Chemical Immobilization of Impala Antelope
The impala (Aepyceros melampus) is a widely-distributed African ungulate comprised of several subspecies. The most common in East Africa is Aepyceros melampus rendilis; Aepyceros melampus petersi, the black-faced impala, is distributed further south. There are other proposed subspecies that are less well known and controversy as to their official designation is ongoing.1
Known for their spectacular leaps when alarmed, clearing obstacles or pursued by predators, impala are one of the most dominant species of antelope on the African savannah. They are most often found in Kenya, Zimbabwe, Uganda, Zambia, Botswana, and Southern Angola to northern South Africa.
Impala are fawn-colored with white underparts and a black stripe that extends from the top of the rump down the back of each thigh.2 Rams have horns which are lyre-shaped, and reach a length of around 30 inches. The impala stands 28–36 inches high and weighs 88–167 pounds. Impala males are approximately 20% heavier than females.
Impala are able to adapt to different environments of the savannas. In some areas, they are grazers and browsers in others. Impala herds typically stay within a few miles of water sources. Younger rams live in bachelor herds, while those strong enough establish or take over breeding herds.3 They engage in a polygynous mating system, with each male mating with a number of females.2 Breeding activity spans from March through May. During this time, pregnant females live in isolation to give birth. After gestating from 190 to 200 days, a single calf is born. Shortly thereafter, the calf and the mother rejoin the herd.3
The Need for Chemical Immobilization
Veterinarians, wildlife managers and researchers sometimes need to immobilize impala to mark them for identification, to provide veterinary treatment or to relocate them. While the term “immobilization” references any forced restriction of movement of all or part of an animal’s body, 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).
The immobilization of large or potentially dangerous wild animals may pose significant challenges with risks for both operators and target animals, and this is where immobilization via chemical means is useful, since most wild animals will act defensively when cornered or restrained. An animal’s threshold of tolerance refers to the point at which a trapped animal will become aggressive upon human approach.1 All antelope are prey animals, and have evolved with instincts and behaviors which help them survive, and this must be considered in any capture scenario where impala are involved.
Sedation and Anesthesia of Impala
While the impala is not among the largest of African antelopes, they are large enough to be considered difficult to handle. In these cases, chemical agents (e.g., sedatives, anesthetics) may be delivered by hand to a restrained impala by using a pole syringe, or at a distance by using a capture gun (either a handgun or long gun) loaded with a dart. Capture guns are fired by CO2 gas cartridges or with .22 caliber blanks. Darts are loaded through a breech, one shot at a time. The effective range may be up to 60 yards.4 Remote chemical immobilization is carried out by approaching impala and shooting a dart from a helicopter, an off-road vehicle, or from the ground.
Since antelope vary widely in size, each species of antelope has its own anesthesia recommendations with intra-species variations of dosages because of diverse individual responses to anesthetic agents.5,6 An impala may be handled after employing heavy sedation or general anesthesia if invasive surgical procedures are to be performed. Drug choices and combinations must be of proven safety and calculated for the animal’s weight, age, physiological and reproductive status and body condition.
The chemical immobilization of impala 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. Thus, it is extremely important for personnel handling the animal to be familiar with animal handling and immobilization techniques, as well as potential emergency scenarios.
Drugs Used in the Chemical Immobilization of Impala
The possession and use of drugs used to capture wildlife is governed by both federal and state regulations in the U.S. 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. While laws do not mandate that a veterinarian be on site during the immobilization process, they do require that a veterinarian must be involved in the process. That said, non-veterinarians handling or dispensing prescription drugs should receive adequate training in the use of these drugs. Some of the drugs used chemical immobilization are also classified as controlled substances, the possession of which requires a U.S. Drug Enforcement Agency registration number, special record keeping, and special storage requirements.
The classes of immobilizing agents used on impala include:
Paralytic Drugs: 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
The second deficiency is that NMB drugs lack central nervous system effects because of their inability to cross the blood-brain barrier. This means that an animal paralyzed with NMB drugs is conscious, aware of its surroundings and fully sensory. As such, it can feel pain and experience psychogenic stress, yet it is physically unable to react. Because of these deficiencies, NMB drugs should be used judiciously.
Tranquilizers/Sedatives: Tranquilizers are used primarily in wildlife immobilization 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, medetomidine) are potent sedatives and can be completely antagonized. They are often combined with ketamine, Telazol, or carfentanil. By themselves, they are capable of heavily sedating animals, particularly ungulates, to the point of relatively safe handling. Animals sedated with these tranquilizers generally can, however be aroused with stimulation and are capable of directed attack.5 Caution should always be exercised in such animals even though they may appear harmless.
Dissociative Anesthetics: These drugs (e.g., ketamine, tiletamine) are characterized by producing a cataleptic state 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 causes 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.
Opioid Anesthetics: Opioids have been used for animal immobilization since the 1960s and are the most potent drugs available for this purpose. A major 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 to humans. 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 immediately on hand.5
Recovery and Reversal Agents
The duration of anesthesia in impala 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 used, reversal agentsare often required to neutralize sedation or anesthetic agents, thus allowing the animal 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.
In recent years, chemical immobilization protocols and drug development have been refined to keep these within safety margins through the use of novel anesthetics, including combinations of true anesthetics, neuromuscular blockers and tranquilizers.5 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.4-7
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 impala. Some of these are available in kit form, which include both the immobilizing and reversal agents.
4Pennfoster.edu. Animal Handling And Chemical Immobilization.
5Arnemo, Jon & Kreeger, Terry. (2018). Handbook of Wildlife Chemical Immobilization 5th Ed.
6Nielsen, L. Chemical Immobilization of Wild and Exotic Animals. (1999) Ames, Iowa, Iowa State University Press.
7Stoskopf, M. Handbook of Wildlife Chemical Immobilization. Journal of Wildlife Diseases 2014 50:1, 157-157.
About NexGen Pharmaceuticals
NexGen Pharmaceuticals is an industry-leading veterinary compounding pharmacy, offering sterile and non-sterile compounding services nationwide. Unlike other veterinary compounding pharmacies, NexGen focuses on drugs that are difficult to find or are no longer available due to manufacturer discontinuance or have yet to be offered commercially for veterinary applications, but which still serve a critical need for our customers. We also specialize in wildlife pharmaceuticals, including sedatives and their antagonists, offering many unique options to serve a wide array of zoo animal and wildlife immobilization and anesthesia requirements.
Our pharmacists are also encouraged to develop strong working relationships with our veterinarians in order to better care for veterinary patients. Such relationships foster an ever-increasing knowledge base upon which pharmacists and veterinarians can draw, making both significantly more effective in their professional roles.
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