Respiratory Depression in Eld’s Deer During Capture and Chemical Immobilization

The management of both wild and captive deer frequently requires the manipulation of individual animals, and chemical immobilization has proven to be the safest and most effective method of capture in such cases.¹ The reasons for capture typically include research, translocating nuisance animals, or for the treatment of injured or sick animals.

The Eld’s deer (Rucervus eldii), also known as the brow-antlered deer, is deer species that is indigenous to Southeast Asia. They were discovered in the Manipur Valley of India in 1838 by British officer Lieutenant Percy Eld.² Eld’s deer are known for the striking lyre-shaped antlers of the males, which sweep back in a single curve about 40 inches long. A smaller tine also grows toward the front of the head.

The Eld’s deer is a large deer with a regal and graceful physique. Its legs are long and thin, and it has a long, slender body with a large head and ears. Eld’s deer coats are rough, coarse and change color with the season. In summer, they are reddish-brown with paler underparts. In winter, their coats are dark brown. Stags tend to have darker coloring than hinds and possess a thick mane of long hair around the neck. Male Eld’s deer typically grow to about six feet high and weigh around 275 to 385 pounds. They are taller and larger than females, which stand about five feet tall.³

The Eld’s deer was formerly widely distributed from the Manipur region of northeastern India through much of Myanmar, Thailand, Laos, Cambodia and Vietnam to the island of Hainan, China.² It inhabited suitable forest habitats, lowland valleys and plains, avoiding dense forests and coastal areas. Today, Eld’s deer occur in a number of protected areas throughout their range, and have been introduced to several other countries as game animals.

Eld’s Deer, Capture and Chemical Immobilization

For research and wildlife management purposes, many techniques have been used to capture Eld’s deer, 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 a remotely-delivered anesthetic agent has become the preferred capture technique. This is because this method is not only selective, but it reduces the stress of manual capture on deer.

Numerous chemical agents and drug formulations are currently available due to increased scientific and physiological knowledge and the refinement of immobilizing drugs, and these are far safer than drug combinations used in years’ past. Still, chemical immobilization does cause physiological stress to deer. Sedated or anesthetized animals 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.^4,5

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 deer are often 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, 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.

Respiratory Depression Risks in Eld’s Deer

The chemical immobilization of Eld’s deer and other wildlife is associated with numerous 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, 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 impact cardiovascular, respiratory and thermoregulatory functions.¹The most common problems encountered during wildlife immobilization include respiratory depression, cardiovascular disturbances, bloat, impaired thermoregulation, hypoxia and capture myopathy.^4-6

Opioids are often used for the chemical immobilization of deer and other wild herbivores. A disadvantage of using these potent drugs is that they can cause clinically significant respiratory depression which is due to their potent effect on mu-opioid receptors.⁶ 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.⁴ 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

Treating Respiratory Depression in Eld’s Deer

There are several approaches available to reduce the risk of opioid-induced respiratory depression in Eld’s deer 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.⁶

The use of oxygen is recommended during the chemical immobilization of Eld’s deer; this can be combined with a partial opioid reversal to better alleviate hypoxia.⁴ 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.^4,5 Signs of recovery after naltrexone administration typically consist of increased respiratory depth, followed by ear twitching, eye movement and lifting of the head.⁴

Partial mu-receptor antagonists (e.g., butorphanol) can be used to reduce respiratory depression caused by strong mu-agonistic immobilization drugs.^1,7 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.⁴ It should be noted that the respiratory effects of doxapram are usually short-lived.

While safe and effective drug combinations used for darting were not always 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 deer than drugs and combinations used in the past.