Aspiration in Deer During Capture and Chemical Immobilization

Deer are hoofed mammals with an even number of toes on each foot which belong to the order Artiodactyla. This group of animals are also known as cervids. Deer have a four-chambered stomach, and are cud-chewing herbivores (ruminants). Most deer have 32 teeth, although they lack upper incisor teeth, and most species also lack upper canines.¹

Cervids differ from other ruminants in that males grow antlers that are comprised of bone. These are shed and regrown annually, and increase each year in size and intricacy. Except in the case of caribou, female deer do not grow antlers. While the antlers grow, they are covered by a thin layer of downy skin known as velvet. When the antlers are fully grown, the velvet dries and is typically rubbed off by the animal.² Antlers serve as ornamentation for breeding purposes, as well as weapons during the mating season.

There are approximately 40 species of deer globally, most of which are found in Asia.¹ There are seven species of deer that are native to North America, with many existing sub-species. Deer have a range that essentially blankets the North American continent, making them popular among big game hunters and, more recently, deer farmers.

Those in the deer family vary in size and weight from just under 100 pounds to over 1000 pounds (technically, elk and moose are also members of the deer family). The Whitetail Deer (Odocoileus virginianus) is the most widely distributed deer in North America, with a range that extends east from the Rocky Mountains into Mexico.² These animals can reach a length of approximately six feet and a height of approximately three feet at the shoulder. Adult males (bucks) can weigh up to 180 pounds, while females (does) average between 80–120 pounds.

Deer, Chemical Immobilization and Aspiration

Researchers, wildlife managers and deer farmers are often called upon to perform routine procedures on deer, or to administer medical treatment. In nearly all cases, this requires the use of sedative and/or anesthetic drugs. Vomiting is one of the more common post-sedation and post-anesthesia complications in both domestic and exotic animals. Vomiting that occurs during a procedure can pose grave risks due to anesthetic aspiration.

Aspiration is described as when a foreign substance enters the airway or lungs; this can apply to food, liquid, or other materials. Aspiration can give rise to serious health problems, such as aspiration pneumonia. Aspiration can occur when a human or animal has difficulty swallowing normally (which is referred to as dysphagia), but in some instances it can be brought on during or after anesthetic events, as indicated above.

Aspiration pneumonia is a condition that is characterized by inflammation and necrosis of lung tissue due to the inhalation of foreign material. The most common material aspirated in large animals is a large volume of liquid, due to weakness from a pathological condition, during oral administration of fluids or as a result of chemical immobilization. The severity of the inflammatory response depend on the type and volume of material aspirated and the distribution of aspirated material in the lungs.

In severe cases in which large volumes of liquid have been aspirated, death can occur swiftly. Other cases will present later with clinical signs consistent with cranioventral bronchopneumonia.³ Cervids such as deer that are affected with chronic wasting disease may also develop aspiration pneumonia due to central nervous system dysfunction.³

Deer and Chemical Immobilization

At the beginning of the last century, the primary method used for the capture of many large wild animals such as deer was to chase them to the point of near-exhaustion—a labor-intensive method that was impractical and fairly inhumane.³ With the pioneering work on the chemical immobilization of wildlife that took place from the 1950s on, chemical immobilization techniques have improved greatly through the development of increasingly efficacious drugs and equipment.

The field immobilization of wild animals with chemical agents is a method of rendering them tractable while using minimal restraint. Here, the research or wildlife management objectives are usually to measure or weigh the live deer, collection of blood or tissue for research or diagnostics, marking an individual or fitting a radio transmitter for studying migration patterns, range requirements and behavior patterns or the translocation of animals for a variety of reasons.^3,4These requirements have resulted in the development of increasingly safer methods of chemical immobilization.

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 These variations are of course factors in the risk of vomiting and anesthetic aspiration in these species, and attendant factors (e.g., stress, venue, individual animal and field conditions) must also be taken into account.

Chemical Immobilization: Practices and Procedures

The utilization of basic veterinary knowledge can make a substantial contribution to animal safety during capture and chemical immobilization. Teams that are qualified to handle wild mammals should evidence the appropriate expertise in wildlife anesthesia and should include an attending veterinarian when appropriate. A successful chemical restraint exercise is not complete until the subject is fully recovered and (in the case of field operations) back in its environment. The application of appropriate pharmacological principles with an emphasis on drug reversibility will minimize the chances that the animal will be at a competitive disadvantage or inordinately disoriented following its release.^3-5

Hyperthermia and capture myopathy are commonly-encountered problems with deer anesthesia. Intubation has been widely recommended for any anesthetized deer that needs to be transported or anesthetized for greater than one hour. Unfortunately, the literature strongly suggests that intubation can increase the risk of vomiting during an anesthetic event.^1,4

Anesthetic Aspiration in Deer

Until formulated drugs (e.g., combinations of α2-agonists such as medetomidine, detomidine, xylazine and their reversal agents) came into use in recent years, opioids were the mainstay of large animal anesthesia in wildlife and captive care.⁴As with other mammals, problems encountered with certain opioids (such as etorphine or carfentanil, which have been widely used in wildlife chemical immobilization) in deer are known to include vomiting or passive regurgitation that can lead to fatal aspiration pneumonia.

Periprocedural fasting (fasting prior to an anesthetic event) has historically been recommended by clinicians because of the suspected risk of aspiration. Unfortunately, periprocedural fasting is very often impossible under field conditions. Additionally, much of the data on anesthetic aspiration relates to humans receiving general anesthesia, however, deer and other mammals have been known to aspirate during procedures while under sedation and where no intubation or general anesthesia were employed.

For the prevention of anesthetic aspiration the literature recommends histamine (H2) antagonists such as cimetidine, famotidine, nizatidine, and ranitidine and proton pump inhibitors (PPIs) such as dexlansoprazole, esomeprazole, lansoprazole, omeprazole, pantoprazole, and rabeprazole, which have been shown to be effective in increasing the pH and reduce the volume of gastric contents.² Prokinetics (e.g., domperidone, metoclopramide, erythromycin and renzapride) promote gastric emptying and are also believed to reduce the risk of aspiration.⁴

In the event that aspiration occurs during a procedure, the first step in managing the situation is the recognition of gastric content in the oropharynx or the airways.^4-7 The deer should be immediately positioned with the head down and rotated laterally if possible. Orotracheal and endotracheal suctioning is indicated, either before or after orotracheal intubation, depending on whether regurgitation continues and if the airway is visible. It is recommended that the airway be secured as rapidly as possible to prevent further contamination and to facilitate airway clearance.² Flexible bronchoscopy is an important adjunct to orotracheal and endotracheal suctioning. Rigid bronchoscopy may be required if particulate matter is present in the deer’s airway.^2,5