Respiratory Depression in Bongo Antelope During Chemical Immobilization
The chemical immobilization of bongo antelope in the field is associated with more risks than in a clinical or zoo setting. In most cases, these animals cannot be examined with regards to their health status beforehand and they 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.1
Chemical immobilization has become the chief method of capture for large wildlife species for the purposes of translocation, diagnostic testing or medical treatment. Remote drug delivery systems are typically used for the purpose of chemical immobilization of bongo and other antelope species, 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 for chemical immobilization should 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 the capture event.2
Potent opioids are often used for the chemical immobilization of bongo antelope. One chief disadvantage of using these drugs is that they can cause clinically significant respiratory depression which is due to their potent effect on mu-opioid receptors.3
Physiology of Bongo Antelope
The Bongo antelope belongs to the genus Tragelaphus (which includes the Sitatunga, Nyala, Bushbuck, Mountain Nyala, Lesser Kudu and Greater Kudu). Bongo are further catalogued into the two subspecies Tragelaphus eurycerus eurycerus, the lowland or Western Bongo and the more rare Tragelaphus eurycerus isaaci, the mountain or Eastern Bongo, which is restricted to North-Eastern Central Africa.4
Bongo are one of the largest of the African forest antelopes and are considered by many to be the most beautiful of all antelopes. In addition to their deep chestnut color, Bongo have white stripes on their sides to help camouflage them from their enemies. Both males and females have long, spiraled horns.
The western/lowland Bongo faces an ongoing population decline as habitat destruction and hunting pressures increase with the expansion of human settlement. The eastern/mountain Bongo’s survival in the wild is largely dependent on the protection of the surviving remnant populations in Kenya. All Bongo are currently considered endangered species.
Treating Respiratory Depression in Bongo Antelope
The available literature states that each species of antelope has its own anesthesia recommendations, with intra-species variations of dosages because of diverse individual responses to anesthetic agents.2-4 These variations are factors in the risk of potential complications; the attendant factors (e.g., stress, venue, individual animal and field conditions) must also be taken into account.
Respiratory depression is common in both human and animal patients recovering from surgery and anesthesia. Failure to recognize occurrences and lack of timely intervention however, can lead to catastrophic cardiorespiratory arrest, anoxic brain injury, and mortality. Opioid-induced respiratory depression during procedures can also be a factor in postoperative respiratory depression. Respiratory impairment in the postoperative period has been associated with significant morbidity and mortality.3 Other causes of respiratory depression include residual anesthesia, residual muscle paralysis and concurrent use of other sedatives.
Fortunately, there are several approaches available to alleviate opioid-induced respiratory depression in bongo undergoing chemical immobilization. Assisted ventilation and oxygen insufflation can combat hypoxia,1 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 via respiratory stimulants which act on non-opioid receptor systems such as potassium channel blockers, ampakines and serotonin receptor agonists.4
The routine use of oxygen is recommended during wildlife immobilization and can be combined with a partial opioid reversal to better alleviate hypoxia.1 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, nalorphine or butorphanol.2,3 Signs of recovery after naltrexone administration typically consist of increased respiratory depth, followed by ear twitching, eye movement and lifting of the head.1
Partial mu-receptor antagonists such as butorphanol can be used to alleviate respiratory depression in bongo caused by strong mu-agonistic immobilization drugs.1,3 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 is widely used as a respiratory stimulant by veterinarians. It has been shown to increase the minute ventilation in large herbivores immobilized with etorphine.1 It should be noted that the respiratory effects of doxapram are usually short lived.
Whereas efficacious 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. Such formulations are often species-specific, reliable and are less likely to bring about complications such as respiratory depression in bongo antelope than drugs used in the past.
1Arnemo, J. Kreeger, T. (2018). Handbook of Wildlife Chemical Immobilization 5th Ed. Sunquest Publishing, 2007.
2Arnemo, 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.
3Ball, L. Antelope Anesthesia. Wiley Online Library, 25 July 2014.
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