Respiratory Arrest in Nyala Antelope During Chemical Immobilization

The nyala is a member of the spiral-horned antelope genus that includes the kudu and eland. More slightly-built than these antelopes, the nyala is noted for its extreme sexual dimorphism and specialized habitat and feeding preferences that limit its distribution.¹ There are two subspecies of nyala: the mountain nyala (Tragelaphus buxtoni), and the lowland nyala (Tragelaphus agasi).

Nyala bulls have long, inwardly-curved horns and a white chevron on their faces and a charcoal-grey, shaggy coat. They stand approximately 106 cm (42 inches) tall and weigh in at around 98–125 kg. The females are much smaller and do not have horns. Females and young have short, brighter chestnut-colored coats with 8–12 white stripes on the torso, with spots and bands on the legs, chest, and cheeks, and a bushy tail.

The nyala is occupies dense woodlands on the coastal plains and major river valleys of eastern Africa from southern Malawi to Natal.² They are cover-dependent browser and grazers whose habitats include lush grasslands next to the cover where these animals spend the day; at night, they emerge to graze during the rainy season. Females and young live in small herds of five or six animals. The males are nonterritorial and occupy overlapping ranges.

Nyala and Chemical Immobilization

The chemical immobilization of nyala is often necessary for a variety of reasons, including physiological study, research, and for the purposes of wildlife management. However, immobilizing drugs can adversely affect the cardiovascular and respiratory systems of these animals and, in certain circumstances, can lead to complications such as respiratory depression and/or respiratory arrest.

Respiratory arrest and cardiac arrest are different complications however, if left untreated, the former inevitably leads to the latter. Interruption of pulmonary gas exchange (respiration) for more than five minutes can irreversible vital organ damage, particularly in the brain.¹ Cardiac arrest almost always follows without an intervention whereby respiratory function is restored.

Causes of Respiratory Arrest in Nyala

Respiratory arrest during chemical immobilization can occur due to drug overdose, but in many cases, it can come about as a spontaneous adverse reaction to immobilizing drugs (e.g., the animal was inordinately stressed, comorbidities existed, etc.). Central nervous system disorders that affect the brain stem can also cause hypoventilation leading to respiratory arrest, as can compression of the brain stem during a capture event.³

In the case of respiratory arrest brought on by chemical immobilization, the decreased respiratory effort reflects central nervous system (CNS) impairment due to the immobilizing drugs. Drugs that decrease respiratory effort include opioids and certain sedatives. Certain combinations of drugs can increase the risk for respiratory depression, although some of the newer species-specific formulations can actually lower the risk of complications, including respiratory depression and arrest. The risk for opioid-induced respiratory depression (ORID) is usually most common in the immediate postoperative recovery period but it can persist and lead to catastrophic outcomes such as severe brain damage or death.³

Antelope Immobilization

According to the available literature, different species of antelope have their own anesthesia recommendations with dosage variations due to their diverse physiology and individual responses to anesthetic agents.^4,5 These variations are factors in the risk of complications. Monitoring core body temperature is widely recommended in antelope anesthesia, and intubation has also been recommended for any anesthetized antelope that needs to be transported or anesthetized for greater than one hour. Until the more recent use of formulated drugs, opioids were the mainstay of antelope anesthesia in wildlife and captive care.⁵

In wildlife chemical immobilization, the most significant group of drugs that carry the potential to depress ventilation are opioids, which include both the natural derivatives, semisynthetic opioids and synthetic opioids.⁶ When respiratory arrest occurs in an immobilized antelope as a result of immobilizing drugs, the probability is high that this is in reaction to opioids.

Reversing Respiratory Arrest in Nyala

Respiratory depression (hypoventilation) is characterized by reduced and/or ineffective breathing, while respiratory arrest is the cessation of breathing. There are several approaches available to alleviate respiratory arrest in nyala as a result of chemical immobilization. Antagonists, or reversal agents, are some of the notable pharmacological developments to wildlife immobilization that are able to reverse the effects of opioid anesthetics and tranquilizers.^4,6 These drugs are typically able to completely reverse anesthetic effects and return an animal to a normal physiological state. The main benefits of antagonists include preventing predation in the wild after anesthetic events and to avoid or overcome complications. Antagonists also decrease the personnel and equipment time needed for monitoring an immobilized animal through its recovery.

When respiratory arrest occurs, the goal is to restore adequate ventilation and oxygenation without further compromising an already compromised cardiovascular situation.⁷ In the event of respiratory arrest in an immobilized nyala, all immobilizing drugs should be immediately withdrawn. 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 can be used for the reversal of opioids such as diprenorphine, nalorphine or butorphanol.^4,6 Atipamezole is often used as a reversal agent for medetomidine and dexmedetomidine. It has also been used for the reversal of other α2- adrenergic agonists (e.g., xylazine, clonidine, tizanidine and brimonidine).

The literature states that potassium channel blockers such as doxapram can also be used to stimulate breathing in animals suffering from respiratory depression/arrest. Doxapram is widely used as a respiratory stimulant by veterinarians and has been shown to increase the minute ventilation in large herbivores immobilized with etorphine.⁵ Oxygen should be used during antelope immobilization whenever possible, as it can lower the risk of respiratory arrest occurring. Oxygen can also be combined with partial opioid reversal agents to better alleviate hypoxia.³