Found across southern Africa, the nyala is a member of the spiral-horned antelope genus (Tragelaphus) that includes the kudu and eland. There are two subspecies of nyala: the mountain nyala (Tragelaphus buxtoni), and the lowland nyala(Tragelaphus agasi). Thename “nyala” is actually the Swahili name for this antelope, which comes from the Zulu word "inyala."1
Of the African antelopes, the nyala exhibits the greatest level of difference between the males and females (sexual dimorphism) of the species; this is evident in both their size and coloration. The males have striking spiral horns, a grey to dark brown coat and faint white stripes. Females are chestnut colored with white stripes across their backs, and have no horns.
Nyala are mixed grazer-browsers, and eat leaves, twigs, flowers and fruits as well as grass. Despite the fact that baboons have been known to eat nyala young, nyala often associate with them, picking up the remains of wild fruit, berries and leaves discarded by baboons.2 Due to their difference in size, nyala bulls usually eat twice as much as the females.
Nyala are classified on the International Union for the Conservation of Nature’s Red List of Threatened Species. The main threats to this species are poaching and habitat loss as human settlements encroach into their territory.1 The males’ elegant spiral horns have made them prized among game animals.
Chemical immobilization has become the chief method of capture for large wildlife species such as nyala for the purposes of management, translocation, diagnostic testing or medical treatment. In zoos, farms, breeding facilities and even in free-ranging situations, chemical immobilization is usually carried out from the ground. In some circumstances however, wild animals often have to be located and darted from a helicopter.3 All of the above methods of capture can cause significant stress and trauma to these animals, potentially giving rise to complications.
Remote drug delivery systems are typically used for the purpose of chemical immobilization, usually via a dart gun or blowpipe. Drugs are injected by means of a dart syringe which is 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.1 The ideal drugs will also be fast-acting to limit stress and the likelihood of escape following darting. They should also be reversible, since animals are often released back into the wild immediately after a capture event.
Chemical immobilization of nyala and other wildlife is associated with risks. In the majority of cases, animals cannot be examined with regards to their health status beforehand and often cannot receive adequate supportive treatment during immobilization in the field. Additionally, they are often highly-stressed and sometimes run long distances before they are immobilized. Most drugs used for immobilization have side effects; they not only cause sedation by influencing the central nervous system, but also influence cardiovascular, respiratory and thermoregulatory functions.3 The most common problems encountered during wildlife immobilization include respiratory depression, cardiovascular disturbances, bloat, impaired thermoregulation, hypoxia and capture myopathy.3-5
Opioids are often used for the chemical immobilization of antelope and other wild herbivores. One chief disadvantage of using these drugs is that they can cause clinically significant respiratory depression. This 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. Concurrently, activation of these receptors activate other receptors in the brain stem, on the aortic arch and carotid bodies, which depresses normal respiratory function. These 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.3
There are several approaches available to alleviate opioid-induced respiratory depression in nyala undergoing chemical immobilization. Assisted ventilation and oxygen insufflation can combat hypoxia,3 while agents such as opioid antagonists or partial antagonists can be used. Unfortunately, the latter also reduce desirable effects, such as the degree of immobilization, sedation and analgesia. Respiration can also be improved during chemical immobilization events through the use of respiratory stimulants which act on non-opioid receptor systems such as potassium channel blockers, ampakines and serotonin receptor agonists.6
The use of supplementary oxygen is recommended during wildlife immobilization and 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 the animal needs to be released back into the field and must be fully alert. If residual analgesic or sedative effects are required, partial opioid antagonists or mixed agonists/antagonists are used for the reversal of opioids such as diprenorphine or nalorphine.4,5
Partial mu-receptor antagonists such as butorphanol can be used to alleviate respiratory depression caused by strong mu-agonistic immobilization drugs.3,5 Some of these partial antagonists, however, also reduce the immobilization effects of opioids. Potassium channel blockers such as doxapram can also be used to stimulate breathing. Doxapram has been shown to increase the minute ventilation in large herbivores immobilized with etorphine.6 It should be noted that the respiratory effects of doxapram are usually short lived.
While safe and effective drug combinations used for darting were once not commercially available as pre-mixed solutions, many of these can now be purchased as highly-concentrated drug formulations for this purpose from compounding pharmacies. These formulations are often species-specific, reliable and are less likely to bring about complications such as respiratory depression in nyala than the drugs and combinations used in the past.
1britannica.com.