Shock in Nyala Antelope During Capture and Chemical Immobilization
The nyala is a large, slender antelope of that inhabits areas of southeastern Africa. It is a member of the spiral-horned antelope genus Tragelaphini (family Bovidae), which also includes the eland and the kudu. The nyala is noted for its sexual dimorphism and specialized habitat preferences that limit its distribution to the forest fringes of southern Africa. The name “nyala” is the Swahili name for this antelope, which is derived from the Zulu word "inyala."1 The nyala consists of two subspecies, the lowland nyala (Tragelaphus agasi) and the mountain nyala (Tragelaphus buxtoni).
Female and young nyala have short, bright chestnut coats with 8–13 white stripes on the torso, spots and bands on the legs, chest, and cheeks, and a bushy tail with a white underside. Males are much larger, and carry horns with 1.5–2.5 twists. The males gradually turn dark charcoal gray with a shaggy coat as they mature.
Nyala live near areas of dense brush and on the fringes of forests close to water sources. They prefer thick vegetation, although they sometimes venture into open areas to graze on grasses and succulent, high-protein fare.1 It is thought that the range of these animals is limited chiefly due to their dietary preferences. Nyala do not have the breakaway running ability of many other African antelope, so they depend on blending into vegetation to evade predators.2
Chemical immobilization and Shock in Nyala
Shock is a critical condition that is brought on by a sudden drop in blood flow throughout an animal’s body. Shock 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 in shock, its organs are not receiving an adequate amount of blood or oxygen. If untreated, this can lead to permanent organ damage or death.
Unfortunately, the processes surrounding capture and/or chemical immobilization can include extreme physical stress and/or trauma sufficient to induce shock in nyala. The degree of risk is contingent upon factors such as species, 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.
There are three main categories of shock:
Circulatory Shock. This occurs when there is a decrease in the nyala’s 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 the body is unable to maintain the vasoconstriction of blood vessels.3
The other 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.3,4For 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 immobilization events.
Understanding Shock
Almost all of the drugs that produce anesthesia in nyala endanger cardiovascular stability by producing dose-dependent impairment of cardiac function, vascular reactivity and autoregulatory responses.4 Hemoglobin is found within red blood cells and carries oxygen to tissues. Under normal circumstances, the amount of oxygen delivered to the cell is 2 to 4 times the amount required, depending on the tissue, which ensures an adequate supply.4 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) trigger changes in heart rate.3,4 An increase in MAP causes bradycardia and vasodilation, while a decrease produces tachycardia and vasoconstriction.4 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 generally placed on effects impacting venous physiology and venous return.4
Venous circulation represents approximately 70% of an animal’s total blood volume, and is a chief contributor to stroke volume and cardiac output.5 Vasodilation is the primary cause of hypovolemia produced by anesthetic drugs. It is often associated with increased venous compliance, decreased venous return, and reduced response to vasoactive substances.3Depending on things like patient status and monitoring, a state of relative hypovolemia can remain clinically undetected for protracted periods of time.3-5
Diagnosis and Treatment of Shock in Nyala
Clinical signs of shock in nyala can include any combination of the following:
- Unresponsiveness
- Hypothermia
- Tachycardia
- Bradycardia
- Tachypnea
- Bradypnea
- Marked hypotension
- Cyanosis
- Orthopnea
Increasing oxygen delivery to the tissues is an imperative in the treatment of shock. This can be accomplished by providing supplemental oxygen, increasing effective circulating volume, increasing hemoglobin concentration and increasing cardiac output with stimulants.4,6 If possible, an intravenous catheter may be placed for vascular access. If venous access cannot be established, an intraosseous catheter can be placed. Oxygen supplementation, when available, will also provide benefits to the animal experiencing shock. This can be accomplished via flow-by oxygen, mask, nasal cannulas or an oxygen cage.7
Lactated Ringer’s solution, Normosol-R, and Plasma-Lyte are the preferred choices for fluid therapy (increasing vascular volume), as these have been shown to cause fewer complications as well as decrease the risk of mortality3 as compared to other options. Hypertonic saline is also a popular option for fluid therapy. 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 has unwanted side effects, such as a transient, dose-dependent increase in sodium and chloride will occur.3,4
Finally, blood products are an important adjunct for the treatment of shock. In normal patients, anemia can be well-tolerated with oxygen delivery being maintained. In nyala with trauma and acute loss of blood volume however, the associated stressors can contribute to decreased oxygen delivery.4
4Noel-Morgan, J., Muir, W. (2018) Anesthesia-Associated Relative Hypovolemia: Mechanisms, Monitoring, and Treatment Considerations. Frontiers in Veterinary Science, Vol. 5 (53).
5Haller G, Laroche T, Clergue F. Morbidity in anaesthesia: today and tomorrow. Best Pract Res Clin Anaesthesiology (2011) 25(2):123–32.
6Steadman 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).
7Ball, L. Antelope Anesthesia. Wiley Online Library, 25 July 2014,
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