Hypothermia and Hyperthermia in Nyala Antelope During Chemical Immobilization
The nyala is a member of the spiral-horned antelope genus that includes the kudu and eland. More slightly-built than these antelopes, the nyala is noted for its sexual dimorphism and specialized habitat and feeding preferences that limit its distribution.1 name “nyala” is the Swahili name for this antelope, which comes from the Zulu word "inyala."1 The nyala consists of two subspecies, the mountain nyala (Tragelaphus buxtoni), and the lowland nyala (Tragelaphus agasi).
Nyala bulls have long, inwardly-curved horns and a white chevron on their faces and a charcoal-grey, shaggy coat. They stand approximately 106 cm (42 inches) tall and weigh in at around 98–125 kg. The females are much smaller and do not have horns. Females and young have short, brighter chestnut-colored coats with 8–12 white stripes on the torso, with spots and bands on the legs, chest, and cheeks, and a bushy tail.
The nyala is a cover-dependent browser and grazer that occupies dense woodlands on the coastal plains and major river valleys of eastern Africa from southern Malawi to Natal.2 Their habitats include lush grasslands next to the cover where these animals spend the day; at night, they emerge to graze. Females and young live in small herds of five or six animals. The males are nonterritorial and occupy overlapping ranges.
Stress, Hypothermia and Hyperthermia
The capture and chemical immobilization of nyala is a highly stressful event which has the potential to cause capture-induced hypothermia or hyperthermia, either of which can result in morbidity or mortality. The severity of capture-induced hyperthermia has been associated with the likelihood of organ damage, alterations in electrolyte balance that can lead to dehydration events, increased oxidative stress and death.3,4 It has also been called one of the primary factors for the development of capture myopathy.
Each species of antelope has its own anesthesia recommendations, and many species have been reported as notoriously difficult to effectively anesthetize due to their diverse individual responses to anesthetic agents. It has been widely reported that until the advent of potent opiates, some species of antelope were nearly impossible to safely capture or anesthetize.5
The mechanisms underlying the increase in body temperature during capture-induced hypothermia and hyperthermia are not fully understood, but one factor appears to be the sympathetic stress response. Even with animals engaging in low levels of activity during capture with mild ambient temperatures can develop severe hyperthermia.5
The average body temperature for most antelope species is typically between 35o -42o C.3,8 When these exceed more than 2 to 3 degrees higher than lower than the norm during an immobilization event, there is cause for concern and intervention may be required. In the case of capture-induced hypothermia, outward signs may also be evident (see below).
Preventing Hyperthermia and Hypothermia
Measurement of body temperature should be standard procedure during all anesthetic events, and thermometers should be able to read over a wide temperature range. While hypothermia is more common in small animals because of the large surface area-to-volume ratio, instances of both hypothermia and hyperthermia have both been reported during the capture of most antelope species. Some drugs used in chemical immobilization are believed to suppress normal thermoregulatory mechanisms, thereby causing hypothermia or hyperthermia. Hyperthermia however, is also common immediately after immobilization of both captive and free-ranging antelope due to excitement and struggling while darting.3-6
Monitoring core body temperature is essential in antelope anesthesia,5,7 and intubation has been widely recommended for any anesthetized nyala that needs to be transported or anesthetized for greater than one hour. Until the more recent use of formulated drugs (e.g., combinations of α2-agonists such as medetomidine, detomidine, xylazine and their reversal agents), opioids were the mainstay of antelope anesthesia in wildlife and captive care.5
Treating Hyperthermia in Nyala
One method recommended for improving the chances for survival of nyala suffering from hyperthermia is to physically cool the captured animal. Recommendations for cooling captured antelope include placing the animals in the shade and dousing them with water using portable mist sprayers, followed by rapid intravenous (IV) fluid therapy.6 In animals with body temperatures greater than 41°C, the use of cold water enemas and intravenous infusion of cold Ringer’s lactate has been recommended.5,6
Ice packs were also reported to have restored the body temperature of hyperthermic antelope to pre-capture levels.5 The authors point out however, that since carrying water is far less cumbersome and difficult than transporting and maintaining ice-packs in the field, they recommend that water-dousing is the most practical and effective first intervention for cooling nyala with capture-induced hyperthermia.
Treating Hypothermia in Nyala
Hypothermia during anesthetic events is a common adverse effect of anesthesia in many species. Smaller animals are more susceptible to hypothermia during anesthetic events, but even large hoofstock and carnivores can be affected.7 Thus, having thermal support available in the form of external heating devices during and after anesthesia is a wise measure. The time of recovery from anesthesia is typically longer in case of injectable anesthesia rather than inhalant anesthesia.
In addition to abnormally low body temperature, signs of hypothermia can include:
- Stiff muscles
- Pale or gray gums
- Fixed and dilated pupils
- Low heart and breathing rate
With mild hypothermia, shivering may be the only outward symptom. As hypothermia increases in severity, the other usually symptoms become evident. The animal’s vital signs are likely to become increasingly erratic as its body goes into heat conservation mode.3,5 At this point, the animal’s focus is on keeping its vital organs working by restricting the blood flow from all other parts of the body.
Hypothermia can be reversed through the use of water bottles filled with warm water and placed around the animal’s body. External heating devices may also be used, although some of the literature states that heating pads should be used with care, as it is easy to burn an animal’s skin.8 Returning the nyala to a warm environment (if possible) and/or using heat lamps (if available) can also be helpful.
3Ball, L. Antelope Anesthesia. Wiley Online Library, 25 July 2014.
4Sawicka, J. et. al. Efficacy of Different Cooling Methods for Capture-Induced Hyperthermia in Antelope. (2015).
5Haskins, S.C. (1995). Thermoregulation, hypothermia, hyperthermia. In: SJ. Ettinger. & EC. Feldman (Eds), Veterinary internal medicine (4th edition) (pp. 26–30). Philadelphia. U.S.A. W.B Saunders Company.
6Arnemo, J., Fahlman, A. (2008). Biomedical protocols for the free-ranging brown bears, gray wolves, wolverines and lynx. Hedmark University College, Norway and Swedish University of Agriculture Sciences, Sweden.
7Arnemo, Jon & Kreeger, Terry. (2018). Handbook of Wildlife Chemical Immobilization 5th Ed. Sunquest Publishing, 2007.
8Richardson, D. Journal of Mammalogy, Volume 56, Issue 3, 29 August 1975, Pages 698–699.
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