Respiratory Arrest in Barasingha Deer During Capture and Chemical Immobilization

The capture and restraint of wild and captive deer is often necessary for the purposes of physiological study, research and wildlife management. In recent years, deer farming has also become popular in the U.S. and several other nations, which has increased the need for services related to deer husbandry and veterinary care.

Since chemical immobilization using sedative and/or anesthetic drugs has been deemed a safer, superior method to nets, cages and other forms of physical restraint in many cases, it has become the method of choice for capturing deer. The drugs used for immobilizing these animals can adversely affect their cardiovascular and respiratory systems, however. In some circumstances, they can lead to complications such as respiratory depression and/or respiratory arrest.

Respiratory arrest and cardiac arrest are obviously different complications, but if left untreated, the former inevitably leads to the latter. Interruption of pulmonary gas exchange (respiration) for more than a few minutes can lead to irreversible vital organ damage, particularly in the brain.^1,2 Cardiac arrest almost always follows without an intervention in which respiratory function is restored.

Barasingha Deer: Background, Biology and Habitat

The barasingha deer (Rucervus duvaucelii) is one of the most widely-recognized deer in India. It is also called the swamp deer because it is found mostly in damp areas, such as reed beds and marshes. They occasionally dwell in open spaces, but this is usually very close to a source of water. The deer’s name is derived from the Hindi words for “twelve ends”, referencing this deer’s antlers, which can have from 12 to 20 tines.³

Approximately 60 years ago, the numbers of barasingha deer in India decreased drastically due to hunting, habitat loss and disease. Breeding programs and conservation practices were successful in bringing this deer back from the verge of extinction over several decades. Habitat improvement and captive breeding subsequently led to a substantial increase in the population.⁴ Today, their populations are no longer in danger, and the barasingha deer has been introduced to many areas including the U.S. as a game animal. While the barasingha deer is extinct in both Bangladesh and Pakistan, there are now fragmented populations in central and northern India, as well as southwestern Nepal.⁵

The barasingha is a large deer with a short head and long, broad ears. Larger animals may have an overall length of up to 71 inches (180 cm) and weigh up to 620 lbs (280 kg). Their coats are a yellowish-brown color, which darkens during the winter months. Some individuals have yellowish spots scattered over their coats. The hair around the deer’s neck may be longer, forming a shaggy mane. The barasingha deer’s face also has a dark mask.^4,5

In India, there are three subspecies of barasingha deer:

1. Wetland barasingha (Rucervus duvaucelii duvacelii)
2. Hard-ground barasingha (Rucervus duvaucelii branderi)
3. Eastern barasingha (Rucervus duvaucelii ranjitsinhii)³

In their native range, barasingha deer have a number of natural predators including the tiger; thus, they have developed an apprehensive, nervous manner and are always on high alert. They have a loud, barking call that they use to signal danger; this is said to be similar to the alarm call of the roe deer. Barasingha deer feed by day, but can also be found resting during the hottest times of the day. In the winter, large herds aggregate, then dissipate during the summer into smaller herds composed chiefly of females and their fawns.⁴ During the summer, males live separately from females in small bachelor groups.

Female barasingha deer usually have a single fawn after eight months of gestation. Fawns are weaned at around six months, and the female is ready to breed again about a year after giving birth. Stags reach puberty at two years of age, and females reach puberty at 18 to 24 months.^3,4 Barasingha deer have a lifespan of up to 20 years.⁵

Risks and Causes of Respiratory Arrest

During chemical immobilization, respiratory arrest in barasingha deer 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

When respiratory arrest is 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 actually lower the risk of complications, including respiratory depression and arrest. The risk for opioid-induced respiratory depression (ORID) is most common in the immediate postoperative recovery period but it can persist and lead to catastrophic outcomes such as severe brain damage or death.¹

Barasingha Deer and Chemical Immobilization

The various species of deer each have their own anesthesia recommendations with dosage variations due to their varied individual responses to sedative and anesthetic agents.^2,3 These variations are factors in the risk of complications. Monitoring core body temperature is essential in deer sedation and anesthesia, and intubation has also been recommended for any chemically-immobilized barasingha deer that needs to be transported or anesthetized for greater than one hour.³

In the area of wildlife immobilization, the most significant group of drugs that carry the potential to depress cardiopulmonary processes are opioids, which include both the natural derivatives, semisynthetic opioids and synthetic opioids.⁴ When respiratory arrest occurs in an immobilized deer as a result of immobilizing drugs, the probability is high that it is in reaction to opioids.

Respiratory Depression Leading to Respiratory Arrest

Hypoventilation, or respiratory depression is defined as reduced and/or ineffective breathing. Respiratory arrest is the complete cessation of breathing. There are several methods for alleviating respiratory arrest in barasingha deer as a result of chemical immobilization. Antagonists (reversal agents) are among the pharmacological developments to wildlife immobilization that are able to completely reverse the effects of opioid anesthetics and tranquilizers.^2,6 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 avoiding complications and preventing predation of animals released into the wild after immobilization events. Antagonists also decrease the personnel and equipment time needed for monitoring the immobilized animal through its recovery.

In cases of respiratory arrest, the ultimate goal is to restore adequate ventilation and oxygenation without further compromising an already compromised cardiovascular situation.⁷ In the event of respiratory arrest in an immobilized deer, 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. ^2,6 Atipamezole is used as a reversal agent for medetomidine and dexmedetomidine in order to reduce their sedative and analgesic effects. Atipamezole has also been used for the reversal of other α2- adrenergic agonists (e.g., xylazine, clonidine, tizanidine and brimonidine).

Doxapram and other potassium channel blockers may be used to stimulate breathing in barasingha 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 barasingha deer whenever possible, as it can lower the risk of respiratory arrest occurring.¹