Respiratory Depression in Antelope During Chemical Immobilization

Apart from traps, capture with nets and the mass-capture of herds of animals, chemical immobilization has become the chief method of capture for large wildlife species for the purposes of 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.¹ 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 the capture event.

Respiratory Depression Risks

Chemical immobilization of antelope and other wildlife is often associated with risks. In most cases, 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 The most common problems encountered during wildlife immobilization include respiratory depression, cardiovascular disturbances, bloat, impaired thermoregulation, hypoxia and capture myopathy.^1-3

Potent 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 which is due to their potent effect on mu-opioid receptors.³ Activation of mu-opioid receptors in the respiratory centers of animals depresses neurons that generate the normal respiratory rhythm. At the same time, activation of these receptors activate other receptors in the brain stem, on the aortic arch and carotid bodies, which depresses 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.² Alpha-2 agonists such as guanabenz, clonidine, medetomidine, and dexmedetomidine cause reflex bradycardia and hypotension, which can lead to hypoxemia and tissue hypoxia. Hypoxia can give rise to capture myopathy, which can ultimately lead to cardiac arrest and death.¹

Treating Respiratory Depression in Antelope

Fortunately, there are several approaches available to alleviate opioid-induced respiratory depression in antelope 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 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 antelope than drugs and combinations 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.

³Bailey, 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.

⁴Van 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.

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