Axis Deer Chemical Immobilization

Wildlife managers, deer farmers, researchers and veterinarians in zoos are regularly called upon to immobilize deer to mark them for identification, to provide veterinary treatment or to relocate them from dangerous or overpopulated areas. 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.¹ 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).

Deer inhabit almost every continent on the globe. There are around a half-dozen species that are common in North America, as well as other subspecies and non-native species.Interbreeding between species also takes place on occasion. Some species of deer in North America have been introduced, largely for the purposes of game hunting. The axis deer (Axis axis) is one species. Native to the Indian subcontinent, it is a moderately large, spotted deer that was introduced into the U.S. in the early 1900s. Axis deer prefer sparse, secondary forests that supply adequate drinking water and shade, and they tend to avoid rugged terrain. Their food consists largely of grasses.^2,3

Axis deer are highly social, and are usually found in herds ranging from a few animals to more than 100. Herd leaders are usually mature, experienced does. Adult male Axis deer are normally found living with herds of young and old animals of both sexes (unlike native North American deer). Like elk, rutting male Axis deer emit bugle-like bellows, and both sexes can generate alarm calls when threats are present.⁴

The reproductive behavior of Axis deer is similar to that of domestic cattle. In the wild, mature bucks in rutting condition may be found throughout the year, with each buck apparently having an independent reproductive cycle of its own; this may not be synchronized with that of other bucks in the herd.³ Females undergo estrous cycles throughout the year, with each cycle lasting approximately three weeks. While pregnant females may be found year-round, most breeding lasts from mid-May through August.

Axis deer females typically give birth to a single fawn after a 210 to 240-day gestation period. In Texas, which has a robust population of Axis deer, fawns are born in early January to mid-April, although they may arrive in any season. Fawns begin consuming green forage by 5 weeks of age, and are weaned at 4 to 6 months.⁴

Sedation Methods Used for Axis Deer

Most deer species are large enough to be considered difficult to handle. In these 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 can be carried out by approaching Axis deer and shooting a dart from a helicopter, snowmobile, an off-road vehicle, or from the ground.

Depending on the procedure being performed, Axis 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 this species and calculated for the weight, age, physiological and reproductive status and body condition. Deer vary widely in size even within a given species, and each species of deer has its own anesthesia recommendations with intra-species variations of dosages because of diverse individual responses to anesthetic agents.^1,5

The chemical immobilization of hoofstock such as Axis 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.

Drugs Used for Chemical Immobilization

The possession and use of drugs used to capture Axis deer 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 for chemical immobilization are also classified as controlled drugs, the possession of which requires a U.S. Drug Enforcement Agency (DEA) registration number, special record keeping, and special storage requirements.

The classes of agents used to immobilize Axis deer include:

Opioid Anesthetics: This class of drugs has been used for the chemical immobilization of wildlife since the 1960s, and these are the most potent drugs available for this purpose. One 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. An advantage is the reduced volume of drug required for immobilization makes them the only class of drugs capable of remote immobilization of large animals. A disadvantage is that they are potentially toxic; when death occurs, it is almost always due to respiratory failure. Opioid immobilizing agents should never be used while working alone or without having an antagonist on hand.²

Tranquilizers/Sedatives: Tranquilizers are used primarily as adjuncts to primary anesthetics (e.g., ketamine, carfentanil) in wildlife immobilization 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 are able to be completely antagonized. These drugs are often combined with ketamine, Telazol, or carfentanil.

Used singly, these drugs are capable of heavily sedating animals to the point of relatively safe handling. However, animals sedated with tranquilizers can be aroused with stimulation and are capable of directed attack. Even though they may appear harmless, caution should always be exercised when handling such animals.

Paralytic Drugs: The neuromuscular blocking (NMB) or paralytic drugs were 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.

• 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.
• 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.^1,7

Dissociative Anesthetics: These drugs produce a cataleptic state (a partial 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.

Recovery and Reversal Agents

In Axis 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.

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.⁶ The use of reversal agents for sedatives/ anesthetics is now widely employed, as this avoids the undesirable and potentially harmful effects of drugs and facilitates speedy recovery from chemical immobilization events.^6,7 This can be 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.

Additionally, 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 Axis deer. Some of these are available in kit form, which include both the immobilizing and reversal agents.