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:
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.
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.
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.
1britannica.com.