Respiratory Arrest in Axis Deer During Capture and Chemical Immobilization

The Axis deer (Axis axis) is a deer species that is native to the Indian subcontinent and Sri Lanka, where it is called a chital. It has also been introduced in several nations across the globe. In the U.S., Axis deer have been introduced into areas in Hawaii, Texas, California, Mississippi and Alabama.¹

Some species such as the Axis deer were originally introduced outside of their native ranges as game animals and are now considered invasive species.² This is because with fewer predators and more resources in their new environments, these animals are often able to out-breed and outcompete local species.

The Axis deer in easily identified by the white spots on its orange-brown coat. Mature male and female Axis deer also have white patches on their throats, and mature males have antlers which they use to compete with other males for mating access to females.^2,3 Males grow antlers on a yearly basis, which they use to compete with other males for mating access to females. Axis deer are herd animals with a diet of grasses and vegetation.

Unlike other deer species, Axis deer typically form mating pairs instead of engaging in polygyny. Females can reproduce multiple times each year; thus, in their non-native environments, offspring are more likely to survive until adulthood, so Axis deer populations tend to grow very quickly.

Axis deer are social animals, and are usually found in herds ranging from a few individuals to more than 100. Herd leaders are usually 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

Risks for Respiratory Arrest in Axis Deer

The chemical immobilization of Axis deer is sometimes necessary for the purposes of physiological study, research and wildlife management. The drugs used for immobilizing these animals can adversely affect their cardiovascular and respiratory systems, however. In some circumstances, these drugs can lead to a variety of complications, respiratory depression and/or respiratory arrest among them.

Respiratory arrest and cardiac arrest are singular complications, however, if left untreated, the former inevitably leads to the latter. Interruption of pulmonary gas exchange (respiration) for more than a few minutes can irreversible vital organ damage, particularly in the brain.⁴ Without an intervention where respiratory function is restored, cardiac arrest almost always follows.

Causes of Respiratory Arrest

Respiratory arrest in Axis deer during chemical immobilization can occur due to drug overdose, but it can also come about as a spontaneous adverse reaction to immobilizing drugs. Central nervous system disorders that affect the brain stem can also cause hypoventilation leading to respiratory arrest, as can compression of the brain stem during a capture event.¹ In the case of respiratory arrest brought on by 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. Certain 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. 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.⁴

Axis Deer and Chemical Immobilization

According to the available literature, the various species of deer and other wild hoofstock each have their own anesthesia recommendations with dosage variations due to their varied individual responses to anesthetic agents.^5,6 These variations are factors in the risk of complications. Monitoring core body temperature is essential in the sedation and anesthesia of Axis deer, and intubation has also been recommended for any anesthetized deer that needs to be transported or anesthetized for greater than one hour. Until the more recent use of formulated drugs, opioids were the mainstay of deer anesthesia in wildlife and captive care.⁶

Opioids are the most significant group of drugs that carry the potential to depress cardiopulmonary processes; these include both the natural derivatives, semisynthetic opioids and synthetic opioids.⁷ When respiratory arrest occurs in an immobilized deer as a result of sedative/anesthetic drugs, the probability is high that it is in reaction to opioids.

Treating Respiratory Arrest in Axis Deer

Respiratory depression, or hypoventilation, is defined as reduced and/or ineffective breathing, whereas respiratory arrest is the complete cessation of breathing. There are several approaches to alleviating respiratory arrest in Axis deer as a result of chemical immobilization. Reversal agents (antagonists) are some of the notable pharmacological developments to wildlife immobilization that have come about; these are able to quickly reverse the effects of anesthetics and tranquilizers.^5,7 Antagonists are typically able to completely reverse sedative and anesthetic effects and return an animal to a normal physiological state. The chief benefits of antagonists include preventing predation in the wild after immobilization events and to avoid complications. Antagonists also decrease the personnel and equipment time needed for monitoring an immobilized Axis deer through its recovery.

Should respiratory arrest occur in a chemically-immobilized Axis deer, the ultimate goal will be to restore adequate ventilation and oxygenation without further taxing an already compromised cardiovascular situation.⁵ In the event of respiratory arrest, the administration of all immobilizing drugs should be immediately 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.^5,7 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 α2- adrenergic agonists (e.g., xylazine, clonidine, tizanidine and brimonidine).

Potassium channel blockers such as doxapram may also be used to stimulate breathing in Axis deer 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.⁶ Oxygen is recommended during the immobilization of Axis deer whenever possible, as it can lower the risk of respiratory arrest occurring.⁴