Respiratory Depression in Alpaca During Chemical Immobilization

Today, chemical immobilization—sedation and/or anesthesia—is the chief method of capture for large wildlife species. This is because it has proven to be a safer, superior method to traps, capture with nets and the mass-capture of herds of animals. In zoos, farms, breeding facilities and some free-ranging situations, chemical immobilization is usually carried out from the ground. In some circumstances, wild animals may be located and darted from a helicopter. Any of the above capture methods however, can cause significant stress and trauma to target animals, potentially giving rise to complications.

In most cases in the field, remote drug delivery systems are used for the purpose of chemical immobilization, usually via a dart gun or blowpipe. Drugs are injected by means of a dart syringe which is fired from the dart gun at a distance. Since dart volume can be a limiting factor, immobilizing drugs must be highly potent and concentrated. They must also have a high therapeutic index and wide safety margin since animals cannot be examined and weighed prior to immobilization.

Alpacas and Chemical Immobilization

The alpaca (Vicugña pacos or Lama pacos) is the smallest relative of the camel. Alpacas are lamoids, which represent several species of South American camelids (Camelidae). Lamoids are also referenced as New World camelids. This particular group of animals includes llamas, alpacas, vicuñas, and guanacos. Like camels, these animals are believed to have originated in North America some 40 million years ago, with lamoids migrating to South America and camels migrating east via the Bering Strait, becoming extinct in North America later on.¹

Alpacas are the most limited in range and the most specialized of the four species of lamoids, being adapted to altitudes from 13,000 to 15,500 feet.² It is believed that alpacas were developed through selective breeding more than 6,000 years ago by people in the Andes for their wool. Distinguished from llamas (the largest New World camelids) by their smaller size, they are the smallest of the domesticated lamoids. The weight of an adult alpaca ranges from 120 to 150 lbs, with a height ranging from 2 to 3 feet. Alpacas have slender bodies, small heads, a short tail and large, pointed ears.

The alpaca encompasses two breed types: the huacaya and the suri. Huacayas are the more common type, and account for about 90% of all alpacas.² The two breed types differ primarily in terms of the properties of their fleece. Alpacas are the most widely-used lamoids for fleece production.

The fleece of the alpaca is lightweight, strong, high in insulation value and very resistant to moisture. It has been reported that during the height of the Incan civilization, the wearing of robes made of alpaca fleece was reserved for the nobility and royalty.³ Alpaca wool fibers are hollow, which gives them the ability to insulate very well, and to absorb moisture. As a result, alpaca farming has become a worldwide industry.

Alpacas are pseudo-ruminants; this means that they have a single stomach that is divided into three compartments instead of four, like other ruminants). They produce rumen, chew cud and are able to process food very efficiently. In the field, alpacas graze on grasses and plants. On farms, alpacas will eat grass or hay. They consume approximately two pounds per 125 pounds of body weight daily in hay or fresh pasture. Some farmers feed extra alfalfa to alpacas that are leaner, or those which live in very cold temperatures.³

Risks for Respiratory Depression

The chemical immobilization of alpacas carries the risk of a variety of complications. Since alpaca are sufficiently domesticated and easily handled, the administration of immobilizing agents may often be accomplished by hand. In some cases however, a pole syringe or remote drug delivery will be the preferred method of administration. Even in the case of highly-domesticated animals such as alpacas, animals are often highly-stressed by this process and can run long distances before they are immobilized.

Additionally, most of the drugs used for immobilization have side effects. These drugs not only cause sedation by influencing the central nervous system, but also influence cardiovascular, respiratory and thermoregulatory functions.⁵The most commonly-encountered problems during wildlife immobilization events include respiratory depression, cardiovascular disturbances, bloat, compromised thermoregulation, hypoxia and capture myopathy.^5-8

Potent opioids are often used for the chemical immobilization of alpaca and other wildlife. A chief disadvantage when using these drugs is that they cause clinically significant respiratory depression due to their potent effect on mu-opioid receptors.⁵ Activation of mu-opioid receptors in the respiratory centers of animals depresses neurons that generate the normal respiratory rhythm. At the same time, activation of these receptors activate other receptors in the brain stem, on the aortic arch and carotid bodies, which depresses normal respiratory function.⁶ Other classes of drugs also have the potential to compromise respiratory function (e.g., through causing hypoxia).

Treating Respiratory Depression in Alpaca

There are several approaches that are typically used to mitigate opioid-induced respiratory depression in alpacas undergoing chemical immobilization. Assisted ventilation and oxygen insufflation can combat hypoxia brought on by some immobilizing agents,⁵ while agents such as opioid antagonists or partial antagonists can be used. Unfortunately, the latter also reduce desirable effects, such as the degree of immobilization, sedation and analgesia. Respiration can also be improved via respiratory stimulants which act on non-opioid receptor systems such as potassium channel blockers, ampakines and serotonin receptor agonists.⁷

Oxygen supplementation is recommended during wildlife immobilization and can be combined with a partial opioid reversal agent to better alleviate hypoxia.⁵ Naltrexone may be used to fully reverse opioid-based immobilization after capture, especially if the animal needs to be released back into the field and must be fully alert.

Partial opioid antagonists or mixed agonists/antagonists may be used for the reversal of opioids such as diprenorphine, nalorphine or butorphanol if residual analgesic or sedative effects are required.^6,7 Signs of recovery after naltrexone administration typically consist of increased respiratory depth, followed by ear twitching, eye movement and lifting of the head.⁵

Partial mu-receptor antagonists such as butorphanol can be used to alleviate respiratory depression caused by strong mu-agonistic immobilization drugs.^5,8 Some of these partial antagonists, however, also reduce the immobilization effects of opioids. Potassium channel blockers such as doxapram can also be used to stimulate breathing. Doxapram is widely used as a respiratory stimulant by veterinarians. It has been shown to increase the minute ventilation in large herbivores immobilized with etorphine.⁵

Drug combinations used for darting were not always commercially available as pre-mixed solutions, but many of these can now be purchased from compounding pharmacies as highly-concentrated drug formulations. Many of these are species-specific, more reliable and are less likely to bring about complications such as respiratory depression in alpaca than drugs and combinations used in the past.