Respiratory Depression in Sable Antelope During Chemical Immobilization
The sable (Hippotragus niger) is an African antelope that belongs to the Hippotraginae subfamily, largely owing to its horse-like, stout build and stance.1 Both males and females have long, ringed horns that rise and curve backward. Apart from the horns, the sable displays significant sexual dimorphism, with the extent varying depending on the location and subspecies. The females and young are a bright chestnut to dark brown color; mature males are dark chestnut to jet black. The sable’s white belly contrasts with the back and sides, and they possess a white face with a black facial mask.
The sable’s range covers the southern savanna from central Tanzania to South Africa.2 Sable herds typically congregate near water, in areas with good drainage and grazing opportunities. They eat mostly grass, but will also eat herbs and leaves from shrubs and trees. The social structure of this antelope consists of small female herds with territorial males. Herds usually have home ranges that encompass several male territories which often give the impression of a far larger herd. Dominant males are known to forcefully defend their territories from rivals and predators using their scimitar-shaped horns.
There are four subspecies of sable:
- Black sable (also Mastitis sable)—This is the most widespread sable. Its habitat covers south of the Zambezi River through Zimbabwe and Botswana into South Africa.
- Giant Sable (also Royal sable)—These have the longest horns of any sable subspecies.
- Common (also southern sable)—This antelope is sometimes called the West Zambian sable.
- Eastern sable—This is the smallest of the sable family, with a range that includes the costal lands of southern Kenya, eastern Tanzania and into Mozambique.2
Capture Methods and Complications
Chemical immobilization has become the chief method of capture for large wildlife species for the purposes of translocation, diagnostic testing or medical treatment. Chemical immobilization is usually carried out from the ground in zoos, farms, breeding facilities and free-ranging situations. In some circumstances however, wild animals are located and darted from a helicopter.3 All methods of capture, whether chemical or manual (e.g., traps, nets) can cause significant stress to these animals, potentially giving rise to complications.
Remote drug delivery systems are used for the purpose of chemical immobilization, usually via a commercial dart gun or blowpipe. Drugs are injected via a dart syringe fired from the dart gun at a distance. Since dart volume can be a limiting factor, immobilizing drugs must be highly potent and concentrated. They must also have a high therapeutic index and wide safety margin since animals cannot be examined and weighed prior to immobilization.3 The ideal drugs will also be fast-acting to limit stress and the likelihood of escape following darting, and should also be reversible since animals are often released back into the wild immediately after the capture event.
Respiratory Depression and Risks in Sable
The chemical immobilization of any animal is associated with risks. Under most circumstances and particularly in the field, animals cannot be examined with regard to their health status beforehand and often cannot receive comprehensive supportive treatment during immobilization. Additionally, they are often highly-stressed and sometimes run long distances before they are immobilized. Most drugs used for immobilization also have side effects; they not only cause sedation by influencing the central nervous system, but also influence cardiovascular, respiratory and thermoregulatory functions.3
Opioids are routinely used for the chemical immobilization of antelope and other wild herbivores since they provide many of the effects desired in successful chemical immobilization events. A chief disadvantage of opioids is that they can cause clinically significant respiratory depression which is due to their potent effect on mu-opioid receptors.5 Activation of mu-opioid receptors in the respiratory centers of animals depresses neurons that generate the normal respiratory rhythm. At the same time, their activation potentiates other receptors in the brain stem, on the aortic arch and carotid bodies, which depress normal respiratory function. These processes in turn lead to a reduction of the respiratory frequency and tidal volume, as well as pulmonary vasoconstriction which decreases pulmonary perfusion.4 Alpha-2 agonists such as guanabenz, clonidine, medetomidine, and dexmedetomidine cause reflex bradycardia and hypotension, which can lead to hypoxemia and tissue hypoxia.
Treating Respiratory Depression in Sable
There are a number of approaches available to lessen opioid-induced respiratory depression in sable undergoing chemical immobilization. Oxygen insufflation and assisted ventilation can combat hypoxia,3 while agents such as opioid antagonists or partial antagonists can be used. Unfortunately, the latter also reduce desirable effects and can limit the degree of immobilization, sedation and analgesia. Respiration can also be improved during chemical immobilization events via respiratory stimulants which act on non-opioid receptor systems such as potassium channel blockers, ampakines and serotonin receptor agonists.6
The routine use of oxygen is advisable during wildlife immobilization; this can be combined with a partial opioid reversal to better alleviate hypoxia.3 Naltrexone is frequently used to fully reverse opioid-based immobilization after capture, especially if an animal needs to be fully alert when it is released back into the field. If analgesic or sedative effects are still required, partial opioid antagonists or mixed agonists/antagonists are used for the reversal of opioids such as diprenorphine, nalorphine or butorphanol.4,5 Signs of recovery after naltrexone administration consist of increased respiratory depth, followed by ear twitching, eye movement and lifting of the head.3
Partial mu-receptor antagonists (e.g., butorphanol) can be used to alleviate respiratory depression caused by mu-agonistic immobilization drugs.3,5 It should be noted that some of these also reduce the immobilization effects of opioids. Potassium channel blockers such as doxapram can also be used to stimulate breathing. Doxapram is widely used as a respiratory stimulant by veterinarians. It has been shown to increase the minute ventilation in large herbivores immobilized with etorphine.3
Efficacious drug combinations used for darting were not always commercially available as pre-mixed solutions, but many of these can now be purchased as highly-concentrated drug formulations from compounding pharmacies. These formulations are often species-specific, reliable and are less likely to bring about complications such as respiratory depression in sable than the drugs and combinations used in the past.
3Arnemo, J. Kreeger, T. (2018). Handbook of Wildlife Chemical Immobilization 5th Ed. Sunquest Publishing, 2007.
4Arnemo, J., et. al. Field Emergencies and Complications. In: G. West, D. Heard, & N. Caulkett, eds. Zoo Animal and Wildlife Immobilization and Anaesthesia. Oxford: Wiley Blackwell, pp. 139–147.
5Bailey, P.L., et. al. (1985) The ED50 of carfentanil for elk immobilization with and without the Tranquilizer R51703. The Journal of Wildlife Management, 49(4), pp.931–934.
6Van der Schier, R., et. al. (2014) Opioid-induced respiratory depression: reversal by non-opioid drugs. F1000 Prime Reports, 6, pp.1–8.
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.
Disclaimer
The information contained in this blog post is general in nature and is intended for use as an informational aid. It does not cover all possible uses, actions, precautions, side effects, or interactions of the medications shown, nor is the information intended as medical advice or diagnosis for individual health problems or for making an evaluation as to the risks and benefits of using a particular medication. You should consult your veterinarian about diagnosis and treatment of any health problems. Information and statements have not been evaluated by the Food and Drug Administration ("FDA"), nor has the FDA approved the medications to diagnose, cure or prevent disease. Medications compounded by NexGen Pharmaceuticals are prepared at the direction of a veterinarian. NexGen Pharmaceuticals compounded veterinary preparations are not intended for use in food and food-producing animals.
NexGen Pharmaceuticals, LLC does not recommend, endorse or make any representation about the efficacy, appropriateness or suitability of any specific dosing, products, procedures, treatments, services, opinions, veterinary care providers or other information that may be contained in this blog post. NEXGEN PHARMACEUTICALS, LLC IS NOT RESPONSIBLE NOR LIABLE FOR ANY ADVICE, COURSE OF TREATMENT, DIAGNOSIS OR ANY OTHER INFORMATION, SERVICES OR PRODUCTS THAT YOU OBTAIN THROUGH THIS BLOG POST