Respiratory Depression in Axis Deer During Capture and Chemical Immobilization

In the areas of research, wildlife management and deer farming, many techniques have been employed to capture animals, including live traps, drop nets, drive nets and rocket nets. Since these methods tend to limit the ability to select specific animals for capture, chemical immobilization using remotely-delivered sedative or anesthetic agents has become the preferred capture technique. This is because this method is not only selective, but it reduces the stress of manual capture on animals. The reasons for capture typically include research, translocating nuisance animals, or for the treatment of injured or sick animals.

There are seven species of deer and many sub-species that are native to North America. Deer belong to the order Artiodactyla, and to the family Cervidae. Cervids differ from other ruminants in that males grow antlers that are comprised of bone.¹ These are shed and regrown annually. The axis deer (Axis axis)—also known as the spotted or chital deer—is a species that is native to the Indian subcontinent. A moderately large, spotted deer, it was introduced into the in the early 1900s. Axis deer inhabit sparse forested areas with ample drinking water and shade, tending to avoid rugged terrain. For food, these deer graze primarily on grasses.^2-4

Axis deer are social animals, and are usually found in herds ranging from a few individuals to more than 100. Herd leaders are mature, experienced does. Unlike native North American deer, adult male axis deer are normally found living with herds of young and old animals of both sexes. Like the native elk, rutting male axis deer emit bugle-like bellows, and both sexes can produce alarm calls.³

Chemical Immobilization and the Risk of Complications

Today, there are many more and far safer chemical agents and drug formulations for the chemical immobilization of wildlife than in decades past. This is the result of increased scientific and physiological knowledge and the refinement of immobilizing drugs. That said, chemical immobilization does cause physiological stress to the target animals. Anesthetized Axis deer will be at risk of complications such as cardiovascular or respiratory depression and disruption of the thermoregulatory system. These effects can require supportive treatment by the attending veterinarian or support staff, or the initiation of anesthetic reversal prior to completion of the procedure.^5,6

For the purpose of chemical immobilization, remote drug delivery systems are typically used. These usually consist of a dart gun or blowpipe. With the former (which generally has greater range and accuracy than the latter), 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 need to be highly potent and concentrated. They must also have a high therapeutic index and wide safety margin, since animals often cannot be examined and weighed prior to immobilization.⁵ The ideal drugs should also be fast-acting to limit stress and the likelihood of escape following darting. Finally, the drugs should be reversible, since in many cases, deer are released back into the wild immediately after the capture event.

In zoos, deer farms, breeding facilities and even in free-ranging situations, chemical immobilization is usually carried out from the ground. In some circumstances however, Axis deer have to be located and darted from a helicopter.⁶ All of the methods of capture mentioned can cause significant stress in these animals, potentially giving rise to complications such as respiratory depression.

Respiratory Depression Risks in Axis Deer

It is unavoidable that the chemical immobilization of Axis deer and other wildlife is associated with risks. In many 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, animals are often highly-stressed and sometimes run long distances before they are immobilized.

Most of the drugs used for chemical immobilization carry side effects; they not only cause sedation by influencing the central nervous system, but also influence cardiovascular, respiratory and thermoregulatory functions.⁵ The most common problems encountered during wildlife immobilization include respiratory depression, cardiovascular disturbances, bloat, impaired thermoregulation, hypoxia and capture myopathy.^2-4

Potent opioids are often used for the chemical immobilization of Axis deer and other wild herbivores. A disadvantage of using these drugs is that they are known to cause clinically significant respiratory depression which is due to their potent effect on mu-opioid receptors. 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.⁶

In turn, these processes lead to a reduction of the respiratory frequency and tidal volume, as well as pulmonary vasoconstriction which decreases pulmonary perfusion.⁷ 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 cause capture myopathy, which can ultimately lead to cardiac arrest and death.^5,8

Treating Respiratory Depression in Axis Deer

Several approaches are available to mitigate the risk of opioid-induced respiratory depression in Axis deer undergoing chemical immobilization. Assisted ventilation and oxygen insufflation can combat hypoxia,⁶ while agents such as opioid antagonists or partial antagonists can be used. The latter also reduce desirable effects of immobilizing drugs however, 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; these include potassium channel blockers, ampakines and serotonin receptor agonists.⁷

Whenever possible, the use of oxygen is recommended during the chemical immobilization of Axis deer; this can be combined with a partial opioid reversal to better alleviate hypoxia.^6,8 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.^7,8 Signs of recovery after naltrexone administration typically consist of increased respiratory depth, followed by ear twitching, eye movement and lifting of the head.⁶

Butorphanol or other partial mu-receptor antagonists can be used to reduce respiratory depression caused by strong mu-agonistic immobilization drugs.^6,7 Some of these also reduce the immobilization effects of opioids, however. 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.⁷ It should be noted that the respiratory effects of doxapram are usually short-lived.

As mentioned previously, while safe and effective drug combinations used for darting were not always commercially available as pre-mixed solutions, many 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 Axis deer than the drugs and combinations used in the past.