Bloat in Deer During Capture and Chemical Immobilization

Deer are included in over 40 species of hoofed ruminants belonging to the order Artiodactyla. This group encompasses numerous species—among them pigs, camelids, deer, giraffe, antelopes, sheep, goats, and cattle.¹ Deer are cervids (family Cervidae), which are noted for having two large and two small hooves on each foot. They are native to all continents except Australia and Antarctica, although many species have been introduced outside of their original habitats as game animals.²

The deer family is further divided into two groups—Old World deer (subfamily Cervinae) and New World deer (subfamily Capreolinae). This distinction primarily references the differing foot structures of the two subfamilies, rather than where these species originated. In Old World deer, the second and fifth hand bones (metapodia) have almost completely disappeared except for proximal, terminal remnants. In New World deer, these remnants are distal.^2,3

All of the cervids occurring naturally in the Western Hemisphere are New World deer. These deer evolved from forebears that colonized North America, South America and Eurasia. Among these are the moose (Alces alces), the white-tailed deer (Odocoileus virginianus), the elk (Cervus canadensis), the caribou (R. tarandus), the mule deer (O. hemionus) and nearly a dozen other native species.¹

Deer are specialized herbivores who feed on young grasses, herbs, lichens, foliage, buds, aquatic plants, woody shoots and fruit. They do not rely much on coarse, fibrous grasses, since they have not evolved a digestive system comparable to those found in bovids.² Other distinguishing characteristics of deer include scent glands and lack of a gall bladder.¹

Chemical Immobilization and Bloat Risks in Deer

The management and research of deer often requires chemical immobilization. Unfortunately, these animals tend to be prone to a variety of capture‐induced risks while immobilized, including bloat.⁴ Bloat is a serious condition that is fatal if left untreated. In veterinary medicine, it is frequently seen in large dogs and is known as gastric dilatation-volvulus, or GDV, and can occur spontaneously.

Bloat occurs when an animal's stomach fills with gas, food, or fluid and subsequently twists. Stomach distension alone is often referred to as dilatation, or “simple bloat” that can occur spontaneously and can resolve on its own.¹ Bloat without twisting (GDV) can be life threatening, but the risk is predicated upon the severity and duration. Bloat is a condition that has been reported in many species of deer; it has been known to occur spontaneously, but is most closely associated with capture and chemical immobilization events.

The Mechanism of Bloat

In instances of bloat, it is the twisting and flipping of the stomach that precipitates a life-threatening condition. When the stomach becomes severely distended with gas, fluid or food, it puts pressure on the surrounding organs and decreases blood flow to and from these organs. The twisted stomach is more severe, as it completely obstructs blood supply to major organs and can impact blood flow throughout the whole body, resulting in shock.¹

As a deer’s stomach expands, it exerts pressure on the large abdominal arteries and veins. The blood supply is cut off to the stomach; subsequently, toxic products build up and tissues begin to die. Deer (or any animal experiencing bloat) can go into shock very quickly, and extended periods without treatment increase the risk of further damage and death.^1,2

Each species of deer has its own anesthesia recommendations, with intra-species variations of dosages due to the diverse individual responses to anesthetic agents.^5,6 These variations are factors in the risk of bloat and other potential complications. Other factors such as stress, venue, individual animal and field conditions must also be taken into account.

Every anesthetic event carries risk, since sedation and anesthesia represent a controlled intoxication of the central nervous system. Since sedatives and anesthetic drugs are never completely devoid of toxicity, the induction of anesthesia invariably carries a risk even to the life of healthy animals.⁴

Drugs Classes and Procedures

Opioids are one of the staple drug classes used in the field immobilization of hoofstock. Unfortunately, opioids can affect thermoregulation, the emetic center (potentially causing nausea and vomiting), and the respiratory system. At a cardiovascular level, bradycardia can occur, as well as arterial hypertension (or hypotension in some cases). Ruminants are thought to be predisposed to gastrointestinal complications (such as bloat) associated with the administration of opioids.¹

When performing procedures on chemically-immobilized deer, a sternal recumbency position is vital whenever possible, as bloat can more readily occur with animals in lateral recumbency. Other causes of bloat include the use of immobilization drugs such as the α-2-agonists, which can result in a ruminal atony and subsequently, bloat.¹

Alpha-2-agonists and opioids used together have a synergistic effect. These drugs inhibit the norepinephrine release by binding with the α-2-adrenoreceptors. Activity in the Sympathetic Nervous System (SNS) is reduced and it results in a decreased heart rate and blood pressure. They induce muscle relaxation, sedation and analgesia, and reduce the stress response. In higher doses, they can induce vomiting because of the activation of the chemoreceptor trigger zone, hypothermia, miosis and hypoxemia. Via inhibition of antidiuretic hormones, an animal usually has an increase of urine production and a decrease of gastrointestinal motility which is thought to result in bloat and colic, mainly in herbivores.¹

Xylazine was the first α-2-agonist to be used in veterinary medicine. Today, it is used in many species, and is easily available and inexpensive. It promotes good muscle relaxation, sedation and a short period of analgesia. It can, however, cause hyper salivation, muscle tremors in some species and GIT motility suppression. The latter can give rise to ruminal atony and bloat.^1-3

Resolving Bloat in Deer

If an immobilized deer starts to bloat, all administration of immobilizing drugs must be suspended. The animal should be re-positioned into sternal recumbency with the neck extended and the head with the nose pointing down. Intubation of the animal to relieve gases inside may be done; in some cases, trocharization of the rumen is recommended.¹ If the veterinarian has high confidence that the bloat is being caused by the anesthetic agents, he or she may employ the available reversal agents to antagonize their effect.^4,5

Reversal drugs (e.g., diprenorphine, naltrexone, naloxone) should be given as quickly as possible to avoid the side effects of the immobilizing agents, which may include respiratory depression and cardiovascular issues, among others. Intravenous catheters should be placed and fluid therapy begun, as bloat can cause the heart rate to race at a rate sufficient to cause heart failure.

Medication for shock and electrolytes are all essential in stabilizing the animal. Premature ventricular contraction (PVC) is often associated with bloat. If this arises, intravenous medications will also be needed to stabilize the heart rhythm. If the disturbed heart rhythm is noted early on, the animal’s prognosis for recovery is generally good.¹ In the event of a chemically-immobilized deer’s death, post-mortem gas formation in the alimentary tract should be distinguished from ante-mortem bloating, which itself can be a cause of death.^5,6