Deer are among the most well-known of the large wildlife species in North America. The Eld’s deer (Rucervus eldii) is a deer that is native to the forested areas, lowland valleys and plains of southeast Asia. Due to hunting and habitat conversion, populations of this deer have significantly declined in their native range and are now severely fragmented.1Today, Eld’s deer occur in several protected areas throughout their former range and have been introduced into numerous other countries as game animals, including the United States.
There are three subspecies of Eld’s deer that are currently recognized:
The Eld’s deer is a large deer that is considered majestic in appearance. Similar in size to white-tailed deer, Eld’s deer differ somewhat in body type. Their legs are long and thin, and they have slender bodies with a large head and ears. Their rough coats change color with the season. In summer, they are reddish-brown, and dark brown in winter. Stags often have darker coloring than hinds (females) and have a thick mane of long hair around the neck.1
Eld’s deer graze and browse on wild fruits and cultivated crops such as rice, lentils, corn, peas and rape from nearby fields.2 The Eld’s deer is often associated with areas that are seasonally burned, since they are often found eating new grasses as they emerge after a burn.1 Their feeding may vary seasonally with food availability and with reproductive considerations; during rut, males often experience a decline in body weight, probably due to a decrease in their food intake.
Eld’s deer are usually solitary, but they will form large groups or herds on occasion as an adaptation to avoid predators. These larger groups decrease the risk of predation, both by increasing the chance that a predator will target other animals rather than a lone individual, and by the increased vigilance for predators which can be provided by all members of the group.2 The most common predators of Eld’s deer are tigers and leopards.1
As is the case with most cervids, Eld’s deer mothers hide their fawns in underbrush immediately after birth. Females give birth during the cool-dry season when the flood waters have receded and vegetation has begun to grow, which provides the young with shelter and helps to conceal them. Fawns are weaned after 4 to 5 months, which allows them to have sufficient time to increase their mobility and ability to travel with the herd.1,2
The sedation of Eld’s 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.
Further 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
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.
Other risks associated with the chemical immobilization of Eld’s 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):
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.3,4
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.6
The drugs used for sedating Eld’s deer will vary depending upon 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 animal.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.