Hypothermia and Hyperthermia in Impala During Chemical Immobilization

The impala (Aepyceros melampus) is one of the most well-recognized and widely-distributed antelopes of the African savannah. Known for their spectacular and iconic leaps when alarmed or pursued by predators, the impala is comprised of of several subspecies. The most common in East Africa is Aepyceros melampus rendilis; Aepyceros melampus petersi, the black-faced impala, is distributed further south. There are other proposed subspecies (up to 4 additional) that are less well known; controversy as to their designation is ongoing.¹

Impala are found in Kenya, Zimbabwe, Uganda, Zambia, Botswana, and Southern Angola to northern South Africa. Impala are fawn-colored with white underparts and a black stripe that extends from the top of the rump down the back of each thigh.² Rams have horns which are lyre-shaped, and reach a length of around 30 inches. The impala stands 28–36 inches high and weighs 88–167 pounds. Impala males are approximately 20% heavier than females.

Impala are able to adapt to different environments of the savannas. In some areas, they are grazers and browsers in others. Impala herds typically stay within a few miles of water sources. Younger rams live in bachelor herds, while those strong enough establish or take over breeding herds.³ Impala are also a very popular prey for predators such as lions, leopards, cheetahs and hyenas.

Hypothermia and Hyperthermia

The capture of impala—or any wild animal—is a highly stressful event which has the potential to cause capture-induced hypothermia or hyperthermia. Either of these can result in morbidity or mortality from a variety of causes. The severity of the capture-induced hyperthermia has been associated with the likelihood of organ damage, rhabdomyolysis, alterations in electrolyte balance (possibly leading to dehydration events), increased oxidative stress and death.⁵ It has also been called one of the causative factors for the development of capture myopathy.

The mechanisms underlying the increase in body temperature during capture-induced hypothermia and hyperthermia are not fully understood, although 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.⁶

The body temperature for most antelope species typically averages between 35^o -42^o C.^4,9 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.

Preventing Hypothermia and Hyperthermia

During all anesthetic events, measurement of body temperature should be standard procedure; thermometers should be able to read over a wide temperature range. Hypothermia is reportedly more common in small animals because of the large surface area-to-volume ratio, but instances of both hypothermia and hyperthermia have both been reported during the capture of impala. Some drugs used in chemical immobilization suppress normal thermoregulatory mechanisms, thereby causing hypothermia or hyperthermia. It should be noted that hyperthermia is also common immediately after immobilization of both captive and free-ranging antelope due to excitement and struggling after being darted.^4-7

Monitoring core body temperature is essential in antelope anesthesia,^6,8 and intubation has been widely recommended for any anesthetized antelope 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.⁶

Treating Hyperthermia in Impala

Given that capture-induced hyperthermia in impala may be severe, one method recommended for improving their chances for survival is to physically cool captured animals. Recommendations for cooling captured animals include placing them in the shade and dousing with water using portable mist sprayers, followed by rapid intravenous (IV) fluid therapy.⁷ In impala with body temperatures greater than 41°C, the use of cold water enemas and intravenous infusion of cold Ringer’s lactate has been recommended.^6,7

Ice packs are also reported to be helpful in restoring the body temperature of hyperthermic animals to pre-capture levels.⁶However, since carrying water is far less cumbersome and difficult than transporting and maintaining ice-packs in the field, water-dousing may be the most practical and effective first intervention for cooling an antelope with capture-induced hyperthermia.

Treating Hypothermia in Impala

Hypothermia is a common adverse effect of anesthesia in many species. Smaller animals are susceptible to hypothermia during anesthetic events, but even large hoofstock and carnivores can be affected.⁸ Thus, having thermal support available (e.g., external heating devices) during and after anesthesia is prudent. The time of recovery from anesthesia is typically longer in case of injectable anesthesia versus inhalant anesthesia.

Apart from abnormally low body temperature, signs of hypothermia can include:

• Shivering
• Stiff muscles
• Pale or gray gums
• Fixed and dilated pupils
• Low heart and breathing rate

In the case of 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.^4,6 At this point, the animal’s body is trying to keep 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 the literature states that heating pads should be used with care, as it is easy to burn an animal’s skin.⁷ Returning the animal to a warm environment and/or the use of heat lamps may also be helpful.