Antagonists Used in the Chemical Immobilization of Wildlife
Immobilization is the restriction of movement of all or part of an animal’s body by physical or chemical means. It is used to facilitate management of animals and is a common practice in many animal management procedures.
Wildlife researchers often need to immobilize wild animals to mark them for later identification or to provide veterinary treatment. Veterinarians working in zoos routinely immobilize animals for the purpose of medical treatment; researchers and biologists employ immobilization techniques in order to capture or restrain animals for study, while animal control personnel are often called upon to relocate animals from dangerous or overpopulated areas.
Chemical immobilization has become an accepted, safe and relatively easy model for the restraint of free-ranging and captive exotic (wild) animals. Chemical immobilization is achieved using various classes of drugs which have a wide range of intended effects. Some of the drugs used produce a widespread muscular paralysis while the animal is fully conscious, while others produce unconsciousness with anesthesia and the attendant lack of sensation and pain.1
Classes of Immobilizing Drugs
The immobilization of large and potentially dangerous wild animals poses significant risks for both humans and target animals. The drugs and combinations used must be of proven safety for each species and calculated for the weight, age, physiological and reproductive status and condition of the target animal.2,3 If the induction of a sedative or anesthetic is slow, the risk of physical injury such as lacerations, limb injuries, head trauma or other injuries will be increased.
The drug classes typically used in the chemical immobilization of wildlife are as follows:
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.1
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.
Dissociative Anesthetics: This group of drugs, which includes ketamine and tiletamine, 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).1
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 in wildlife is carfentanil. Opioids provide potent analgesia, and concurrent administration can lower the dose of inhalant or barbiturate general anesthetics for surgery.
Drug Delivery Methods
Oral delivery of immobilizing agents is rare with free-ranging wildlife, but some specific applications do exist. When using oral delivery in the field, it can be difficult to target specific animals, as animals of different sizes or different species can accidentally take baits.
Intramuscular hand injection can be used when working with smaller mammalian species that have been trapped in box traps or leg-hold traps. When hand injecting, rapid delivery while minimizing risk to the handler and animal is very important. The use of syringes that require needles to be twisted on will help prevent needles from coming off the syringe during injection will increase safety.
Drug delivery by pole syringe requires manual injection facilitate IM injection at a safer distance. The exception to this is the use of commercial spring loaded or air pressurized pole syringes. As with hand injections, larger bore needles are needed to ensure complete drug delivery.
Remote delivery of anesthetics to wildlife is most often accomplished using a variety of syringe darts that are propelled by blow pipes or a variety of dart guns. These range from very heavy duty rifles that use a .22-caliber system, to very lightweight pistols that powered by air or CO2. Dart types range from lightweight air-pressured systems to charge-powered injection systems. The lightest dart possible for a specific operation and the lowest velocity possible to accurately and precisely hit a target should be used.2
Chemical Immobilization Antagonists
Antagonists, or reversal agents, have been amongst the most important pharmacological developments to wildlife immobilization. Antagonists reverse the anesthetic effects of tranquilizers, sedatives and anesthetic agents. The objective in using antagonists is to return immobilized animals more quickly to physiological normalcy. This is particularly important when treating animals in the field, as this reduces the chances of injury, predation and other hazards upon being released. Antagonists also reduce the risks of complications such as bloat and cardiorespiratory distresses due to long periods of immobilization. They are also useful the cases of accidental overdose, and decrease the personnel and equipment time for monitoring immobilized animals.
What is an example of an antagonist used when chemically immobilizing wildlife? There are different antagonists available for the various immobilizing drugs and classes of drugs. There are also immobilizing drugs for which there are no known antagonists, or those which are not easily antagonized.
Alpha 2–adrenergic antagonists (for the reversal of α-2 adrenergic agonists like xylazine, medetomidine and dexmedetomidine) such as yohimbine hydrochloride, tolazoline hydrochloride and atipamezole are safe and are frequently used in wild animals. Tolazoline has been found to be more effective than yohimbine in some animals. In the chemical immobilization of wild species using opioids, naltrexone is the reversal agent of choice. Generally, slower recoveries occur with antagonists when given via the IM route. It is therefore a common practice to give equal doses of the antagonist both IV and IM or a single IV dose.2
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1West, G., et. al. Zoo Animal and Wildlife Immobilization and Anesthesia, Second Edition, John Wiley & Sons, Inc., July 2014.
2Kreeger, T., Arnemo, J. Handbook of Wildlife Chemical Immobilization. Fifth edition, 2018
3Nielsen, L. Chemical immobilization of wild and exotic animals, Ames: Iowa State University Press, 1999.
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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