Eld’s Deer Chemical Immobilization

Immobilization via chemical means has become an indispensible tool for researchers and wildlife managers because the immobilization of large or potentially dangerous wild animals poses significant challenges with risks for both operators and target animals. Additionally, most wild animals will act defensively when cornered or restrained in any kind of a trap. An animal’s threshold of tolerance refers to the point at which a trapped animal will become aggressive upon human approach.¹

Wildlife managers, researchers and veterinarians in zoo settings are regularly called upon to immobilize deer to mark them for identification, to provide veterinary treatment or to relocate them from dangerous or overpopulated locations. While the term “immobilization” references any forced restriction of movement of all or part of an animal’s body—including traps of varying design—chemical immobilization is achieved using drugs which have a range of intended effects. These may involve widespread muscular paralysis while the animal is fully or partially conscious (sedation), to those which produce unconsciousness with lack of sensation (anesthesia).

Eld’s Deer: Biology and Background

Eld's deer (Rucervus eldii) are indigenous to Southeast Asia. Their name arises due to their discovery by Lt. Percy Eld in the Manipur Valley of India in 1838. There are three recognized subspecies of R. eldii, which include:

• Rucervus eldii eldii (native to Manipur),
• Rucervus eldii thamin (native to Burma/Myanmar), and
• Rucervus eldii siamensis, (native to Thailand, Annam, and Hainan island).

The Eld’s deer is a large deer that is considered very regal and graceful in appearance. They are similar in size to white-tailed deer, but differ somewhat in appearance. Their legs are long and thin, and they have slender bodies with a large head and ears. Their rough coats change color with the season. In summer, they are reddish-brown, and dark brown in winter. Stags often have darker coloring than hinds (females) and have a thick mane of long hair around the neck.²

Eld’s deer stags have large bow- or lyre-shaped antlers; these sweep back in a curve of about 40 inches in length. One smaller tine grows toward the front of the head. Antlers are shed every year and reach their largest size during the breeding season.^2,3 Male Eld’s deer grow to about 71 inches in length and weigh from 276 to 386 pounds. They are taller and larger than the hinds, which stand about 60 inches tall.

In their native ranges, Eld’s deer inhabit suitable forest habitats, lowland valleys and plains, avoiding dense forests and coastal areas. This also includes monsoonal forests. Today, they occur in a number of protected areas throughout these areas and have been introduced to numerous countries as game animals, including the United States.²

Sedation Methods Used for Eld’s Deer

Nearly all deer species are large enough to be considered difficult to handle, and the Eld’s deer is certainly among these. In such cases, chemical agents (sedatives and/or anesthetics) may be delivered by hand to a restrained deer by using a pole syringe, or by using a capture gun (either a handgun or along gun). Capture guns are powered by CO₂ gas cartridges or with .22 caliber blanks. Syringes (often called darts) are loaded through a breech, one shot at a time. The effective range may be up to 60 yards.¹ Remote chemical immobilization is often carried out by approaching deer and shooting a dart from a helicopter, snowmobile, an off-road vehicle, or from the ground.

Depending on the procedure being performed, a deer may be handled using heavy sedation or general anesthesia in the case of invasive surgical procedures. Drug choices and combinations must be of proven safety for each species and calculated for the weight, age, physiological and reproductive status and body condition. Deer vary widely in size, and each species of deer has its own anesthesia recommendations with intra-species variations of dosages because of diverse individual responses to anesthetic agents.^4,5

The chemical immobilization of Eld’s deer carries inherent risks. These include, but are not limited to capture myopathy, hypothermia, hyperthermia, respiratory depression/arrest, aspiration and cardiac arrest. Additionally, if the onset (induction) of anesthesia is slow, the risk of physical injury such as lacerations, limb injuries, head trauma etc. is increased. It is therefore extremely important for personnel in the field and/or support staff to be familiar with animal handling and immobilization techniques, as well as potential emergencies.

Chemical Immobilization Agents

The classes of immobilizing agents typically used on Eld’s deer include:

Opioid Anesthetics: Opioids have been used for the chemical immobilization of wildlife since the 1960s and are the most potent drugs available for this purpose. The most commonly used opioid is carfentanil. An advantage in the use of opioids is the availability of specific antagonists. The potency of opioids, such as etorphine and carfentanil, is both an advantage and disadvantage. The advantage is the reduced volume of drug required for immobilization makes them the only class of drugs capable of remote immobilization of large animals. The disadvantage is that they are potentially toxic. Death is almost always due to respiratory failure. Opioid immobilizing agents should never be used while working alone or without having an antagonist immediately on hand.⁴

Paralytic Drugs: The neuromuscular blocking (NMB) or paralytic drugs are some of the earliest drugs used for the chemical immobilization of wildlife. Despite their long history of use, NMB drugs are generally inferior to modern drugs. There are two major deficiencies of NMB drugs. One is that NMB drugs have a very low safety margin and dosage errors of only 10% can result in either no effect (underdosing) or death by asphyxia (overdosing). Mortality rates as high as 70% have occurred.^4,5

The second deficiency is that NMB drugs are virtually devoid of central nervous system effects because of their inability to cross the blood-brain barrier. Thus, an animal paralyzed with NMB drugs is conscious, aware of its surroundings, fully sensory, and can feel pain and experience psychogenic stress, yet is physically unable to react. Because of these deficiencies, NMB drugs should be used judiciously.

Tranquilizers/Sedatives: In wildlife immobilization, tranquilizers are used primarily as adjuncts to primary anesthetics (e.g., ketamine, carfentanil) to hasten and smooth induction and recovery and to reduce the amount of the primary agent required to achieve immobilization. Valium is used primarily for small mammals as an anticonvulsant adjunct to ketamine anesthesia and it is also an excellent muscle relaxant. The α-adrenergic tranquilizers (e.g., xylazine or Rompun, medetomidine) are potent sedatives and can be completely antagonized. They are often combined with ketamine, Telazol, or carfentanil.

As single drugs, they are capable of heavily sedating animals, particularly ungulates, to the point of relatively safe handling. However, animals sedated with tranquilizers can be aroused with stimulation and are capable of directed attack. Caution should always be exercised in such animals even though they may appear harmless.

Dissociative Anesthetics: These drugs are characterized by producing a cataleptic state (a malleable rigidity of the limbs) in which the eyes remain open with intact corneal and light reflexes. Ketamine is probably one of the most widely used drugs for wildlife immobilization because of its efficacy and safety. Tiletamine is unavailable as a single product and it is combined in equal proportions with the diazepinone tranquilizer, zolazepam (e.g., Telazol).When used singly, ketamine usually cause rough inductions and recoveries, and convulsions are not uncommon. Because of this, they are usually administered concurrently with tranquilizers or sedatives. There is no complete antagonist for ketamine or Telazol.

The possession and use of drugs used to capture Eld’s deer and other animals is governed by both federal and state regulations in the United States. All drugs currently used to sedate or immobilize wild animals are prescription drugs and must be used by or on the order of a licensed veterinarian. This requires a veterinarian must be involved in the process, but it does not mandate that a veterinarian be on site during the immobilization process. Non-veterinarians using prescription drugs should receive adequate training in their use. Some drugs used chemical immobilization are also classified as controlled drugs, the possession of which requires a U.S. Drug Enforcement Agency registration number, special record keeping, and special storage requirements.

Recovery and Reversal Agents

In Eld’s deer, the duration of sedation or anesthesia will be influenced by the drugs used, age, sex, body weight, procedure performed and the amount of stimulus during the procedure. Whether sedation or general anesthesia has been employed, reversal agents are often required to neutralize sedation or anesthetic agents, thus allowing the deer to completely recover from being anesthetized. This is even more important in the field than in a clinic or zoo setting, because a chemically-compromised animal will be in danger of injury, predation and other hazards.

Conservation and humane treatment concerns have helped to bring about the refinement of chemical immobilization protocols and drug development to keep these within safety margins through the use of novel anesthetics, including combinations of true anesthetics, neuromuscular blockers and tranquilizers.² The use of antagonists to anesthetics is now widely employed, as this avoids the undesirable and potentially harmful effects of drugs and facilitates speedy recovery from chemical immobilization events.^1,6

Veterinary custom compounding pharmacies have widely expanded the variety, availability and efficacy of immobilizing drugs through the development of custom formulations for wildlife such as Eld’s deer. Some of these are available in kit form, which include both the immobilizing and reversal agents.

¹Pennfoster.edu. Animal Handling And Chemical Immobilization.
²animaldiversity.org.
³nationalzoo.si.edu.
⁴Arnemo, Jon & Kreeger, Terry. (2018). Handbook of Wildlife Chemical Immobilization 5th Ed.
⁵Nielsen, L. Chemical Immobilization of Wild and Exotic Animals. (1999) Ames, Iowa, Iowa State University Press.
⁶Stoskopf, M. Handbook of Wildlife Chemical Immobilization. Journal of Wildlife Diseases 2014 50:1, 157-157.