Mammal Chemical Immobilization—Inhalant Anesthesia for Mink
Mink are members of the weasel family and are native to the Northern Hemisphere. Historically, both the American mink (Neovison vison) and the European mink (Mustela lutreola) have been valued for their fur. The American mink is a staple of the fur industry and has been raised in captivity throughout the world for many decades. In the wild, mink are rarely seen due to their nocturnal habits, living in close proximity to water.
Both the American and European mink measure about 12–20 inches (30–50 cm) in length, not including a long tail, and weigh around 4.5 pounds (2 kg), with females being smaller. Like weasels, mink have short legs, a long, thick neck, and a broad head with short, rounded ears.1 Their coats are a deep brown.
Mink Habits and Habitats
Mink are semiaquatic, obtaining most of their food from near the water’s edge. Carnivorous animals, mink eat frogs, salamanders, fish, crayfish, muskrats, mice, voles, aquatic birds and their eggs.1 Occasionally, mink will prey upon larger animals, such as rabbits. Mink are strong, agile swimmers and can dive to investigate underwater areas when hunting prey.
Mink are solitary animals except during the mating season in spring. Both males and females may mate with several individuals with the females raising the young alone. The gestation period of the American mink lasts for approximately 50 days; this can vary however, as implantation of fertilized eggs can be delayed for 1–14 days. The litter size of mink ranges from two to eight. Young mink are independent after six months.
Originally, the American mink was found throughout North America, except in the more arid regions. Today the American mink inhabits many areas of Europe, Scandinavia, Russia and South America.1 Unfortunately, the American mink has become a problem in areas where it has been introduced into the European mink’s habitat. Since the latter is less aggressive and less adaptable, it is now rare or threatened in many parts of Europe where it was once abundant.
American mink are raised in captivity for fur. Typically, these animals are bred during early spring, with the harvest of pelts occurring when the animals reach adult size, usually when they are six to eight months old.
Mink Chemical Immobilization
In most cases, the chemical immobilization of mink is carried out in the setting of fur farms, and less frequently in research. Very recently however, the response of wild and captive mink populations to the COVID-19 virus has elicited new waves of research into this phenomenon as researchers study the worldwide coronavirus pandemic.3
When inhalant anesthesia is used in mink, the process will be a function of balanced anesthesia. In general, balanced anesthesia is a term used for anesthetic protocols that are employed in order to minimize patient risk and maximize patient comfort and safety, and typically involve the use of two or more drugs and/or drug formulations to bring about sedation prior to inhalant anesthetics being used. The objectives of balanced anesthesia are to calm the animal, minimize pain, and reduce the potential for adverse effects associated with procedures and the effects of anesthetic drugs. Calming small animals such as mink is important to allow for ease in handling and to decrease stress, which can bring about tachycardia, tachypnoea, hypertension, and other adverse effects,2 all of which can all be detrimental to the anesthetized animal.
For small wild animal species, sedatives (e.g., medetomidine) and tranquilizers (e.g., acepromazine, diazepam, midazolam) are often part of the premedication protocol. It should be noted that while the terms tranquilization and sedation are often used interchangeably, tranquilizers and sedatives are two different classes of drugs. Tranquilization and sedation carry dose-dependent effects of drugs in both classes.2 Low-dose acepromazine can be used to tranquilize or calm a nervous patient, whereas a higher dose can be used to sedate a patient for a minor procedure. Acepromazine has antiemetic, antihistamine, and antidysrhythmic properties.4
Medetomidine is an α-2 agonist that has sedative, analgesic, and muscle-relaxant properties.4 The advantages of this drug are that it decreases the dose requirements of other anesthetic agents, has excellent analgesic properties, and is reliably reversible by atipamezole. It can also be titrated from light to heavy sedation because it is dose dependent. Medetomidine should be used for premedication only after its advantages and disadvantages are carefully weighed.
Opioids (e.g., morphine, oxymorphone, buprenorphine, butorphanol and hydromorphone) are also frequently used in the chemical immobilization of small mammals. When selecting an opioid, the drug's onset and duration of action should always be considered. Buprenorphine has a long duration of analgesic action similar to, but with far less respiratory and cardiovascular depression than pure mu agonists.4 Butorphanol, which is neither a pure mu agonist nor a partial mu agonist, is an agonist-antagonist and displays agonist activity at the k receptors and antagonizes the pure mu agonists.2
In order to reach the maximum effect of anesthesia in the perioperative phase in mink patients, neuroleptanalgesia should be achieved in the premedication phase.5 The combination of an opioid and a major tranquilizer provides more sedation and analgesia than either drug would if used alone and provides mild to marked effects.
Induction and Maintenance of Anesthesia
When premedication has reached its full effect, the induction phase, provides a smooth transition to general anesthesia in the mink. The goal of this phase is to quickly anesthetize the patient for procedures. Here, maintenance of a safe and consistent level of anesthesia, can be achieved with an inhalant anesthetic agent (e.g., isoflurane, sevoflurane, halothane). Although isoflurane and sevoflurane are used during mask inductions, sevoflurane does not irritate the airways and has little or no noxious odor.4,6 Isoflurane is the most commonly used inhalant anesthetic in most countries; sevoflurane and desflurane produce a more rapid onset/offset of anesthetic effects compared to isoflurane because they are less soluble.
All inhalant anesthetics cause dose-dependent cardiovascular and respiratory depression in all animals. In comparisons isoflurane and sevoflurane, cardiopulmonary effects in small animals such as mink are distinguishable.5 Isoflurane has a higher potency and a lower minimum alveolar concentration value compared with sevoflurane.6 These attributes of sevoflurane help induce anesthesia in certain species (e.g., rabbits) that tend to hold their breath.4 Halothane produces more cardiovascular depression, is more likely to produce arrhythmias, and undergoes a greater percentage of hepatic metabolism than isoflurane, sevoflurane, or desflurane.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.
The final phase of the anesthetic event includes recovery and postoperative analgesia. Here, the goal is to provide the mink with continued analgesia and a recovery that is as calm and quiet as possible.
2Quandt J, Trim C: Lecture notes. Course 530A. Anesthesiology. Unpublished. Athens, GA, University of Georgia Veterinary Teaching Hospital, September 2002.
4Plumb’s Veterinary Drugs.
5Arnemo, Jon & Kreeger, Terry. (2018). Handbook of Wildlife Chemical Immobilization 5th Ed. Sunquest Publishing, 2007, 432 pages.
6Arnemo, Jon & Soli, Ema. (1992). Immobilization of mink (Mustela vison) with medetomidine-ketamine and remobilization with atipamezole. Veterinary research communications.
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