Capture Myopathy in Elk During Capture and Chemical Immobilization

Capture myopathy is a serious condition that can occur in both wild and domestic animals. In most cases, it occurs as the result of muscle damage following extreme exertion, struggle, or stress. This condition is also known as exertional myopathy, overstraining disease and exertional rhabdomyolysis.¹ Capture myopathy most often occurs as a result of capture, transport or chemical immobilization, but it can also be the result of other natural causes of stress, such as in prey animals attempting to avoid or struggling with predator animals.² Capture myopathy is of particular concern in cases when it is a cause of death in wild animals that are handled by humans.

Capture myopathy can occur in any animal under extreme stress, although it is thought that some species may be more predisposed to it than others due to their natural temperament and physiological characteristics.³ Capture myopathy has been most widely studied in ungulates and birds, although it is believed to potentially affect any captured wildlife species. It has also been observed in coyotes, badgers, primates, and many other species.²

Ungulates in North America that have been reported with this condition have included white-tailed deer, mountain goats, bighorn sheep, pronghorn antelope, bison, moose, and elk.^2,6

Elk Background and Biology

The elk (Cervus elaphus canadensis) is the second-largest wild cervid after the moose. The North American elk (Cervus elaphus) is further split by some biologists into six subspecies, two of which are now considered extinct:

• Rocky Mountain (Rocky Mountain West) – these elk have the largest antlers of all subspecies
• Roosevelt’s (Coastal Pacific Northwest) – these are the largest in body size of all subspecies
• Tule (Central California) – these have the smallest body size of all subspecies
• Manitoban (northern Great Plains)
• Merriam’s (Southwest and Mexico) – Extinct
• Eastern (east of the Mississippi) – Extinct⁴

Elk are popular as game animals; its antlers are used commercially and in traditional Eastern medicine. The word “elk” historically had a meaning like “large deer.” In the 17th Century, English-speaking people in North America were familiar with the elk’s relative, the red deer (Cervus elaphus) but not the moose, so they gave the name “elk” to Cervus canadensis, also referred to as “red deer.”⁵ Elk are related to a breed of red deer in Asia, and are still called red deer in these areas. They are believed to have come to North America across the Bering Strait over 100,000 years ago, as did the caribou and other animals. Between 5,000 to 10,000 years ago, they advanced as far as what is now the southwestern U.S., and it is believed that there may have been as many as 10 million elk in North America when Europeans began to settle the continent.⁴

The elk’s native habitat is Eastern Asia and North America, but these animals were once introduced into Central Asia. They are believed to have had a wider habitat range in prehistoric times. Elk typically live in mountain meadows, forests, and forest edges.

Male elk are called bulls and the females are called cows. Male elk have antlers that are made of bone; these grow about an inch a day and can grow over 20 inches in length. As they grow, a layer of velvet covers. This is shed in the spring after the antlers have finished developing. Elk rub against trees and other objects to shed their winter coats in early summer. Like some deer species, they migrate into higher altitudes in the spring and retreat in the fall, creating summer and winter ranges.⁴

Elk are some of the most social cervids, with group sizes during the summer reaching up to 400 individuals.^4,5 During most of the year, bulls and cows segregate themselves into different herds, with female herds being larger and males living in small groups or being solitary. Young bulls often live in female herds or with older, less aggressive bulls.⁵ During the mating season, bulls engage in rutting behavior and compete for females.

Clinical Signs of Capture Myopathy

Capture myopathy can occur naturally when an elk is attempting to avoid predation, but here, capture myopathy will be the result of these animals being captured and/or immobilized with or without chemical means being involved. Elk are adapted to escape from predators, but they are not adapted to struggle for long periods of time in human-constructed restraints.³ When animals overexert themselves (e.g., struggling in a trap) to the extent that physiological imbalances develop and result in severe muscle damage, capture myopathy can result.²

Clinical signs of capture myopathy in elk can vary depending on various conditions and the cause of exertion.² The method of capture and restraint is also a determinant in occurrences of capture myopathy. The available literature states that capture myopathy may result in sudden death, or that clinical signs may develop hours, days, or up to two months following capture.⁴ The clinical signs during early onset include elevated respiratory rate, heart rate, and body temperature.^1,3 Body temperature increases during exertion, with higher temperatures being associated with death due to capture myopathy. The increase in body temperature can be above 42°C.⁴Muscle spasms, stiffness and lameness are also clear signs of capture myopathy. Animals often become recumbent and may stumble. If dark red-colored urine is noted, this is an indication that the animal's muscles are breaking down and that its kidneys have been severely affected.^2-4 Death of the animal usually follows. Upon necropsy, light-colored skeletal and cardiac muscle is indicative of capture myopathy being the cause of death.

Preventing Capture Myopathy in Elk

There is no treatment for capture myopathy. Thus, prevention is the best method of avoiding this condition. Care should be taken in case of handling of animals that tend to be more susceptible to capture myopathy. An anesthetic protocol consisting of good anesthetic agents can aid significantly in preventing capture myopathy in elk. In the case of wild elk (as opposed to a zoo or farm), the remote delivery of anesthetic agents is considered a superior methodology to trapping prior to the anesthetic event.

The team in the field should be thoroughly aware of the risks of capture myopathy and make every effort to prevent its occurrence. Elk should only be captured when necessary, and the negative effects that capture may have on an animal's health should always be considered before beginning a capture or initiating an anesthetic event.⁵ Capture methods that minimize animal stress, struggling and handling time should be utilized.

Appropriate protocols for chemical immobilization may vary depending on the subspecies, so research can be helpful in identifying the ideal capture method. It has been reported that using a combination of Xylazine HCL and Ketamine HCL can decrease the chance of capture myopathy, but this is not a guarantee of avoiding capture myopathy in elk.³