Chemical Immobilization for Moose
Moose (Alces alces) is the largest member of the deer family; they are found in the northern regions of Eurasia and North America. While the Eurasian and American moose are very similar, the American moose is somewhat larger and is considered by some to be a separate species, Alces americana. The moose inhabits the coniferous forests of Canada and the northern United States; the Eurasian moose is found from Scandinavia to Eastern Siberia. The largest variety is the Alaska moose, with adult male weighing from 1,000 to 1,800 pounds and standing as much as 7 ½ feet high at the shoulder.1 Moose have a heavy body with humped shoulders and long, relatively slender legs. They have a thick, overhanging muzzle and a short neck. A flap of skin covered with long hair, called a bell, hangs from the throat. Male moose have broad, flattened antlers, with a spread of up to 6 feet. The lifespan of a moose in the wild is roughly 15 to 25 years.
Moose are browsers rather than grazers, eating leaves, twigs, buds, and the bark of some woody plants, as well as lichens, aquatic plants, and some of the taller herbaceous land plants.1 Moose live in small groups during the summer, sometimes forming large herds in the winter. Although moose are generally timid, the males become bolder and even aggressive during the Fall breeding season. During breeding season, males compete for females by fighting with their antlers and hoofs. Female moose typically begin to breed between two and three years of age. Mothers give birth to one, or occasionally two, calves in Spring, with the gestation period lasting 216 to 240 days.2 Female moose are extremely protective of their young.
Habitats and Concerns
In North America, landscape changes are occurring at an unprecedented pace, and together with levels of wildlife harvesting, humans are having a large effect on moose populations. The clearing of northern forests for settlement and agriculture have led to the range of the white tailed deer expanding northward. Where their ranges overlap, moose may become infected by parasites carried by the deer such as brain worm (Parelaphostrongylus tenuis) and winter ticks (Dermacentor albipictus), which can be fatal to moose. Moose also visit residential areas in search of food, and motorists occasionally collide with them; consequently, hundreds of moose calves are orphaned every year due to the death of their mothers.1 Thus, the management of moose populations has become a concern amongst conservationists and wildlife managers.
Moose are chemically immobilized for various research and management purposes: radio transmitter deployment, collection of biological materials, morphometry, health examination, and translocation. Most moose are approached with a helicopter or occasionally by snowmobile, all-terrain vehicle, car, boat, or on foot, and drugs are administered by projectile darts fired from a dart gun. The first chemical immobilization of free-ranging moose was carried out in Alaska in1957-58 with nicotine as a neuromuscularblocking (NMB) agent.3 Since that time, a broad range of drugs and drug combinations have been used to capture moose in North America and Europe, including other NMB agents, tranquilizers, sedatives, and anesthetics. Helicopter net-gunning has been successfully used on moose, with an immediate capture mortality rate of less than 1%, but mortality rates as high as 14% have been reported from other projects using this method.4
Drugs for the Immobilization of Moose
There are three major groups of drugs currently used for wildlife capture: α-2 adrenoceptor agonists, opioid agonists, and cyclohexanes. According to Kreeger, et al., the NMB agents are “a fourth group that was extensively used during the pioneer days of chemical immobilization.”6 These drugs cause muscular paralysis, leaving the animal conscious, aware of its surroundings and fully sensory, and with the ability to feel pain and experience psychogenic stress. Due to the narrow range between effective immobilizing doses and lethal doses, mortality rates as high as 70% have occurred with NMB agents.6 Although inferior to modern immobilizing drugs, the NMB agent succinylcholine has been widely used for moose capture in recent years, with a mortality rate due to respiratory paralysis of 7%, but with only 63% of the immobilization attempts being successful.5 Due to animal welfare considerations and the low therapeutic index, succinylcholine and other NMB agents are no longer recommended for moose immobilization.
Today, potent opioids such as etorphine, carfentanil and thiafentanil are generally considered the drugs of choice for immobilization of free-ranging moose,3 although many wildlife managers and veterinarians opt for the more versatile drug combinations including α-2 adrenoceptor agonists, opioid agonists, and cyclohexanes.
α -2 adrenoceptor agonists include xylazine, romifidine, detomidine, and medetomidine. These agents induce dose-dependent sedation and analgesia and they possess anxiolytic and muscle relaxing properties.
Opioid agonists used for wildlife immobilization include carfentanil, etorphine, fentanyl, thiafentanil and sufentanil.6 In moose and other cervids, carfentanil and etorphine have been the most widely-used opioids, either alone or in combination with xylazine. Carfentanil and etorphine both have high therapeutic indices in moose. The effects of opioids can be reversed by several opioid antagonists, with naltrexone being the preferred agent due to its potency and long duration. Other opioid antagonists include naloxone, nalmefene, nalbuphine, and diprenorphine.6,7
Cyclohexanes (also known as NMDA antagonists) include ketamine and tiletamine. These drugs are general anesthetics however, due to severe side effects such as muscle rigidity, frequent convulsions, and rough recoveries, they should only be used in combination with an α-2 agonist or another tranquilizing or sedative drug.6
In general, it is recommended that antagonists should be administered intramuscularly, since intravenous injection of reversal agents will cause complete recovery in less than one minute in animals immobilized with opioids alone.5 Such rapid recoveries may be stressful to the animals and could jeopardize the safety of both animals and handlers.
In Chemical Immobilization of Free-Ranging Moose (Arnemo, et. al., Alces, 2003), the authors offer the following:
“For routine immobilization of free-ranging moose, we recommend carfentanil at 0.01 mg/kg or etorphine at 7.5 mg total dose per adult. At these doses most animals are able to maintain sternal recumbency. We do not advocate combining opioids with xylazine or other sedative drugs because this will often induce lateral recumbency and thereby increase the risk for tympany, regurgitation, and aspiration of rumen contents. Carfentanil and etorphine have a wide safety margin in moose and the risk of severe anesthetic side effects during immobilization is minimal. Medetomidine-ketamine is a useful non-opioid alternative. Neuromuscular blocking agents should never be used in moose.”5
Handbook of Wildlife Chemical Immobilization (Arnemo & Kreeger, 2018) recommends etorphine at 7.5 mg total dose for adult moose, and 3.75 mg total dose for calves. The authors recommend repeating the full dose if the animal is not down within 15 minutes. This is reversed with diprenorphine at 15 mg total dose. Alternately, thiafentanil may be used at 0.03 mg/kg, and reversed with 0.6 mg/kg of naltrexone.6
1Ofstad, E., et. al. Opposing fitness consequences of habitat use in a harvested moose population. Journal of Animal Ecology, 27 March 2020.
2Evans, A., et. al. Physiological evaluation of free-ranging moose (Alces alces) immobilized with etorphine-xylazine-acepromazine in Northern Sweden. Acta Veterinaria Scandinavica Volume 54, Article 77 (2012).
3Rausch, R.A., Ritcey, R.W. (1961) Narcosis of moose with nicotine. Journal of Wildlife Management 25:326-328.
4Carpenter, L.H., Innes, J. I. (1995) Helicopter netgunning: a successful moose capture technique. Alces 31:181-184.
5Arnemo, J., Kreeger, T., Soveri, T. Chemical Immobilization of Free-Ranging Moose. Alces Vol. 39: 243-253 (2003).
6Arnemo, J., Kreeger, T. (2018). Handbook of Wildlife Chemical Immobilization 5th Ed.
7Nielsen L. Chemical immobilization of wild and exotic animals. Iowa: Iowa State University Press, 1999; 227-281.
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