for diagnostics.²

Some animals require special preparation and handling, and some animals also require special drugs. To deliver an appropriate dose to a large, fractious animal remotely, the veterinarian will require a formulation that is potent and of small volume. Ideally, the drug will also have a very short induction time, stopping the animal before it escapes; it would also be reversible. Here, the ideal class of drugs would be the ultra-potent opioids.² These act quickly to induce anesthesia and provide pain control, which is essential for surgical procedures. They are also completely reversible with the administration of antagonists. Once the procedure is completed, the opioid antagonist is administered and, within a few minutes, the animal is standing, able to interact normally with other animals and to defend itself.

Ideal capture sites—which, admittedly are not always available—should allow good visualization of the animal during the induction period. Hyperthermia is a serious complication that can be difficult to treat in field situations.^3,4 Equipment for airway support if applicable, including appropriately sized-endotracheal tubes and a bag valve mask to enable mechanical ventilation should be included in the field emergency kit. Physical capture, such as net gunning, rocket nets, drive nets, traps, and snares, can induce greater stress than chemical immobilization, but these may be necessary with some species.²It should be noted that remote drug delivery systems do carry the potential to cause serious injury or death if complications arise or if used inappropriately. The major reported sources of injury arise from dart impact trauma, high velocity injection of dart contents, and inaccurate dart placement.² Careful planning, close monitoring, and early intervention are essential for preventing morbidity and mortality.