Nyala Antelope Chemical Immobilization
The nyala is a large antelope that is found in the eastern part of southern Africa. There are two subspecies of nyala; the lowland nyala (Tragelaphus agasi) and the mountain nyala (Tragelaphus buxtoni). “Nyala” is the Swahili name for this antelope, which is derived from the Zulu word "inyala."1 These antelope live near areas of dense brush and on the fringes of forests, usually very close to water sources. Nyala prefer thick vegetation, although they will venture into open areas to graze on grasses, fruit and higher-protein fare.1 It is thought that their range is limited due to their dietary preferences. Finally, nyala do not have the breakaway running ability of many other African antelope, so they depend on blending into vegetation to escape the notice of predators.2
Males, females and calves have vertical white stripes along the torso and flanks. The males are substantially larger than the females, with a coat that becomes grey in color as they reach maturity. Females and calves are a reddish-brown. The males also have spiral horns with 1.5 to 2 twists and a crest of longer hair down the back of the neck, throat and under the abdomen.1
Wildlife managers, researchers and veterinarians in zoo settings sometimes need to immobilize nyala to mark them for identification, to provide veterinary treatment or to relocate them. While the term “immobilization” references any forced restriction (physical or chemical) 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 a 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 the immobilization of nyala via chemical means is useful, since most wild animals will act defensively when cornered or restrained in any kind of a trap. All antelope are prey animals, and have evolved with instincts and behaviors gauged to help them survive; this must be considered in any capture scenario involving nyala.
Sedation and Anesthesia of Nyala
Nearly all antelope 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 nyala by using a pole syringe, or by using a capture gun (either a handgun or along gun). Capture guns are fired 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.3 Remote chemical immobilization is carried out by approaching nyala and shooting a dart from a helicopter, an off-road vehicle or from the ground.
Depending on the procedure(s) being performed, nyala may be handled using heavy sedation or general anesthesia (for 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. Antelope vary widely in size, and each species of antelope has its own anesthesia recommendations with intra-species variations of dosages due to the diverse individual responses to anesthetic agents.4,5
The chemical immobilization of nyala 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 extremely important for personnel in the field and support staff to be familiar with animal handling and immobilization techniques, as well as potential emergencies.
Drugs Used to Chemically Immobilize Nyala
In the U.S., the possession and use of drugs used to capture nyala is governed by both federal and state regulations. 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 that a veterinarian must be involved in the process and the disposition of immobilizing drugs, but it does not necessarily mandate that a veterinarian be on site during the immobilization process. Non-veterinarians using prescription drugs must 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.
The classes of immobilizing agents used on nyala 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 and drug formulations. 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.3
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, as such, can feel pain and experience psychogenic stress yet is physically unable to react. Because of these deficiencies, NMB drugs should be used judiciously.4
Tranquilizers/Sedatives: Tranquilizers are used in wildlife immobilization 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.4 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. However, animals sedated with these tranquilizers generally can be aroused with stimulation and are capable of directed attack. Caution should always be exercised in such animals even though they appear harmless.5
Dissociative Anesthetics: This group of drugs which includes ketamine and 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).5,6 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.
Opioid Anesthetics: The opioids have been used for animal immobilization since the 1960s and are the most potent drugs available for this purpose. The most commonly used opioid is carfentanil. A major advantage in the use of opioids is the availability of specific antagonists. The potency of opioids 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, even via incorrect handling. 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
Drug Recovery and Reversal Agents
The duration of anesthesia in nyala is influenced by the drugs used for immobilization, age, sex, body weight, procedures performed and the amount of stimulus during procedures. Whether sedation or general anesthesia has been employed, reversal agents are 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 nyala 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.4-6 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.3,4
Veterinary custom compounding pharmacies have also expanded the variety, availability and efficacy of immobilizing drugs through the development of custom formulations for wildlife such as nyala. Some of these are available in kit form, which include both the immobilizing and reversal agents.
3Pennfoster.edu. Animal Handling And Chemical Immobilization.
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.
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