Tolazoline vs. Yohimbine for Wild and Exotic Animals
There are numerous drug and drug combinations that are used for the remote immobilization of wildlife species, but only a few have become standards amongst veterinarians and wildlife managers. For years, many large wildlife species have been remotely immobilized using ketamine plus xylazine however, long flight distances are often a problem with certain species, such as deer.1 One of the more widely recommended drug combinations for the remote immobilization of wild deer is Telazol®. This is a combination of tiletamine hydrochloride and zolazepam hydrochloride. Tiletamine is a cyclohexanone dissociative anesthetic with no known antagonist. Zolazepam, a benzodiazepine, has been successfully antagonized with flumazenil and sarmazenil.2 Shorter flight distances after darting have thus been reported with the use of Telazol® rather than ketamine plus xylazine.
Then, there are problems which are inherent in remote immobilization; among them:
- Environmental obstacles: Mountainous terrain, flora, swamps etc. pose problems because they can result in animals can be difficult to track, or they may become immobilized in an area where an animal is difficult to reach.
- Lack of supportive equipment: Anesthesia in a controlled environment requires equipment to supply oxygen, support ventilation, blood pressure etc. This equipment is not available in the field, increasing the risk of complications in the animal
- Weather conditions: Many anesthetic agents can impair an animal’s ability to thermoregulate. Animals can become hyperthermic in a hot environment or hypothermic with prolonged exposure to cold environments.
- Predators: Sleeping animals left unattended are prone to predation.
- Injection site: The drug injection site can influence absorption of the drug and time to induction of anesthesia, and the body target area of choice can not always be accurately darted.
To Dart or Not to Dart?
For some wildlife species, and in instances wherein remote immobilization is not necessary, xylazine still remains a drug of choice in many applications. Xylazine is a non-narcotic compound that is commonly used as a sedative, analgesic, and as a muscle relaxant. Xylazine has a long-standing use in small animal practices, but it is also used in large animal and wildlife applications. The sedative and analgesic activity of xylazine is related to central nervous system depression. Its muscle relaxant effect is based on inhibition of the intraneural transmission of impulses in the central nervous system.
Adverse effects of xylazine can include muscle tremors, seizures or slowed heart rate with partial heart block and slowed breathing rate. Ruminants have increased sensitivity to the sedative/hypnotic effects of α2-agonists compared to other species.3 Being an α-2 adrenergic agonist, xylazine’s effects can be antagonized with the α-antagonists.
In cases where remote immobilization is a necessity, and in general with wildlife in the field, drugs with fast induction and shorter recovery times, or drugs that can be readily reversed are ideal. Thus, the search for superior reversal agents for wildlife species is an ongoing proposition.
Yohimbine
Yohimbine is an α-2 adrenergic antagonist that antagonizes the effects of xylazine and other α-2 adrenergic agonists. A study in horses that received detomidine 0.03 mg/kg IV followed 15 minutes later by yohimbine 0.2 mg/kg IV found that yohimbine rapidly reversed the sedative effects of detomidine, effectively returned heart rate and the percent of atrioventricular conduction disturbances to pre-detomidine values, and effectively reduced detomidine-induced hyperglycemia.5
Tolazoline
Tolazoline is an α-antagonist labeled for human use. It is FDA-approved and indicated for the reversal of effects associated with xylazine in horses. It also will reverse the analgesic effects of α-agonists. Tolazoline antagonizes the effects of detomidine more completely, hastens recovery, and lasts longer than atipamezole.
Tolazoline is rapid acting, but may not fully reverse effects on sedation or heart rate and rhythm.
While both yohimbine and tolazoline have been evaluated as reversal agents for xylazine and other α-2 adrenergic agonists, there are few comparative studies of these where wild and exotic species are concerned.6 Since xylazine is common in veterinary regimens for sedation and anesthesia, the best α-antagonists would be of significant value to shorten the α2-agonist-induced sedation, to eliminate their effects when adverse cardiopulmonary responses occur, and to treat animals receiving an overdose. Based on the available literature, the decision as to whether to choose yohimbine or tolazoline as a reversal agent is largely based upon the species being immobilized, and the experience and preference of the attending veterinarian.
1Kilpatrick, H.J., Spohr, M.. 1999. Telazol-xylazine versus ketamine-xylazine: A field evaluation for immobilizing white-tailed deer. Wildlife Society Bulletin 27: 566–570.
2Kreeger, T. J. 1996. Handbook of wildlife chemical immobilization. Wildlife Pharmaceuticals, Laramie, Wyoming, 340 pp.
3Schwartz DD, Clark TP. Selectivity of atipamezole, yohimbine and tolazoline for α-2 adrenergic receptor subtypes: implications for clinical reversal of α-2 adrenergic receptor mediated sedation in sheep. J Vet Pharmacol Ther. 1998 Oct;21(5):342-7.
5Knych H, et al. Effect of yohimbine on detomidine induced changes in behavior, cardiac and blood parameters in the horse. Vet Anaesth Analg. 2012;39(6):574-583.
6Hartsfield, S.M., et. al. Comparison of The Effects of Tolazoline, Yohimbine, and Doxapram in Cats Medicated With Xylazine. Journal of Veterinary Anaesthesia, Volume 18, Supplement 1, August 1991, Pages 71-73.
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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