Mammal Chemical Immobilization—Inhalant Anesthesia for Mustelids
The family Mustelidae comprises stoats, polecats, mink, fishers, wolverines, weasels, martens, badgers, and otters, and is the largest family within the order Carnivora. Mustelids have a global distribution, with members found on every continent except Australia and Antarctica. Mustelids are free-ranging, cover terrestrial and marine environments, are kept as pets, farmed, used as laboratory animals, and kept in zoos. Mustelids contain around 55–60 species across eight subfamilies. Skunks were once members of this family but have been recategorized into the family Mephitidae.1
Mustelids have existed in North America since the early Oligocene period.2 There are currently 11 species of mustelids that are native to North America:
- American Badger
- American Marten
- American Mink
- Black-footed Ferret
- Ermine
- Fisher
- Least Weasel
- Long-tailed Weasel
- River Otter
- Sea Otter
- Wolverine
In Europe, mustelid species also include the Stoat, the European polecat, the Eurasian badger, the Eurasian otter and the European pine marten. Mustelids have become particularly successful in North America, with taxa ranging across the entire continent. However, the fossil record suggests that mustelids arose in Europe, where several fossil taxa dating to the Middle Oligocene have been uncovered.2
A diverse group of animals, there are several characteristics that are common to all in the family Mustelidae. In general, male members are larger than females. Their bodies are usually generally long and thin, although wolverines and badgers are stockier. Most species have short ears and limbs. Mustelids are diurnal or nocturnal and live in crevices and burrows. Many have the ability to sit up to look around. Mustelids have anal glands that secrete in a defensive measure.3 As carnivores, mustelids prey upon other animals. Some can feed on plants, and some are omnivorous. They have keen eyesight and hearing, but generally hunt by smell.
Mustelids are mostly solitary except for Eurasian badgers, sea otters, and some northern river otters. In the more solitary species, association between males and females during the mating season is brief. Mating occurs mostly in the spring, and in many species ovulation is induced during copulation. Delayed implantation of the fertilized egg occurs in many mustelids. Females raise the young alone. Only the least weasel produces two litters yearly; other species produce annually. In most mustelids, the young become sexually mature at about 10 months of age.3
Evaluating Mustelid Species for Sedation and Anesthesia
Given their ubiquitous nature, anesthetizing mustelid species is often necessary in wildlife veterinary medicine. Certain species, such as ferrets are also popular pets, and many mink are raised worldwide for their fur. Research and wildlife management also account for the necessity of chemical immobilization of individual mustelids.
Special challenges exist when anesthetizing smaller mammals such as mustelids. First off, the risk for anesthesia-related death in small mammals is higher than in larger animals, even companion animals such as dogs and cats. The attendant stress can bring about tachycardia, tachypnoea, hypertension, and other adverse effects, some of which can lead to death if too severe. Further, many smaller mammals cannot be intubated during general anesthesia, including most mustelid species.
As in other animals, a thorough history and physical examination are important whenever possible. Knowledge of the animal’s diet and feeding frequency can provide information about nutritional status, and cursory observation by a veterinarian’s trained eye can tell a great deal about an individual animal’s general health. Additional information regarding diet, socialization and temperament may also be relevant. Obtaining an accurate weight is essential for appropriate drug administration.
Due to the unique anatomy and physiology of small animals, preoperative fasting times vary and are generally much shorter than in companion animals. “For example, guinea pigs— which have limited reserves and a high metabolic rate—may be fasted for 2 hours or less; however, the pharyngeal pouches may need to be cleaned out prior to anesthesia. Rabbits do not vomit, but fasting up to 4 hours may help limit stomach volume and lower GI tract fermentation to enable easier breathing during anesthesia. Clinicians should be aware that prolonged fasting in small mammals can increase the risk for ileus and hypoglycemia. Ferrets may regurgitate during anesthesia, so longer fasting times of up to 6 hours in otherwise healthy animals are recommended.”4 In the field however, preoperative fasting may not always be possible, which can necessarily affect which anesthetic protocol is used.
Anesthetic Protocols for Mustelids
When dealing with smaller mammal species such as mustelids, the use of insulin and tuberculin syringes can help deliver drugs at appropriate doses.4 Dilution of some drugs may be necessary. To achieve sedation appropriate for the patient, subcutaneous, intramuscular or intravenous premedication can be implemented and can reduce patient stress during subsequent manipulations, such as placement of an IV catheter or monitoring equipment.
Although the drugs used for sedation in small mammals are similar to those used in other species, doses can vary widely due to pharmacokinetic factors, and drug effects can also vary with sex and strain in some species. Sedative and tranquilizer drugs commonly used in mustelids include α-2 agonists (e.g., xylazine, dexmedetomidine), acepromazine, and midazolam. Opioids are commonly combined with sedative and tranquilizing drugs to provide periprocedural analgesia, although clinicians often develop a drug preference based on their experience related to efficacy and adverse effects. Drug formulations offered by custom compounding pharmacies are being increasingly used for these applications since they are specially-formulated for many small mammal species.
Because of their large surface area-to-body-weight ratio, small mammals tend to lose body heat rapidly after receiving sedatives and anesthetics.4 The literature recommends that to minimize energy use and ensure a timely recovery, external heat support should be maintained from premedication into recovery, and body temperature should be monitored and maintained at a range appropriate for the species. Heating pads, forced air units, heat lamps, and warmed breathing circuits may be used to facilitate patient warming. The placement of anesthetic monitoring devices following sedation is recommended.
Inhalant Anesthetics for Mustelids
Maintenance of a safe and consistent level of anesthesia, can be achieved in mustelids with an inhalant anesthetic agent (e.g., isoflurane, sevoflurane, halothane). Isoflurane is the most commonly used inhalant anesthetic however, sevoflurane and desflurane are known to produce a more rapid onset/offset of anesthetic effects compared to isoflurane because of their relative insolubility. All inhalant anesthetics cause dose-dependent cardiovascular and respiratory depression in all animals. Isoflurane has a higher potency and a lower minimum alveolar concentration value compared with sevoflurane.5
Nitrous oxide is an inhaled gas that is categorized as an inhalant anesthetic, but which lacks the potency to produce general anesthesia in animals when used alone. The choice of tranquilizers, sedatives and inhalant anesthetic agents are generally predicated upon the preference and experience of the attending veterinarian.
As with many species, anesthetic hypotension may be observed in mustelids and is further influenced by sedative and anesthetic medications.4 Management includes decreasing the anesthetic dose and providing an appropriate fluid bolus. Emergency medications (e.g., epinephrine) should be kept readily available to facilitate rapid intervention when needed. Come clinicians prefer to have these additional emergency medications drawn up along with reversal agents for sedative/anesthetic drugs when available.5
1Williams, B., et. al. Mustelids. Pathology of Wildlife and Zoo Animals, 2018.
2Paterson, R., et. al. The earliest mustelid in North America. Zoological Journal of the Linnean Society, Volume 188, Issue 4, April 2020, Pages 1318–1339.
5Arnemo, Jon & Soli, Ema. (1992). Immobilization of mink (Mustela vison) with medetomidine-ketamine and remobilization with atipamezole. Veterinary research communications.
About NexGen Pharmaceuticals
NexGen Pharmaceuticals is an industry-leading veterinary compounding pharmacy, offering sterile and non-sterile compounding services nationwide. Unlike other veterinary compounding pharmacies, NexGen focuses on drugs that are difficult to find or are no longer available due to manufacturer discontinuance or have yet to be offered commercially for veterinary applications, but which still serve a critical need for our customers. We also specialize in wildlife pharmaceuticals, including sedatives and their antagonists, offering many unique options to serve a wide array of zoo animal and wildlife immobilization and anesthesia requirements.
Our pharmacists are also encouraged to develop strong working relationships with our veterinarians in order to better care for veterinary patients. Such relationships foster an ever-increasing knowledge base upon which pharmacists and veterinarians can draw, making both significantly more effective in their professional roles.
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