Frostbite in Nyala Antelope During Chemical Immobilization

The Nyala is a large antelope that inhabits the dense forest and thick bush in areas of southern Africa. Since they do not have the running ability of other African antelopes, they are uncomfortable in open spaces and most often seen near water holes. The nyala comprises two subspecies, the mountain nyala (Tragelaphus buxtoni), and the lowland nyala(Tragelaphus agasi).¹These antelope typically live alone or in small family groups of up to 10 individuals.

Nyala females have a chestnut-brown shaggy coat that is marked with white vertical stripes and spots on the flanks. Nyala males are more charcoal grey in color, and have long, inwardly curving horns and a white chevron on the face. They also have a ridge of long hairs along their undersides and a mane of thick, black hair from the head along the spine to their rear.

Nyala feed by both grazing and browsing, typically feeding on leaves, fruit and flowers. It is thought that this variety in their diet is a factor that has contributed to their survival.² Nyala are very secretive and more easily seen at night. They are not territorial, with both sexes having overlapping ranges.

Frostbite Risk for Nyala

The chemical immobilization of nyala and other antelope can require extended periods of immobility in the captured animal. While hypothermia and frostbite in African antelope such as nyala are lower risk complications than with antelope in other geographic areas, risk factors do exist, particularly at higher elevations and accompanying inclement weather conditions in the field.

Frostbite is generally divided into four overlapping phases of pathology:

• Prefreeze
• Freeze–thaw
• Vascular stasis
• Late ischemic³

Prefreeze is described by tissue cooling with accompanying vasoconstriction and ischemia but without ice crystal formation. The freeze–thaw phase is represented by the intracellular or extracellular formation of ice crystals. This can cause protein and lipid derangement, cellular electrolyte shifts, cellular dehydration, cell membrane lysis and cell death. In the vascular stasis phase, blood vessels fluctuate between constriction and dilation, and blood may leak from vessels or coagulate within them. The late ischemic phase results from progressive tissue ischemia and infarction from a cascade of events, including inflammation, vasoconstriction and emboli.⁴

Classifications of Frostbite

Frostbite is also classified into four degrees of injury; these follow the classification schemes for thermal burn injuries. Early stages of frostbite are differentiated from frostnip, which is a superficial nonfreezing cold injury associated with intense vasoconstriction on exposed skin. Frostnip may, however, precede frostbite.^5,6

• First-degree frostbite causes numbness and erythema. A white or yellow, firm, and slightly raised plaque develops in the area of injury. There may be slight epidermal sloughing and mild edema is common.
• Second-degree frostbite injury causes superficial skin vesiculation. A clear or milky fluid will be present in superficial blisters surrounded by erythema and edema.
• Third-degree frostbite causes deeper hemorrhagic blisters, indicating that the injury has extended into the reticular dermis and beneath the dermal vascular plexus.
• Fourth-degree frostbite extends completely through the dermis and involves the comparatively avascular subcutaneous tissues, with necrosis extending into muscle and bone.³

A variation favored by McIntosh, et. al., involves a 2-tier classification model:

• Superficial—Minimal anticipated tissue loss, corresponding to first- and second-degree injury.
• Deep—Anticipated tissue loss corresponding to third- and fourth-degree injury.³

The authors note that the severity of frostbite may vary within a single extremity.

Prevention of Frostbite in Nyala

Most of the literature suggests that prevention is a far better methodology than treatment for frostbite, which is usually preventable but often not improved by treatment. Underlying medical problems and the chemical immobilization event itself can increase risk of frostbite in nyala, so prevention must address both health-related and environmental areas. Frostbite injury usually occurs when tissue heat loss exceeds the ability of local tissue perfusion to prevent freezing of soft tissues; thus, the team in the field must ensure adequate perfusion and minimize heat loss to prevent frostbite.⁵

Preventive measures to ensure local tissue perfusion include:

• Maintaining adequate core temperature
• Maintaining adequate body hydration
• Minimizing the effects of any known diseases that might decrease perfusion
• Covering the body and head to insulate from the cold
• Minimizing any blood flow restriction
• Using supplemental oxygen in severely hypoxic conditions^3,6

Measures should also be taken to minimize exposure of the animal’s tissues to cold, such as:

• Avoid environmental conditions that predispose to frostbite if possible (e.g., below -15°C, even with low wind speeds
• Protecting exposed skin from moisture, wind, and cold
• Avoiding perspiration or wet extremities
• Increasing insulation and skin protection
• Using chemical and/or electric warmers to maintain peripheral warmth (These should be close to body temperature before being activated and must not be placed directly against skin or constrict flow)
• Regularly checking the animal’s temperature
• Recognizing frostnip or superficial frostbite before it becomes more serious
• Minimizing duration of cold exposure^3,7

Treatment of Frostbite in Nyala

It is not known how long an animal’s extremities can remain numb before developing frostbite. Thus, any extremity at risk for frostbite (indicated by pale color) should be warmed.³ If part of a nyala’s extremity is frozen in the field, the frozen tissue should be protected from further damage.^3,4 A decision must be made whether to thaw the tissue. If environmental conditions are such that thawed tissue could refreeze, it is safer to keep the affected part frozen until a thawed state can be maintained.

Hypothermia frequently accompanies frostbite and causes peripheral vasoconstriction that impairs blood flow to the extremities. Mild hypothermia may be treated concurrently with frostbite injury. Moderate and severe hypothermia should be treated effectively before treating frostbite injury.⁶

Frostbite and Hydration

Frostbite injury in nyala can result in vascular stasis, so appropriate hydration and avoidance of hypovolemia are important for frostbite recovery. Intravenous normal saline should be given to maintain normal urine output. IV fluids should optimally be warmed before infusion and infused in small, rapid boluses, as slow infusion can result in fluid cooling and even freezing as it passes through tubing. Fluid administration should be optimized to prevent clinical dehydration.^5,6

Treatment With NSAIDS

Nonsteroidal anti-inflammatory drugs (NSAIDs) block the arachidonic acid pathway and decrease production of prostaglandins and thromboxanes. These can lead to vasoconstriction, dermal ischemia, and further tissue damage.³ No studies have demonstrated that any particular anti-inflammatory agent or dosing is clearly related to outcome, however. One study showed a 23% tissue survival with aspirin vs 0% in the control group.⁶ However, aspirin theoretically blocks production of certain prostaglandins that are beneficial to wound healing.⁷ The authors of this study recommended the use of ibuprofen rather than aspirin.

Treatment With Low Molecular Weight Dextran

Intravenous low molecular weight dextran (LMWD) decreases blood viscosity by preventing red blood cell aggregation and formation of microthrombi and can be given in the field once it has been warmed. In some studies, the extent of tissue necrosis was found to be significantly less than in control subjects when LMWD was used, and was more beneficial if given early.^3,7

The use of LMWD has not been evaluated in combination with other treatments such as thrombolytics. LMWD should be given if the animal is not being considered for other systemic treatments, such as thrombolytic therapy.⁴

¹britannica.com.

²animalia.bio.

³McIntosh, S., et. al. Clinical Practice Guidelines for the Prevention and Treatment of Frostbite: 2019 Update. Wilderness Medical Society Clinical Practice Guidelines, Volume 30, Issue 4, Supplement S19-S32, December 01, 2019.

⁴McIntosh, S.E., et. al. Wilderness Medical Society practice guidelines for the prevention and treatment of frostbite: 2014 update.Wilderness Environ Med. 2014; 25: S43-S54

⁵Mazur P. Causes of injury in frozen and thawed cells. Fed Proc. 1965; 24: S175-S182

⁶Lange K., et. al. The functional pathology of frostbite and the prevention of gangrene in experimental animals and humans.Science. 1945; 102: 151-152.

⁷Cauchy E., et. al. Retrospective study of 70 cases of severe frostbite lesions: a proposed new classification scheme. Wilderness Environ Med. 2001; 12: 248-255.

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