Deer Sedation

Deer are among the most well-known of the large wildlife species in North America. These animals belong to the order Artiodactyla and the family Cervidae; thus, deer are also known as cervids. There are around a half dozen deer species that are common in North America, as well as other subspecies and non-native species.^1,2 There are over 60 species of deer that are recognized worldwide, and deer inhabit all of the continents except Antarctica.

Deer occupy a range of diverse habitats, from rainforests to temperate zones to deserts. Male deer are referred to as bucks, although males of larger species are often called stags. Female deer are called a does or hinds, and young deer are called fawns. Bucks and does can be distinguished from each other by the presence or absence of antlers. Female Caribou (also known as Reindeer) are the only female deer which grow antlers.

Deer have a four-chambered stomach, and are cud-chewing herbivores, or ruminants. Cud-chewing is a process of partially chewing food, swallowing and regurgitating it, then chewing it again to make it easier to digest.

There are an estimated 35 million deer in the U.S., with the white-tailed deer (or whitetail deer, Odocoileus virginianus)being the most common. These deer weigh from 150-300 pounds, and stand from 37-87 inches at the shoulder.¹

White-tailed deer are distributed east of the Rocky Mountains and in most of Mexico, although in recent years, they have expanded farther west in the United States and into western Canada. Also called the Virginian deer, whitetails get their name from the white underside of their tail. Upon seeing predators, they raise their tail and flash the white underside to let the predator know it has been seen, reducing the possibility of an actual attack.²

Anesthesia vs. Sedation in Deer

Whether in the field, zoos or farms, the sedation of deer is often required for the purposes of research, wildlife management and medical care. First, it is necessary to distinguish between sedation and anesthesia. While “sedation” and “anesthesia” are often used interchangeably, these are two discrete modalities of chemical immobilization.

Anesthesia is a pharmacologically induced reversible state of amnesia, analgesia, loss of responsiveness, and loss of skeletal muscle reflexes. Sedation is a drug- induced depression of consciousness during which an animal cannot be easily aroused, but may respond following repeated or painful stimulation.

The relative safety of sedation over general anesthesia in both animal and human patients is well-established. Sedation is associated with decreased risk in other species as well; therefore, sedation is often considered in place of general anesthesia whenever possible.³ Until the advent of potent opiates, some cervid species were known to be very difficult to safely capture or immobilize chemically.⁴ The focus on the use of sedation in exotic animals such as deer is a direct result of the perception of greater anesthetic risk in these patients, especially in those that are ill or debilitated.

Other advantages of sedation include a general reduction of anxiety and stress related to disease processes such as respiratory disease, and for diagnostic sampling and therapeutics. In some cases, the risk of handling must be weighed against the risk of foregoing diagnostic testing or procedures, or risk of general anesthesia.³

Risks Associated With Chemical Immobilization

A variety of physiological and metabolic changes occur in a deer’s body as a result of chemical immobilization, not all of which are caused by the immobilizing agents. Many of these changes are due to the capture of the animal itself, the most serious of which can bring about a condition called capture myopathy. This is believed to be related to a metabolic acidosis caused by extreme exertion over a short period of time, resulting in necrosis of the large muscle masses of the legs and other areas. This leads to the collapse of the animal and eventual death from acidosis, predation or other factors.³Since all deer are prey animals and have evolved with instincts and behaviors gauged to help them survive, this is an inherent risk in deer capture events of any kind.

Other risks associated with the chemical immobilization of deer include vomiting and aspiration, hypothermia, hyperthermia, frostbite, bloat, respiratory depression/arrest and cardiac arrest. Each species of deer has its own anesthesia recommendations with intra-species variations of dosages because of diverse individual responses to anesthetic agents.^3,4

Disadvantages of sedation can include incomplete elimination of patient movement, patient semi-awareness, and lack of complete analgesia. So, while drugs and lower dosages used for sedation are linked with greater patient safety, they are not entirely without risk. These disadvantages can be overcome with careful dosing and monitoring, effective patient handling and efficient use of analgesics when handing or procedures are expected to produce discomfort.³ The American College of Veterinary Anesthetists (ACVA) has published recommendations for monitoring animals that are sedated without general anesthesia (ACVA recommendations):

• Palpation of pulse rate, rhythm and quality
• Observation of mucous membrane color and CRT
• Observation of respiratory rate and pattern
• Auscultation
• Pulse oximetry Supplemental oxygen, and endotracheal tube (where applicable) and materials to obtain vascular access should be readily available³

Agents for the Sedation of Deer

Sedatives produce calmness, loss of aggression and loss of alertness which are generally required during transportation. In this condition, animals are not immobilized fully and can be aroused by various disturbances. Therefore, they are usually used singly for only very minor procedures, or as adjuncts to dissociative anesthetics for hastening smoother induction and to reduce the quantity of anesthetic for achieving more effective immobilization. The combined synergistic effect of tranquilizers and anesthetic is far greater than the individual effect of either of the two drugs with respect to smooth induction, good muscle relaxation and smoother recovery.³

Sedatives commonly used in veterinary medicine include drugs such as midazolam, alprazolam, amitriptyline, buspirone, clomipramine, dexmedetomidine, diazepam, fluoxetine, lorazepam, paroxetine, sertraline, or trazodone. Acepromazine is also widely used in veterinary medicine as a sedative. It is a member of the phenothiazine class of sedatives and works primarily by blocking dopamine receptors within the brain, thereby depressing certain brain functions.³

Midazolam is often used in human and veterinary medicine for the purposes of pre-anesthesia and sedation and has a wide margin of safety in many species. When combined with an opioid, its effects are synergistic, allowing a reduction of the amount of either drug.⁴ Dosages most commonly used are 0.5-10 mg/kg, combined with an opioid (butorphanol, buprenorphine, hydromorphone, other). Effects are variable, from slight decrease in activity to lateral recumbency. These effects are likely related to species variability in response and the varying dose rates suggested for different species. In all cases, patients still react somewhat to handling and certain stimuli.¹

When midazolam is used alone, sedation may be adequate in ruminants, camelids and several other species. When used in combination with other drugs (e.g., opioids, ketamine, acepromazine, dexmedetomidine), midazolam provides more reliable sedation. It should be noted that use of sedation and manual restraint alone is inappropriate for any procedure expected to produce discomfort. In mammals, additional sedation can be provided with sub-anesthetic dosages of ketamine, 2-7 mg/kg, or alfaxalone, 1 mg/kg IM. If additional immobilization is essential, low concentrations of inhalant gas can be considered.¹

The drugs used for sedating deer will vary depending upon the species, as well as the preference and experience of the veterinarian or wildlife management personnel. The literature suggests that dosing higher when in doubt is in fact safer than dosing conservatively, in that there is more risk to a partially-immobilized animal and to human handlers than there is to a heavily-dosed deer.^4,5 This is largely due to the relative safety of modern drug formulations, which allow for much more latitude in dosing without putting an animal’s health or life at risk.