The bongo (Tragelaphus eurycerus) is an African antelope species found in the lowland forests from Sierra Leone in West Africa, through Central Africa and as far as southern Sudan in east Africa. Small populations also live in the montane (highland forests) of Kenya. They are characterized by prominent colors and long, spiral horns. They have a reddish-brown coat with 10-15 vertical white stripes, and a thin mane along their backs. Their legs have black and white bands and its long tail ends in a tuft.
The bongo has large ears and its tongue is long and prehensile. These antelopes are unusual in their genus in that both sexes have horns. Females weigh in at between 210 kg - 235 kg and males at 240 kg - 405 kg. Unlike other antelope species, bongos live in herds, ranging between 5 and 50 individuals foraging for food together.1 The bongo is classified as Lower Risk/ Near Threatened by the International Union for Conservation of Nature (IUCN)'s Red List of Threatened Species.1
The chemical immobilization of bongo is often necessary for a variety of reasons (e.g., physiological study, research, 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.
While respiratory arrest and cardiac arrest are different complications, 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.2 Cardiac arrest almost always follows without an intervention whereby respiratory function is restored. 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.2
When respiratory arrest is brought on during 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. Some 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.2
Antelope belong to the families Antilocapridae and Bovidae. Prior to the advent of potent opiates, certain antelope species were notorious for being very difficult to safely capture or anesthetize.3 According to the available literature, the varied species of antelope each have their own anesthesia recommendations with dosage variations due to their diverse individual responses to anesthetic agents.3,4 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. Opioids were the mainstay of antelope anesthesia until the more recent use of formulated drugs.4
In the area of wildlife immobilization, the most significant group of drugs that carry the potential to depress ventilation are the opioids, which include both the natural derivatives, semisynthetic opioids and synthetic opioids.5 When respiratory arrest occurs in an immobilized bongo as a result of immobilizing drugs, the probability is high that this is in reaction to opioids.
Respiratory depression (hypoventilation) is characterized by reduced and/or ineffective breathing. Respiratory arrest is the cessation of breathing. There are several approaches available to alleviate respiratory arrest in antelope 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.3,5 These drugs are typically able to completely reverse anesthetic effects and return an animal to a normal physiological state. The chief 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 the immobilized animal through its recovery.
When respiratory arrest occurs, the ultimate goal is to restore adequate ventilation and oxygenation without further compromising an already compromised cardiovascular situation.5 In the event of respiratory arrest in an immobilized antelope, of course the administration of all immobilizing drugs should be ceased. 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.3,5 Atipamezole is often used as a reversal agent for medetomidine and dexmedetomidine in order to reduce their sedative and analgesic effects. It has also been used for the reversal of other alpha-2 adrenergic agonists (e.g., xylazine, clonidine, tizanidine and brimonidine).
Potassium channel blockers such as doxapram can also be used to stimulate breathing in an antelope 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.4 The use of oxygen is recommended during antelope immobilization whenever possible, as it can lower the risk of respiratory arrest occurring. It can also be combined with partial opioid reversal agents to better alleviate hypoxia.2
1wwf.org.