Shock in Axis Deer During Capture and Chemical Immobilization

Shock is a critical condition that is caused by a sudden drop in blood flow throughout an animal’s body. This can be the result of a wide variety of conditions or circumstances, including extreme physical stress, trauma, disease, heatstroke, blood loss, allergic reactions or severe infection. When an animal is suffering from shock, its organs are not receiving an adequate amount of blood or oxygen. If untreated, this condition can lead to permanent organ damage or death. The processes surrounding capture and/or chemical immobilization can include extreme physical stress and/or trauma sufficient to induce shock in many animals.

The axis deer (Axis axis) is a deer species that is native to India. It is a moderately large, spotted deer that was introduced into Texas in the early 1900s. These deer prefer sparse, secondary forests that supply adequate drinking water and shade, tending to avoid rugged terrain. Their food consists largely of grasses.^1,2

Like all deer, axis deer are cervids (family Cervidae). Deer are native to all continents except Australia and Antarctica, although many species such as Axis deer have been introduced outside of their original habitats as game animals. In all but one species of deer, only the males carry antlers; in the reindeer or caribou (Rangifer tarandus), both sexes have antlers.³

Axis deer are gregarious, and are usually found in herds ranging from a few individuals to more than 100 animals. The herd leaders are usually mature, experienced does. Unlike native North American deer, adult male axis deer are normally found living with herds of young and old animals of both sexes. Like elk, rutting male axis deer emit bugle-like bellows, and both sexes can produce alarm calls.¹

The reproductive behavior of Axis deer is said to be akin to that of domestic cattle. In the wild, mature bucks in rutting condition may be found throughout the year, with each buck apparently having an independent reproductive cycle of its own, which may not be synchronized with that of other bucks in the herd.¹ Females experience estrous cycles throughout the year, with each cycle lasting approximately 3 weeks. While pregnant females may be found year-round, most breeding lasts from mid-May through August. Bucks make no attempt to collect or retain harems of does.

After a 210 to 240-day gestation period, typically only one fawn will be born. In Texas (which has the largest population of Axis deer in the U.S.), Axis deer fawns are born in early January to mid-April, although they may arrive in any season. Fawns begin consuming green forage by 5 weeks of age, and are weaned at 4 to 6 months.²

How Shock Arises in Axis Deer

The degree of risk for shock in Axis deer is dependent upon factors such as sex, age, overall health, environmental factors, length of immobilization, the degree of stress involved in the capture/immobilization event itself, the specific chemical agents involved in immobilizing the animal and others. It should be noted that cervids are known to be susceptible to a variety of complications during chemical immobilization, including shock.

The main categories of shock include:

Circulatory Shock. This occurs when there is a significant decrease in effective circulating blood volume. This category is further divided into the three subcategories of cardiogenic, hypovolemic and distributive shock. Cardiogenic shock occurs when the circulating volume of blood decreases despite normal or increased blood volume. Hypovolemic shock occurs when blood volume is decreased through hemorrhage, third space fluid distribution, or dehydration. Distributive shock occurs when an animal’s body is unable to maintain the vasoconstriction of blood vessels.⁵

The last two categories of shock are hypoxic shock and metabolic shock. Hypoxic shock results from impaired oxygen delivery to cells, while metabolic shock involves cells that have become unable to utilize oxygen for energy production.^5,6For the purposes of this discussion, the types of shock being discussed are the subcategories of circulatory shock and hypoxic shock, which are the most likely to be brought on due to capture and/or chemical immobilization events.

The Mechanics of Shock in Axis Deer

Even under ideal circumstances, chemical immobilization is a risky proposition in deer as well as in humans. This is because nearly all of the drugs that produce sedation and/or anesthesia compromise cardiovascular stability by producing dose-dependent impairment of cardiac function, vascular reactivity and autoregulatory responses.⁶ Hemoglobin is found within red blood cells and carries oxygen to tissues. In normal circumstances, the amount of oxygen delivered to cells is 2 to 4 times the amount required, which ensures an adequate supply.² However, if tissues are not adequately perfused with blood, the oxygen fails to get to the cells, regardless of the oxygen content in the blood.⁵

Significant changes in the mean arterial pressure (MAP) will trigger changes in heart rate.^6,7 An increase in MAP also causes bradycardia and vasodilation, while a decrease produces tachycardia and vasoconstriction.^5,6 While anesthesia-related depression of cardiac function and arterial vasodilation are adverse effects that are well-recognized as contributing to anesthetic risk, far less emphasis is usually placed on effects impacting venous physiology and venous return.²

Approximately 70% of an animal’s total blood volume is represented by venous circulation, and this is a chief contributor to stroke volume and cardiac output.⁵ Vasodilation is the primary cause of hypovolemia produced by sedative/anesthetic drugs. It is often associated with increased venous compliance, decreased venous return, and reduced response to vasoactive substances.⁶ Depending on factors like patient status and monitoring, a state of relative hypovolemia can remain clinically undetected for long periods of time.^5-7

Diagnosis and Treatment of Shock in Axis Deer

The clinical signs of shock in Axis deer can include any combination of the following:

• Unresponsiveness
• Hypothermia
• Tachycardia
• Bradycardia
• Tachypnea
• Bradypnea
• Marked hypotension
• Cyanosis
• Orthopnea

The treatment of shock in Axis deer should focus on increasing oxygen delivery to the tissues. This can be accomplished by providing supplemental oxygen, increasing effective circulating volume, increasing hemoglobin concentration and increasing cardiac output with stimulants.^5,7 An intravenous catheter should be placed for vascular access if possible. If venous access cannot be established, an intraosseous catheter can be placed. Oxygen supplementation, when available, will also provide benefits to the deer suffering from shock. This can be accomplished via flow-by oxygen, mask, nasal cannulas or an oxygen cage.⁶

Fluid Therapy for Shock

Lactated Ringer’s solution, Normosol-R, and Plasma-Lyte are the preferred choices for fluid therapy in treating shock in deer, as these have been shown to cause fewer complications as well as decrease the risk of mortality⁵ as compared to other options. Hypertonic saline is also a popular option for fluid therapy (as this increases vascular volume). Hypertonic saline increases plasma osmolarity, pulling water into the vascular space from the interstitial space, thereby expanding plasma volume. It should be noted that hypertonic saline does have unwanted side effects, such as a transient, dose-dependent increase in sodium and chloride.^6,7

Blood products are an important adjunct for the treatment of shock in Axis deer. In normal patients, anemia can be well-tolerated with oxygen delivery being maintained. In patients with trauma and acute loss of blood volume however, the associated stressors can contribute to decreased oxygen delivery.⁷

¹J. Schmidly, J., Bradley, R. The Mammals of Texas, Seventh Edition 1994, University of Texas Press.
²tsusinvasives.org.
³animalia.bio.
⁴todaysveterinarynurse.com.
⁵Noel-Morgan, J., Muir, W. (2018) Anesthesia-Associated Relative Hypovolemia: Mechanisms, Monitoring, and Treatment Considerations. Frontiers in Veterinary Science, Vol. 5 (53).
⁶Haller G, Laroche T, Clergue F. Morbidity in anaesthesia: today and tomorrow. Best Pract Res Clin Anaesthesiology (2011) 25(2):123–32.
⁷Steadman J, Catalani B, Sharp CR, Cooper L. Life-threatening perioperative anesthetic complications: major issues surrounding perioperative morbidity and mortality. Trauma Surg Acute Care Open (2017).