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Euthanasia of Food Animals
Anesthesia
0749-0720/86 $00.00 + $.20
Euthanasia of Food Animals John C. Thurmon, D.V.M., M.S.*
This article has been extracted, in major part, from the 1986 report of the AVMA Panel on Euthanasia. Panel members include A. W. Smith, D.V.M., Ph.D. (Chairman), Oregon State University College of Veterinary Medicine, Corvallis, Oregon; K. A. Houpt, V.M.D., Ph.D., New York State College of Veterinary Medicine, Cornell University, Ithaca, New York; R. L. Kitchell, D.V.M., Ph.D., University of California School of Veterinary Medicine, Davis, California; D. F. Kohn, D.V.M., Ph.D., University of Texas Medical School at Houston, Houston, Texas; L. E. McDonald, D.V.M., Ph.D., University of Georgia College of Veterinary Medicine, Athens, Georgia; M. Passaglia, Jr., M.S., former Executive Director, The American Humane Association, 1979-1985; J. C. Thurmon, D.V.M., M.S., University of Illinois College of Veterinary Medicine, Urbana, Illinois; and E. R. Ames, D.V.M., Ph.D., Staff Coordinator, American Veterinary Medical Association, Schaumburg, Illinois. For a thorough coverage of the subject on euthanasia, the reader is encouraged to review the panel's full report in J. Am. Vet. Med. Assoc., 188:252-268, 1986. Euthanasia is the act of inducing rapid, painless death. The criterion to be considered for a painless death is rapidly occurring unconsciousness followed by cardiac or respiratory arrest. For pain to be experienced, the cerebral cortex and subcortical structures must be functional. An unconscious animal can not experience pain because the cerebral cortex is not functioning. If the cerebral cortex is rendered nonfunctional by any means, such as hypoxia, depression by drugs, electric shock, or concussion, pain is not experienced. Stimuli that might evoke pain in a conscious animal may elicit only reflex responses manifested by movement in an unconscious animal. For this reason, nonpurposeful movements of an animal are not reliable indicators of pain perception. Conversely, an animal can experience pain although no body movements occur in response to nox-
* Diplomate,
American College of Veterinary Anesthesiologists; Professor, and Head of AnestheSiology Section, Department of Veterinary Clinical Medicine, University of Illinois College of Veterinary Medicine, Urbana, Illinois
Veterinary Clinics of North America: Food Animal Practice-Vol. 2, No, 3, November 1986
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ious stimuli if the animal is given muscle-paralyzing agents such as curare, succinylcholine, gallamine, pancuronium, nicotine, or decamethonium. These muscle-paralyzing agents do not induce unconsciousness or depress the cerebral cortex or any neural mechanism involved in pain perception. As with other procedures applied to animals, euthanasia requires some physical control over the animal. The degree of control and kind of restraint needed are determined by the animal species, breed, size, state of domestication, presence of painful injury or disease, degree of excitement, and method of euthanasia. Suitable control is vital to minimize pain in animals, to assure safety of the person performing euthanasia, and frequently to protect other animals and people. Selection of the most appropriate method of euthanasia in any given situation depends on the species of animal involved, available means of animal control, skill of personnel, numbers of animals, economic factors, and other considerations.
MODES OF ACTION OF EUTHANATIZING AGENTS Euthanatizing agents terminate life by three basic mechanisms: (1) hypoxemia, direct or indirect; (2) direct depression of neurons vital for life function; and (3) physical damage to brain tissue. Agents that produce death by direct or indirect hypoxia can act at various sites and can cause different times of onset of unconsciousness. With some agents, unconsciousness may occur before cessation of motor activity (muscle movement). Thus, although animals demonstrate muscular contractions, they are not perceiving pain. The uninformed observer may find this difficult to accept. Conversely, muscle relaxants induce a flaccid muscular paralysis and the animal remains conscious until death eventually occurs as a result of hypoxemia and hypercapnia. Although the animal appears relaxed, it is actually in a state of panic and can feel pain. Outwardly this appears to be an ideal form of euthanasia, but it is not. Agents that do not induce rapid loss of consciousness prior to death include curare, succinylcholine, gallamine, nicotine, magnesium sulfate, or potassium chloride, pancuronium, decamethonium, and strychnine.
Use of any of these agents alone for euthanasia is absolutely condemned. The second group of euthanatizing agents depresses nerve cells of the brain first, blocking apprehension and pain perception, and then causes unconsciousness and death. Some of these agents "release" muscle control during the first stage of anesthesia, resulting in a socalled excitement or delirium phase, during which there may be vocalization and some muscle contraction. 3 Although these responses appear to be purposeful, they are not. Death is due to hypoxemia and direct depression of respiratory centers. With the third group, physical damage to the brain, concussion, or direct electrical current flOW 42 through the brain produces instant
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unconsciousness. Exaggerated muscular activity may follow unconsciousness. When electrocution is properly performed, loss of motor function occurs concomitantly with loss of consciousness; with the other methods in this group, muscular contraction may occur. Although this may be unpleasant to the observer, the animal is not suffering. Criteria for Judging Methods of Euthanasia Several criteria are used in evaluating methods of euthanasia: (1) ability to produce death without causing pain, (2) time required to produce loss of consciousness, (3) time required to produce death, (4) reliability, (5) safety of personnel, (6) potential for minimizing undesirable psychologic stress on the animal, (7) nonreversibility, (8) compatibility with requirement and purpose, (9) emotional effect on observers or operators, (10) economic feasibility, (11) compatibility with histopathologic evaluation, and (12) drug availability and human abuse potential.
BEHAVIORAL CONSIDERATIONS The facial expressions and body postures that indicate various emotional states have been described. 18,23 Behavioral and physiologic responses to noxious stimulation include distress vocalization, struggling, attempts to escape, defensive or redirected aggression, salivation, urination, defecation, pupillary dilatation, tachycardia, sweating, and reflex skeletal muscle contractions causing shivering, tremors, or other muscular spasms. Some of these responses can occur in unconscious as well as conscious animals. Fear can cause immobility or freezing in certain species, particularly rabbits and chickens. This immobility response should not be interpreted as unconsciousness when the animal is in fact conscious. 23 In very young animals, autonomic and reflexive reactions are evident, although overt behavioral reactions may differ from those of adults. The need to minimize fear and apprehension must be considered in determining the method of euthanasia. Distress vocalizations, fearful behavior, and release of certain odors or pheromones by a frightened animal may cause anxiety and apprehension in others. Therefore, whenever possible, animals should not observe euthanasia of one another, especially those of their own species. This is particularly important when vocalization or release of pheromones may occur during induction of unconsciousness. Gentle restraint, preferably in a familiar environment, and careful handling during euthanasia often have a calming effect on companion animals. However, some of these methods may not be operative with wild animals or animals that are injured or diseased. When capture or restraint may cause pain, injury, or anxiety to the animal or danger to the operator, the use of tranquilizing or immobilizing drugs (phase I) may be necessary before the euthanatizing agent is given (phase II).
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Inhalant Agents The inhalant anesthetics (for example, ether, halothane, methoxyflurane) are used only infrequently for euthanasia of food-producing animals. On the other hand, carbon dioxide (C0 2 ) has been used more frequently. Inhalation of 60 per cent CO 2 results in loss of consciousness within 45 seconds and respiratory arrest within 5 minutes. 17 Carbon dioxide has been used to euthanatize pigs, chickens, and rabbits 3 , 7, 24, 26, 27 and for humane slaughter of swine for human consumption. 22 , 25 According to Croft, animals do not detect the CO 2 immediately, and its depressant action takes place almost unnoticed. 10 Studies in day-old chickens have shown that CO 2 is an effective euthanatizing agent. Inhalation of CO 2 caused little distress to the birds. It suppressed nervous activity and induced death rather quickly. Because respiration begins during embryonic development, the unhatched chicken's environment may normally have a CO 2 concentration as high as 14 per cent. Thus, CO 2 concentration for euthanasia for baby chickens and other neonates should be especially high. A CO 2 concentration of 60 to 70 per cent with a 5-minute exposure time appears to be optimal. 26 Carbon dioxide is used for preslaughter anesthesia of pigs. The undesirable side effect of CO 2 , as used in commercial slaughterhouses, is that mature swine experience a stage of excitement with vocalization for about 40 seconds before they lose consciousness. 22 ,29 For that reason, CO 2 preslaughter anesthesia may appear less humane than other techniques. The signs of effective CO 2 anesthesia are those associated with deep surgical anesthesia, such as loss of withdrawl and palpebral reflexes. 3 Advantages. (1) The rapid depressant and anesthetic qualities of CO 2 are well established; (2) CO 2 may be purchased in cylinders or in a solid state as "dry ice"; (3) CO 2 is inexpensive, nonflammable, and nonexplosive and presents minimal hazard to personnel when used with properly designed equipment; (4) CO 2 does not result in accumulation of tissue residues in food-producing animals; and (5) CO 2 euthanasia does not distort cellular architecture. 14 Disadvantages. (1) Because CO 2 is heavier than air, incomplete filling of a chamber may permit a tall animal to avoid exposure and survive, and (2) in immature animals, the time required for euthanasia may be substantially prolonged. Recommendations. Carbon dioxide is recommended for piglets, birds, and rabbits. Chamber design should allow for precharging with CO 2 and should enable cleaning and removal of dead animals with minimal loss of CO2 • Compressed CO 2 gas in cylinders is preferable to dry ice. Inflow to a euthanatizing chamber can be regulated precisely with compressed CO 2 • The optimal flow rate appears to be one that will displace approximately 20 per cent of chamber volume per minute. 2o When using compressed CO 2 , oxygen can be added (for example, 30 percent O 2 , 70 per cent CO 2 ), decreasing the discomfort of hypoxia prior to onset of narcosis and anesthesia. If dry ice is used, animal contact must be avoided to prevent freezing or chilling.
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N oninhalant Pharmacologic Agents Noninhalant agents that can be used for euthanasia are widely diverse in chemical composition. 19 , 31 Although death can be induced by administering these drugs via many routes (intravenous, intracardiac, intraperitoneal, intrathecal, intramuscular, intrathoracic, subcutaneous, and oral), intravenous administration is preferred because the effect is most rapid and reliable. Intrapulmonic injection should be avoided. Oral, rectal, and intraperitoneal routes of administration of drugs for euthanasia are inadvisable because of prolonged onset of action, wide range in lethal doses, and potential irritation of tissues. An hour or more may elapse from the time of administration until death. Some drugs, such as chloral hydrate and T-61, when given intraperitoneally, are irritating and cause abdominal pain. Others may produce tissue changes, depending on the dose and route of administration. 14, 40 Because crying and struggling may follow improper intracardiac injection, this route of administration is objectionable. Skill is required to penetrate the heart of an animal with one thrust of a hypodermic needle, especially if the animal is not properly restrained. Intracardiac injection of drugs is not recommended for euthanasia, except in depressed, anesthetized, or comatose animals. Intrathecal use of drugs in unanesthetized animals is not recommended because puncture of the cisterna magna without causing pain and struggling is not possible. If the animal to be euthanatized is excitable or vicious, use of analgesics, tranquilizers,12 narcotics, ketamine, xylazine, or other depressants for phase I of euthanasia is recommended before administration of the euthanatizing agent, phase II.
BARBITURIC ACID DERIVATIVES Barbiturates depress the central nervous system in descending order, beginning with the cerebral cortex. Within seconds of intravenous administration, unconsciousness is induced and it progresses to deep anesthesia. 35 Apnea occurs owing to depression of the respiratory center, and cardiac arrest quickly follows. Several barbiturates are acceptable, but pentobarbital sodium is most commonly used. Advantages. (1) A primary advantage of barbiturates is speed of action. This effect depends on the dose, concentration, and rate of injection; (2) the barbiturates induce euthanasia smoothly, with minimal discomfort to the animal, and favorably impress the observer because the animal dies quietly. A cost comparison by Lumb and Moreland34 (Table 1) indicates that barbiturates are less expensive than most other injectable agents. Disadvantages. (1) Intravenous injection is necessary for best results, necessitating trained personnel; (2) each animal must be restrained; (3) current federal drug regulations require strict accounting of the barbiturates and, by necessity, these must be used under the
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Relative Cost of Injectable Euthanasia Products* APPROXIMAL PENTOBARBITAL
NAME
MANUFACTURER
Euthanasia solution
Prepared in houset
Sleepaway Fatal-Plus
Ft. Dodge North American Pharmacal Somlethol Med-Tech Beuthanasia- Burns-Biotic D Special T-61 National Laboratories
(CRlML)
COST/LB OF COST
AMIMAL
2
Pentobarbital powder, $100/kg
$0.003
4 6
$ 5.95/100 ml $15.95/250 ml
$0.006 $0.006
6 6
$ 7.25/100 ml $28.00/100 ml
$0.007 $0.28
$55.00/250 ml
$0.22
* From Lumb, W. V., and Moreland, A. F.: Chemical methods of euthanasia. Lab. Anim., 11 :29-35, 1982; with permission. t "Homemade" euthanasia solution: Pentobarbital sodium, powder 259.2 Ethyl alcohol, 95% 400.0 Sodium hydroxide solution, 10% 80.0 Hot water, q.s. 2000.0
gm ml ml ml
Give 1 mll5 lb of body weight intravenously, more if necessary. Note: This is twice anesthetic strength.
supervision of personnel registered with the U.S. Drug Enforcement Agency; and (4) an aesthetically objectionable terminal gasp may occur in the unconscious animal. Recommendations. The advantages of using barbiturates for euthanasia outweigh the disadvantages. The intravenous injection of a barbiturate is satisfactory for large and small food animals not intended for human consumption. Nervous or vicious animals may require tranquilization or sedation before injection of a barbiturate.
SECOBARBITAL-DIBUCAINE COMBINATION* This mixture contains a short-acting barbiturate to induce anesthesia. The cardiotoxic effects of dibucaine cause cardiac arrest. Limited trials suggest this combination is an acceptable barbiturate euthanatizing agent when given intravenously to dogs and cats. 21 , 41 It would appear to be acceptable in larger species. * Each milliliter contains 400 mg secobarbital and 25 mg dibucaine. Dosage for dogs and cats is 0.22 mg per kg, intravenously. Repose, Diamond Laboratories, Des Moines, Iowa.
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T-61 T -61 is an injectable nonbarbiturate, nonnarcotic mixture of three drugs used to induce euthanasia. These drugs provide a combination of general anesthetic, curariform, and local anesthetic actions. Death results from severe central nervous system depression, hypoxia, and circulatory collapse. T-61 has been used in Germany since 1962 and became available in the United States soon thereafter. 13, 28, 32, 36 T-61 has been withdrawn from the market in the United Kingdom since 1976, but because it is not currently listed as a controlled substance under federal drug regulations, it has gained some popularity in the United States in recent years. A recent survey39 and review 2 indicate a need for further controlled studies on the efficacy and humaneness of T -61 as a euthanatizing agent in several species. Advantages. (1) The terminal gasp15 that may accompany pentobarbital euthanasia in unconscious animals is not evident with T61, and (2) it is not regulated or controlled by the U.S. Drug Enforcement Agency. Disadvantages. (1) T -61 must be administered intravenously because it is painful when administered extravascularly; (2) if T -61 is injected at too rapid a rate, the animal may appear to experience pain or discomfort immediately before becoming unconscious; (3) doses larger than recommended may cause pulmonary edema and other tissue lesions; and (4) T-61 is not approved for use in animals intended for human consumption. Recommendations. Ifbarbiturates cannot be used, T-61 is a substitute, but only when it is administered intravenously by a highly skilled person at recommended doses and at proper injection rates. 2 Intracardiac or intrathoracic administration is not recommended.
CHLORAL HYDRATE Because chloral hydrate depresses the cerebrum slowly, restraint during induction may become a problem in some animals. Death is due to hypoxemia caused by progressive depression of the respiratory center. Death may be preceded by gasping, muscle spasms, and vocalization. Recommendation. Chloral hydrate can be recommended for euthanasia of large food-producing animals when administered intravenously.
CHLORAL HYDRATE, MAGNESIUM SULFATE, AND SODIUM PENTOBARBITAL This combination has been used for anesthesia of large animals and may be administered in overdose for euthanasia. 33
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Recommendation. This mixture is an acceptable large-animal euthanatizing agent when administered intravenously.
STRYCHNINE Strychnine or any of its salts, such as strychnine sulfate, increases the excitability of the central nervous system. The animal remains conscious and excessively responsive to stimuli. Strychnine causes violent convulsions and associated painful contraction of skeletal muscles. 16
Recommendation. euthanasia.
Strychnine is absolutely condemned for
MAGNESIUM SULFATE (MgS04 ) OR POTASSIUM CHLORIDE (KCI) Magnesium or potassium ions exert little if any direct central nervous system depressant effect. Doses of MgS04 previously recommended for euthanasia have been shown to cause complete neuromuscular block and death due to hypoxemia. 6 Recommendations. Magnesium or potassium salts must not be used alone for euthanasia because of the lack of analgesic or anesthetic effect. Potassium chloride may be administered to an anesthetized animal as an efficient and inexpensive way to cause cardiac arrest and death.
NICOTINE Nicotine is an alkaloid obtained from the tobacco plant. Nicotine sulfate is an odorless, clear, water-soluble liquid. Lethal doses can be absorbed through mucous membranes and intact skin, making it dangerous for personnel. Nicotine sulfate has been used extensively in the past in capture equipment for immobilizing wild game and feral and domestic dogs. Other drugs have replaced nicotine sulfate because of the high mortality associated with its use. Nicotine sulfate induces a short period of stimulation, followed by blockade of autonomic ganglia. When injected in sufficient quantity, it induces a muscle relaxant effect, with paralysis of the respiratory muscles and, subsequently, hypoxemic death. Salivation, vomiting, defecation, and convulsions commonly occur prior to death. Recommendation. Because nicotine sulfate is an extremely dangerous drug for personnel, is inhumane for animals, and induces serious side effects prior to death, it is absolutely condemned for
euthanasia.
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CURARIFORM DRUGS Drugs such as curare, succinylcholine, pancuronium, guaifenesin, and other neuromuscular blocking agents induce death by immobilizing the respiratory muscles, causing suffocation. There is no depressant action on the brain. Human patients given these drugs have described periods of full consciousness accompanied by complete muscular immobility and intense anxiety.33 Recommendations. When standard methods of restraint are impractical, impossible, or dangerous or when manual capture and restraint may cause pain and injury through struggling and anxiety, the use of immobilizng drugs is justified. Because the immobilized animal is fully conscious and subject to death by suffocation, euthanasia by chemical of physical methods must be accomplished immediately.
Curariform drugs alone are absolutely condemned for euthanasia. OTHER PARENTERAL PREPARATIONS A number of injectable agents are currently available for immobilizing domestic and feral animals. Drugs such as ketamine and xylazine are used for immobilization, analgesia, and anesthesia. The effect occurs within minutes following intramuscular injection, making these drugs useful for restraint (that is, phase I of euthanasia). Currently, these agents are not included as controlled substances by federal drug regulations. Narcotics also may be used for sedation and analgesia, but detailed records must be kept and their use properly supervised by a licensed professional registered with the U.S. Drug Enforcement Agency. Although it is possible to cause death with high doses of these drugs, such use would be impractical and may produce convulsions before death.
PHYSICAL METHODS Physical methods employed in food animals include captive bolt pistol, gunshot, stunning, cervical dislocation, exsanguination, and electrocution. It is a requirement that any method of euthanasia be performed by knowledgeable, well-trained individuals. Because trauma often is associated with physical methods for inducing unconsciousness, this requirement is particularly vital. Physical methods are often aesthetically displeasing; however, when properly performed, unconsciousness is rapid, without distress or pain to the animal. In some applications, the amount of apprehension of the animals is less than if chemical agents are employed.
CAPTIVE BOLT PISTOL This method is used in ruminants, horses, and swine and is being developed for laboratory rabbits. The purpose of both the captive bolt
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and nonpenetrative percussion methods is to damage the cerebral hemisphere and brainstem so that the animal is unconscious. 4 Penetrating or nonpenetrating captive bolts are powered by gunpowder or compressed air. N onpenetrating instruments usually are less effective in inducing unconsiousness. Animals must be adequately restrained to ensure proper placement of the captive bolt. The muzzle of the pistol should be placed at a right angle to the skull and directed toward the center of the brain in ruminant species. A multiple projectile has been suggested as a more effective technique, especially on large cattle. In swine, the captive bolt should be directed to the brain from a central point slightly above a line between the eyes. 9 The signs of effective stunning are immediate collapse and a 15second period of tetanic spasm, followed by slow hindlimb movements of increasing frequency. Advantages. A humane method for use in slaughterhouses and in research facilities when the use of drugs is contraindicated. Disadvantages. (1) Aesthetically displeasing; (2) nonpenetrating captive bolt pistols cannot be used effectively in mature swine, and (3) death may not occur. Recommendations. The captive bolt pistol is more appropriate when there is effective restraint of the animal. Use of the penetrating captive bolt pistol, when followed by exsanguination or pithing, is an acceptable method of euthanasia for horses, ruminants, and swine when chemical agents cannot be used. N onpenetrating captive bolt pistols are not recommended.
GUNSHOT Under some circumstances, gunshot may be the only practical method of euthanasia. It should be performed by highly skilled and trained personnel using a rifle or pistol appropriate for the situation. The projectile should be accurately placed to enter the brain, causing instant unconsiousness. 1 Advantages. (1) Euthanasia is instantaneous; and (2) under field conditions, gunshot may be the only effective method available. Disadvantages. (1) It is dangerous to personnel; (2) it is aesthetically unpleasant; and (3) under field conditions, it may be difficult to hit the brain. Recommendation. When other methods cannot be used, competently performed gunshot is an acceptable method of euthanasia.
STUNNING Stunning can render an animal unconscious; however, unconsciousness will occur only if a blow to the head is properly executed. If not performed correctly, various degrees of consciousness with concomitant pain will ensue.
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Advantages. (1) Stunning is humane when properly performed, and (2) it enables collection of blood and other tissues without chemical contamination. Disadvantages. (1) Stunning is inhumane if improperly performed; (2) it is impossible to ensure constancy of performance by personnel; (3) stunning may be aesthetically displeasing for personnel performing or observing the procedure; and (4) it must be followed by other means to ensure the death of the unconscious animal. Recommendations. Stunning of rabbits by a sharp blow to the head as a method of euthanasia is not encouraged because of the inherent risk of not rendering animals immediately unconscious. Stunning must be followed by other means to ensure death of the unconscious animal.
CERVICAL DISLOCATION Cervical dislocation is used to euthanatize poultry and rabbits. For immature rabbits, the head is held in one hand and the hind limbs in the other. The animal is stretched and the neck is hyperextended and dorsally twisted to separate the cervical vertebrae from the skull. 24 Advantages. (1) Cervical dislocation is a technique that may induce immediate unconsciousness; (2) it does not chemically contaminate tissues; and (3) it is rapidly accomplished. Disadvantages. (1) It may be aesthetically displeasing to personnel, and (2) its use is limited to poultry and immature rabbits. Recommendations. When properly executed, cervical dislocation is a humane technique to euthanatize poultry and for rabbits that weigh less then 1 kg. In heavier rabbits, the greater muscle mass in the cervical region makes cervical dislocation physically more difficult and, accordingly, it should not be performed. Because unconsciousness may not occur immediately, it is preferable to anesthetize lightly or sedate the animal before cervical dislocation.
ELECTROCUTION Electrocution as a form of euthanasia or stunning has been employed for years in species such as dogs, cattle, sheep, and hogs. 8 , 11, 19, 30, 37, 38, 42, 43 Experiments in dogs have shown the necessity of directing the electric current through the brain in order to produce instant stunning with loss of consciousness. In the dog, when electricity passes between fore- and hindlimbs or neck and feet, it causes the heart to fibrillate promptly but does not induce unconsciousness. The dog does not lose consciousness for at least 12 seconds after ventricular fibrillation, which causes cerebral hypoxemia. These data dictate against use of methods that direct current through the heart and not directly through the brain. An apparatus that applies electrodes
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to opposite sides of the head, or in another way directs electric current immediately through the brain, is necessary to induce immediate unconsciousness. The signs of an effective electric stun are extension of the limbs, opisthotonos, downward rotation of the eyeballs, and a tonic spasm changing into a clonic spasm, with eventual muscle flaccidity. This effect should be followed promptly by electrically induced fibrillation of the heart, exsanguination, or other appropriate methods to ensure death. Advantages. (1) Electrocution is humane if current is directed through the brain; (2) it does not chemically contaminate tissues; and (3) it is economical. Disadvantages. (1) Electrocution is hazardous to personnel; (2) it is not a useful method for mass euthanasia because considerable time is required per animal; (3) it is not a useful method for a vicious, intractable animal; (4) because violent extension and stiffening of the limbs, head, and neck occur, electrocution is aesthetically objectionable; and (5) in small animals, electrocution may not result in death because ventricular fibrillation and circulatory collapse do not always persist after cessation of current flow. Recommendations. Electrocution for euthanasia and preslaughter stunning requires special skills and equipment to assure passage of sufficient current through the brain to produce unconsciousness followed by electrically induced fibrillation of the heart. Although the method is acceptable if the above requirements are met, the disadvantages far outweigh its advantages in most applications.
PRECAUTIONS CONCERNING USE OF EUTHANATIZING AGENTS IN ANIMALS INTENDED FOR HUMAN FOOD In euthanasia of animals for human food, agents cannot be used that lead to tissue residues, unless approved by the U.S. Food and Drug Administration. 5 Carbon dioxide is the only chemical currently used in euthanasia of animals (primarily swine) for human food that does not lead to tissue residue. 25 When euthanasia of poikilothermic (cold-blooded) animals and aquatic animals is performed, the differences in their metabolism, respiration, and tolerance to cerebral hypoxemia may preclude methods that are acceptable in terrestrial mammals.
Postface This panel's report summarizes contemporary scientific knowledge on euthanasia in animals and calls attention to the lack of scientific reports assessing pain and discomfort in animals undergoing euthanasia. Many reports on various methods of euthanasia are either anecdotal or testimonial narratives and are, therefore, not cited here. The panel unanimously endorses the need for well-designed experiments to determine more fully the extent to which each procedure meets the criteria used for judging the methods of euthanasia.
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The Panel on Euthanasia is fully committed to the concept that whenever it becomes necessary to kill any animal for any reason whatsoever, death should be induced as painlessly as possible. It has been our charge to develop workable guidelines for addressing this need, and it is our sincere desire that these guidelines be conscientiously used by all who exercise stewardship over animals on earth.
REFERENCES 1. Anis, G. W.: Euthanasia of domesticated animals by shooting. In Humane Destruction of Unwanted Animals. Herts, England: Potters Bar. The Universities Federation for Animal Welfare, 1975. 2. Barocio, L. D.: Review of literature on use of T-61 euthanasia agent. Inst. J. Stud. Anim. Probl., 4:336-342, 1983. 3. Blackmore, D. K., and Newhook, J. C.: The assessment of insensibility in sheep, calves, and pigs during slaughter. In Eikelenboom, G. (ed.): Stunning of Animals for Slaughter. Boston, Martinus Nijhoff, 1983. 4. Blackmore, D. K.: Energy requirements for the penetration of heads of domestic stock and the development of a multiple projectile. Vet. Rec., 116:36-40, 1979. 5. Booth, N. H.: Drug and chemical residues in the edible tissues of animals. In Booth, N. H., and McDonald, L. E. (eds.): Veterinary Pharmacology and Therapeutics. Edition 4. Ames, Iowa State University Press, 1982, pp. 1065-1113. 6. Bowen, J. M., Blackman, D. M., and Heavner, J. E.: Effect of magnesium ions on neuromuscular transmission in the horse, steer, and dog. J. Am. Vet. Med. Assoc., 157:164-173,1970. 7. Breazile, J. E., and Kitchell, R.L.: Euthanasia for laboratory animals. Fed. Proc., 28:1577-1579, 1969. 8. Carding, T.: Euthanasia of dogs and cats. Anim. Reg. Stud., 1 :5-21, 1977. 9. Clifford, D. H.: Preanesthesia, anesthesia, analgesia, and euthanasia. In Fox, Cohen, and Loew (eds.): Laboratory Animal Medicine. New York, Academic Press, 1984, pp. 528-563. 10. Croft, P. G.: Anaesthesia and euthanasia. In UF AW Handbook on the Care and Management of Laboratory Animals. Edition 3. Baltimore, Williams & Wilkins Co., 1967, pp. 170-172. 11. Croft, P. G., and Hume, C. W.: Electric stunning of sheep. Vet. Rec., 68:318-321, 1956. 12. Dallaire, A., and Chalifoux, A.: Premedication of dogs with acepromazine or pentazocine before euthanasia with carbon monoxide. Can. J. Compo Med., 49:171178,1985. 13. Eikmeier, H.: Experience with a new preparation for painless destruction of small animals. (T-61.) Die Blauen Hefte Tieraerztl, 5:22-23, 1962. 14. Feldman, D. B., and Gupta, B. N.: Histopathologic changes in laboratory animals resulting from various methods of euthanasia. Lab. Anim. Sci., 26:218-221, 1976. 15. Fowler, N. G., and Foster, S. J.: The last gasp. Vet. Rec., 86:145, 1970. 16. Franz, D. N.: Central nervous system stimulants. In Goodman, L. S., and Gilman, A. (eds.): The Pharmacological Basis of Therapeutics. Edition 5. New York, Macmillan Publishing Co., 1975, pp. 359-366. 17. Glen, J. B., and Scott, W. N.: Carbon dioxide euthanasia of cats. Br. Vet. J., 129:471479,1973. 18. Hafez, E. S. E.: The Behavior of Domestic Animals. Edition 3. Baltimore, Williams & Wilkins Co., 1975. 19. Hatch, R. C.: Euthanizing agents. In Jones, L. M., Booth, N. H., and McDonald, D. E. (eds.): Veterinary Pharmacology and Therapeutics. Ames, Iowa State University Press, 1977, pp. 1289-1296. 20. Hernett, T. D., and Haynes, A. P.: Comparison of carbon dioxide/air mixture and nitrogen/air mixture for the euthanasia of rodents. Design of a system for inhalation euthanasia. Anim. Technol., 35:93-99, 1984.
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21. Herschler, R C., Lawerence, J. R, and Schiltz, R A.: Secobarbital-dibucaine combination as a euthanasia agent for dogs and cats. Vet. Med. Small Anim. Clin., 76:1009-1012, 1981. 22. Hoenderken, R: Electrical and carbon dioxide stunning of pigs for slaughter. In Eikelenboom, G. (ed.): Stunning of Animals for Slaughter. Boston, Martinus Nijhoff, 1982, pp. 59-63. 23. Houpt, K. A., and Wolski, T. R: Domestic Animal Behavior for Veterinarians and Animal Scientists. Ames, Iowa State University Press, 1982. 24. Hughes, H. C.: Euthanasia of laboratory animals. In Melby, and Altman (eds.): Handbook of Laboratory Animal Science. Volume III. Cleveland, CRC Press, 1976, pp. 553-559. 25. Humane slaughter regulations. Fed. Register, 44(232}:68309-68817, 1979. 26. Jaksch, W.: Euthanasia of day-old male chicks in the poultry industry Int. J. Study Anim. Probl., 2:203-213, 1981. 27. Kotula, A. W., Drewniak, E. E., and Davis, L. E.: Experimentation with in-line carbon dioxide immobilization of chickens prior to slaughter. Poul. Sci., 40:213216, 1961. 28. Kuepper, G.: T-61 used in large animals. Die Blauen Hefte Tieraerztl, 3:32-33, 1964. 29. Laursen, A. M.: Choosing between CO 2 and electrical stunning of pigs. A preliminary examination of stress and ethics. In Eikelenboom, G. (ed.): Stunning of Animals for Slaughter. Boston, Martinus Nijhoff, 1983, pp. 64-72. 30. Loftsgard, G., Braathen, S., and Helgoboetd, A.: Electrical stunning of mink. Vet. Rec., 91 :132-134, 1972. 31. Lumb, W. V.: Euthanasia by noninhalartt pharmacologic agents. J. Am. Vet. Med. Assoc., 165:831-852, 1974. 32. Lumb, W. V., Doshi, K, and Scott, R J.: A comparative study of T-61 and pentobarbital for euthanasia of dogs. J. Am. Vet. Med. Assoc., 172:149-152, 1978. 33. Lumb, W. V., and Jones, E.: Veterinary Anesthesia. Philadelphia, Lea & Febiger, 1985. 34. Lumb, W. V., and Moreland, A. F.: Chemical methods of euthanasia. Lab. Anim., 11 :29-35, 1982. 35. McDonald, L. E., Booth, N. H., Lumb, W. V., et al.: Report of the AVMA panel on euthanasia. J. Am. Vet. Med. Assoc., 173:59-72,1978. 36. Quin, A. H.: Observations on a new euthanasia agent for small animals. Vet. Med., 58:494-495, 1963. 37. Roberts, T. D. M.: Cortical activity in electrocuted dogs. Vet. Rec., 66:561-567, 1954. 38. Roberts, T. D. M.: Correspondence: Electrocution cabinets. Vet. Rec., 95:241-242, 1974. 39. Rowan, A. N.: T-61 use in the euthanasia of domestic animals: A survey. Adv. Anim. Welfare Sci., in press. 40. Sawyer, D. C.: Comparative effects of halothane. Gaines Dog Res. Prog., 1975, pp. 2-3. 41. Wallach, M. B., Peterson, K E., and Richards, R K: Electrophysiologic studies of a combination of secobarbital and dibucaine for euthanasia of dogs. Am. J. Vet. Res., 42:850-853, 1981. 42. Warrington, R: Electrical stunning, a review of literature. Vet. Bull., 44:617-628, 1974. 43. WHO Joint FAOIWHO Expert Committee on Meat Hygiene. Second Report. W.H.O. Tech. Rep. Ser., 1962, p. 241. Department of Veterinary Clinical Medicine College of Veterinary Medicine University of Illinois Urbana, Illinois 61801
0749-0720/86 $00.00 + $.20
Euthanasia of Food Animals John C. Thurmon, D.V.M., M.S.*
This article has been extracted, in major part, from the 1986 report of the AVMA Panel on Euthanasia. Panel members include A. W. Smith, D.V.M., Ph.D. (Chairman), Oregon State University College of Veterinary Medicine, Corvallis, Oregon; K. A. Houpt, V.M.D., Ph.D., New York State College of Veterinary Medicine, Cornell University, Ithaca, New York; R. L. Kitchell, D.V.M., Ph.D., University of California School of Veterinary Medicine, Davis, California; D. F. Kohn, D.V.M., Ph.D., University of Texas Medical School at Houston, Houston, Texas; L. E. McDonald, D.V.M., Ph.D., University of Georgia College of Veterinary Medicine, Athens, Georgia; M. Passaglia, Jr., M.S., former Executive Director, The American Humane Association, 1979-1985; J. C. Thurmon, D.V.M., M.S., University of Illinois College of Veterinary Medicine, Urbana, Illinois; and E. R. Ames, D.V.M., Ph.D., Staff Coordinator, American Veterinary Medical Association, Schaumburg, Illinois. For a thorough coverage of the subject on euthanasia, the reader is encouraged to review the panel's full report in J. Am. Vet. Med. Assoc., 188:252-268, 1986. Euthanasia is the act of inducing rapid, painless death. The criterion to be considered for a painless death is rapidly occurring unconsciousness followed by cardiac or respiratory arrest. For pain to be experienced, the cerebral cortex and subcortical structures must be functional. An unconscious animal can not experience pain because the cerebral cortex is not functioning. If the cerebral cortex is rendered nonfunctional by any means, such as hypoxia, depression by drugs, electric shock, or concussion, pain is not experienced. Stimuli that might evoke pain in a conscious animal may elicit only reflex responses manifested by movement in an unconscious animal. For this reason, nonpurposeful movements of an animal are not reliable indicators of pain perception. Conversely, an animal can experience pain although no body movements occur in response to nox-
* Diplomate,
American College of Veterinary Anesthesiologists; Professor, and Head of AnestheSiology Section, Department of Veterinary Clinical Medicine, University of Illinois College of Veterinary Medicine, Urbana, Illinois
Veterinary Clinics of North America: Food Animal Practice-Vol. 2, No, 3, November 1986
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ious stimuli if the animal is given muscle-paralyzing agents such as curare, succinylcholine, gallamine, pancuronium, nicotine, or decamethonium. These muscle-paralyzing agents do not induce unconsciousness or depress the cerebral cortex or any neural mechanism involved in pain perception. As with other procedures applied to animals, euthanasia requires some physical control over the animal. The degree of control and kind of restraint needed are determined by the animal species, breed, size, state of domestication, presence of painful injury or disease, degree of excitement, and method of euthanasia. Suitable control is vital to minimize pain in animals, to assure safety of the person performing euthanasia, and frequently to protect other animals and people. Selection of the most appropriate method of euthanasia in any given situation depends on the species of animal involved, available means of animal control, skill of personnel, numbers of animals, economic factors, and other considerations.
MODES OF ACTION OF EUTHANATIZING AGENTS Euthanatizing agents terminate life by three basic mechanisms: (1) hypoxemia, direct or indirect; (2) direct depression of neurons vital for life function; and (3) physical damage to brain tissue. Agents that produce death by direct or indirect hypoxia can act at various sites and can cause different times of onset of unconsciousness. With some agents, unconsciousness may occur before cessation of motor activity (muscle movement). Thus, although animals demonstrate muscular contractions, they are not perceiving pain. The uninformed observer may find this difficult to accept. Conversely, muscle relaxants induce a flaccid muscular paralysis and the animal remains conscious until death eventually occurs as a result of hypoxemia and hypercapnia. Although the animal appears relaxed, it is actually in a state of panic and can feel pain. Outwardly this appears to be an ideal form of euthanasia, but it is not. Agents that do not induce rapid loss of consciousness prior to death include curare, succinylcholine, gallamine, nicotine, magnesium sulfate, or potassium chloride, pancuronium, decamethonium, and strychnine.
Use of any of these agents alone for euthanasia is absolutely condemned. The second group of euthanatizing agents depresses nerve cells of the brain first, blocking apprehension and pain perception, and then causes unconsciousness and death. Some of these agents "release" muscle control during the first stage of anesthesia, resulting in a socalled excitement or delirium phase, during which there may be vocalization and some muscle contraction. 3 Although these responses appear to be purposeful, they are not. Death is due to hypoxemia and direct depression of respiratory centers. With the third group, physical damage to the brain, concussion, or direct electrical current flOW 42 through the brain produces instant
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unconsciousness. Exaggerated muscular activity may follow unconsciousness. When electrocution is properly performed, loss of motor function occurs concomitantly with loss of consciousness; with the other methods in this group, muscular contraction may occur. Although this may be unpleasant to the observer, the animal is not suffering. Criteria for Judging Methods of Euthanasia Several criteria are used in evaluating methods of euthanasia: (1) ability to produce death without causing pain, (2) time required to produce loss of consciousness, (3) time required to produce death, (4) reliability, (5) safety of personnel, (6) potential for minimizing undesirable psychologic stress on the animal, (7) nonreversibility, (8) compatibility with requirement and purpose, (9) emotional effect on observers or operators, (10) economic feasibility, (11) compatibility with histopathologic evaluation, and (12) drug availability and human abuse potential.
BEHAVIORAL CONSIDERATIONS The facial expressions and body postures that indicate various emotional states have been described. 18,23 Behavioral and physiologic responses to noxious stimulation include distress vocalization, struggling, attempts to escape, defensive or redirected aggression, salivation, urination, defecation, pupillary dilatation, tachycardia, sweating, and reflex skeletal muscle contractions causing shivering, tremors, or other muscular spasms. Some of these responses can occur in unconscious as well as conscious animals. Fear can cause immobility or freezing in certain species, particularly rabbits and chickens. This immobility response should not be interpreted as unconsciousness when the animal is in fact conscious. 23 In very young animals, autonomic and reflexive reactions are evident, although overt behavioral reactions may differ from those of adults. The need to minimize fear and apprehension must be considered in determining the method of euthanasia. Distress vocalizations, fearful behavior, and release of certain odors or pheromones by a frightened animal may cause anxiety and apprehension in others. Therefore, whenever possible, animals should not observe euthanasia of one another, especially those of their own species. This is particularly important when vocalization or release of pheromones may occur during induction of unconsciousness. Gentle restraint, preferably in a familiar environment, and careful handling during euthanasia often have a calming effect on companion animals. However, some of these methods may not be operative with wild animals or animals that are injured or diseased. When capture or restraint may cause pain, injury, or anxiety to the animal or danger to the operator, the use of tranquilizing or immobilizing drugs (phase I) may be necessary before the euthanatizing agent is given (phase II).
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Inhalant Agents The inhalant anesthetics (for example, ether, halothane, methoxyflurane) are used only infrequently for euthanasia of food-producing animals. On the other hand, carbon dioxide (C0 2 ) has been used more frequently. Inhalation of 60 per cent CO 2 results in loss of consciousness within 45 seconds and respiratory arrest within 5 minutes. 17 Carbon dioxide has been used to euthanatize pigs, chickens, and rabbits 3 , 7, 24, 26, 27 and for humane slaughter of swine for human consumption. 22 , 25 According to Croft, animals do not detect the CO 2 immediately, and its depressant action takes place almost unnoticed. 10 Studies in day-old chickens have shown that CO 2 is an effective euthanatizing agent. Inhalation of CO 2 caused little distress to the birds. It suppressed nervous activity and induced death rather quickly. Because respiration begins during embryonic development, the unhatched chicken's environment may normally have a CO 2 concentration as high as 14 per cent. Thus, CO 2 concentration for euthanasia for baby chickens and other neonates should be especially high. A CO 2 concentration of 60 to 70 per cent with a 5-minute exposure time appears to be optimal. 26 Carbon dioxide is used for preslaughter anesthesia of pigs. The undesirable side effect of CO 2 , as used in commercial slaughterhouses, is that mature swine experience a stage of excitement with vocalization for about 40 seconds before they lose consciousness. 22 ,29 For that reason, CO 2 preslaughter anesthesia may appear less humane than other techniques. The signs of effective CO 2 anesthesia are those associated with deep surgical anesthesia, such as loss of withdrawl and palpebral reflexes. 3 Advantages. (1) The rapid depressant and anesthetic qualities of CO 2 are well established; (2) CO 2 may be purchased in cylinders or in a solid state as "dry ice"; (3) CO 2 is inexpensive, nonflammable, and nonexplosive and presents minimal hazard to personnel when used with properly designed equipment; (4) CO 2 does not result in accumulation of tissue residues in food-producing animals; and (5) CO 2 euthanasia does not distort cellular architecture. 14 Disadvantages. (1) Because CO 2 is heavier than air, incomplete filling of a chamber may permit a tall animal to avoid exposure and survive, and (2) in immature animals, the time required for euthanasia may be substantially prolonged. Recommendations. Carbon dioxide is recommended for piglets, birds, and rabbits. Chamber design should allow for precharging with CO 2 and should enable cleaning and removal of dead animals with minimal loss of CO2 • Compressed CO 2 gas in cylinders is preferable to dry ice. Inflow to a euthanatizing chamber can be regulated precisely with compressed CO 2 • The optimal flow rate appears to be one that will displace approximately 20 per cent of chamber volume per minute. 2o When using compressed CO 2 , oxygen can be added (for example, 30 percent O 2 , 70 per cent CO 2 ), decreasing the discomfort of hypoxia prior to onset of narcosis and anesthesia. If dry ice is used, animal contact must be avoided to prevent freezing or chilling.
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N oninhalant Pharmacologic Agents Noninhalant agents that can be used for euthanasia are widely diverse in chemical composition. 19 , 31 Although death can be induced by administering these drugs via many routes (intravenous, intracardiac, intraperitoneal, intrathecal, intramuscular, intrathoracic, subcutaneous, and oral), intravenous administration is preferred because the effect is most rapid and reliable. Intrapulmonic injection should be avoided. Oral, rectal, and intraperitoneal routes of administration of drugs for euthanasia are inadvisable because of prolonged onset of action, wide range in lethal doses, and potential irritation of tissues. An hour or more may elapse from the time of administration until death. Some drugs, such as chloral hydrate and T-61, when given intraperitoneally, are irritating and cause abdominal pain. Others may produce tissue changes, depending on the dose and route of administration. 14, 40 Because crying and struggling may follow improper intracardiac injection, this route of administration is objectionable. Skill is required to penetrate the heart of an animal with one thrust of a hypodermic needle, especially if the animal is not properly restrained. Intracardiac injection of drugs is not recommended for euthanasia, except in depressed, anesthetized, or comatose animals. Intrathecal use of drugs in unanesthetized animals is not recommended because puncture of the cisterna magna without causing pain and struggling is not possible. If the animal to be euthanatized is excitable or vicious, use of analgesics, tranquilizers,12 narcotics, ketamine, xylazine, or other depressants for phase I of euthanasia is recommended before administration of the euthanatizing agent, phase II.
BARBITURIC ACID DERIVATIVES Barbiturates depress the central nervous system in descending order, beginning with the cerebral cortex. Within seconds of intravenous administration, unconsciousness is induced and it progresses to deep anesthesia. 35 Apnea occurs owing to depression of the respiratory center, and cardiac arrest quickly follows. Several barbiturates are acceptable, but pentobarbital sodium is most commonly used. Advantages. (1) A primary advantage of barbiturates is speed of action. This effect depends on the dose, concentration, and rate of injection; (2) the barbiturates induce euthanasia smoothly, with minimal discomfort to the animal, and favorably impress the observer because the animal dies quietly. A cost comparison by Lumb and Moreland34 (Table 1) indicates that barbiturates are less expensive than most other injectable agents. Disadvantages. (1) Intravenous injection is necessary for best results, necessitating trained personnel; (2) each animal must be restrained; (3) current federal drug regulations require strict accounting of the barbiturates and, by necessity, these must be used under the
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Relative Cost of Injectable Euthanasia Products* APPROXIMAL PENTOBARBITAL
NAME
MANUFACTURER
Euthanasia solution
Prepared in houset
Sleepaway Fatal-Plus
Ft. Dodge North American Pharmacal Somlethol Med-Tech Beuthanasia- Burns-Biotic D Special T-61 National Laboratories
(CRlML)
COST/LB OF COST
AMIMAL
2
Pentobarbital powder, $100/kg
$0.003
4 6
$ 5.95/100 ml $15.95/250 ml
$0.006 $0.006
6 6
$ 7.25/100 ml $28.00/100 ml
$0.007 $0.28
$55.00/250 ml
$0.22
* From Lumb, W. V., and Moreland, A. F.: Chemical methods of euthanasia. Lab. Anim., 11 :29-35, 1982; with permission. t "Homemade" euthanasia solution: Pentobarbital sodium, powder 259.2 Ethyl alcohol, 95% 400.0 Sodium hydroxide solution, 10% 80.0 Hot water, q.s. 2000.0
gm ml ml ml
Give 1 mll5 lb of body weight intravenously, more if necessary. Note: This is twice anesthetic strength.
supervision of personnel registered with the U.S. Drug Enforcement Agency; and (4) an aesthetically objectionable terminal gasp may occur in the unconscious animal. Recommendations. The advantages of using barbiturates for euthanasia outweigh the disadvantages. The intravenous injection of a barbiturate is satisfactory for large and small food animals not intended for human consumption. Nervous or vicious animals may require tranquilization or sedation before injection of a barbiturate.
SECOBARBITAL-DIBUCAINE COMBINATION* This mixture contains a short-acting barbiturate to induce anesthesia. The cardiotoxic effects of dibucaine cause cardiac arrest. Limited trials suggest this combination is an acceptable barbiturate euthanatizing agent when given intravenously to dogs and cats. 21 , 41 It would appear to be acceptable in larger species. * Each milliliter contains 400 mg secobarbital and 25 mg dibucaine. Dosage for dogs and cats is 0.22 mg per kg, intravenously. Repose, Diamond Laboratories, Des Moines, Iowa.
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T-61 T -61 is an injectable nonbarbiturate, nonnarcotic mixture of three drugs used to induce euthanasia. These drugs provide a combination of general anesthetic, curariform, and local anesthetic actions. Death results from severe central nervous system depression, hypoxia, and circulatory collapse. T-61 has been used in Germany since 1962 and became available in the United States soon thereafter. 13, 28, 32, 36 T-61 has been withdrawn from the market in the United Kingdom since 1976, but because it is not currently listed as a controlled substance under federal drug regulations, it has gained some popularity in the United States in recent years. A recent survey39 and review 2 indicate a need for further controlled studies on the efficacy and humaneness of T -61 as a euthanatizing agent in several species. Advantages. (1) The terminal gasp15 that may accompany pentobarbital euthanasia in unconscious animals is not evident with T61, and (2) it is not regulated or controlled by the U.S. Drug Enforcement Agency. Disadvantages. (1) T -61 must be administered intravenously because it is painful when administered extravascularly; (2) if T -61 is injected at too rapid a rate, the animal may appear to experience pain or discomfort immediately before becoming unconscious; (3) doses larger than recommended may cause pulmonary edema and other tissue lesions; and (4) T-61 is not approved for use in animals intended for human consumption. Recommendations. Ifbarbiturates cannot be used, T-61 is a substitute, but only when it is administered intravenously by a highly skilled person at recommended doses and at proper injection rates. 2 Intracardiac or intrathoracic administration is not recommended.
CHLORAL HYDRATE Because chloral hydrate depresses the cerebrum slowly, restraint during induction may become a problem in some animals. Death is due to hypoxemia caused by progressive depression of the respiratory center. Death may be preceded by gasping, muscle spasms, and vocalization. Recommendation. Chloral hydrate can be recommended for euthanasia of large food-producing animals when administered intravenously.
CHLORAL HYDRATE, MAGNESIUM SULFATE, AND SODIUM PENTOBARBITAL This combination has been used for anesthesia of large animals and may be administered in overdose for euthanasia. 33
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Recommendation. This mixture is an acceptable large-animal euthanatizing agent when administered intravenously.
STRYCHNINE Strychnine or any of its salts, such as strychnine sulfate, increases the excitability of the central nervous system. The animal remains conscious and excessively responsive to stimuli. Strychnine causes violent convulsions and associated painful contraction of skeletal muscles. 16
Recommendation. euthanasia.
Strychnine is absolutely condemned for
MAGNESIUM SULFATE (MgS04 ) OR POTASSIUM CHLORIDE (KCI) Magnesium or potassium ions exert little if any direct central nervous system depressant effect. Doses of MgS04 previously recommended for euthanasia have been shown to cause complete neuromuscular block and death due to hypoxemia. 6 Recommendations. Magnesium or potassium salts must not be used alone for euthanasia because of the lack of analgesic or anesthetic effect. Potassium chloride may be administered to an anesthetized animal as an efficient and inexpensive way to cause cardiac arrest and death.
NICOTINE Nicotine is an alkaloid obtained from the tobacco plant. Nicotine sulfate is an odorless, clear, water-soluble liquid. Lethal doses can be absorbed through mucous membranes and intact skin, making it dangerous for personnel. Nicotine sulfate has been used extensively in the past in capture equipment for immobilizing wild game and feral and domestic dogs. Other drugs have replaced nicotine sulfate because of the high mortality associated with its use. Nicotine sulfate induces a short period of stimulation, followed by blockade of autonomic ganglia. When injected in sufficient quantity, it induces a muscle relaxant effect, with paralysis of the respiratory muscles and, subsequently, hypoxemic death. Salivation, vomiting, defecation, and convulsions commonly occur prior to death. Recommendation. Because nicotine sulfate is an extremely dangerous drug for personnel, is inhumane for animals, and induces serious side effects prior to death, it is absolutely condemned for
euthanasia.
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CURARIFORM DRUGS Drugs such as curare, succinylcholine, pancuronium, guaifenesin, and other neuromuscular blocking agents induce death by immobilizing the respiratory muscles, causing suffocation. There is no depressant action on the brain. Human patients given these drugs have described periods of full consciousness accompanied by complete muscular immobility and intense anxiety.33 Recommendations. When standard methods of restraint are impractical, impossible, or dangerous or when manual capture and restraint may cause pain and injury through struggling and anxiety, the use of immobilizng drugs is justified. Because the immobilized animal is fully conscious and subject to death by suffocation, euthanasia by chemical of physical methods must be accomplished immediately.
Curariform drugs alone are absolutely condemned for euthanasia. OTHER PARENTERAL PREPARATIONS A number of injectable agents are currently available for immobilizing domestic and feral animals. Drugs such as ketamine and xylazine are used for immobilization, analgesia, and anesthesia. The effect occurs within minutes following intramuscular injection, making these drugs useful for restraint (that is, phase I of euthanasia). Currently, these agents are not included as controlled substances by federal drug regulations. Narcotics also may be used for sedation and analgesia, but detailed records must be kept and their use properly supervised by a licensed professional registered with the U.S. Drug Enforcement Agency. Although it is possible to cause death with high doses of these drugs, such use would be impractical and may produce convulsions before death.
PHYSICAL METHODS Physical methods employed in food animals include captive bolt pistol, gunshot, stunning, cervical dislocation, exsanguination, and electrocution. It is a requirement that any method of euthanasia be performed by knowledgeable, well-trained individuals. Because trauma often is associated with physical methods for inducing unconsciousness, this requirement is particularly vital. Physical methods are often aesthetically displeasing; however, when properly performed, unconsciousness is rapid, without distress or pain to the animal. In some applications, the amount of apprehension of the animals is less than if chemical agents are employed.
CAPTIVE BOLT PISTOL This method is used in ruminants, horses, and swine and is being developed for laboratory rabbits. The purpose of both the captive bolt
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and nonpenetrative percussion methods is to damage the cerebral hemisphere and brainstem so that the animal is unconscious. 4 Penetrating or nonpenetrating captive bolts are powered by gunpowder or compressed air. N onpenetrating instruments usually are less effective in inducing unconsiousness. Animals must be adequately restrained to ensure proper placement of the captive bolt. The muzzle of the pistol should be placed at a right angle to the skull and directed toward the center of the brain in ruminant species. A multiple projectile has been suggested as a more effective technique, especially on large cattle. In swine, the captive bolt should be directed to the brain from a central point slightly above a line between the eyes. 9 The signs of effective stunning are immediate collapse and a 15second period of tetanic spasm, followed by slow hindlimb movements of increasing frequency. Advantages. A humane method for use in slaughterhouses and in research facilities when the use of drugs is contraindicated. Disadvantages. (1) Aesthetically displeasing; (2) nonpenetrating captive bolt pistols cannot be used effectively in mature swine, and (3) death may not occur. Recommendations. The captive bolt pistol is more appropriate when there is effective restraint of the animal. Use of the penetrating captive bolt pistol, when followed by exsanguination or pithing, is an acceptable method of euthanasia for horses, ruminants, and swine when chemical agents cannot be used. N onpenetrating captive bolt pistols are not recommended.
GUNSHOT Under some circumstances, gunshot may be the only practical method of euthanasia. It should be performed by highly skilled and trained personnel using a rifle or pistol appropriate for the situation. The projectile should be accurately placed to enter the brain, causing instant unconsiousness. 1 Advantages. (1) Euthanasia is instantaneous; and (2) under field conditions, gunshot may be the only effective method available. Disadvantages. (1) It is dangerous to personnel; (2) it is aesthetically unpleasant; and (3) under field conditions, it may be difficult to hit the brain. Recommendation. When other methods cannot be used, competently performed gunshot is an acceptable method of euthanasia.
STUNNING Stunning can render an animal unconscious; however, unconsciousness will occur only if a blow to the head is properly executed. If not performed correctly, various degrees of consciousness with concomitant pain will ensue.
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Advantages. (1) Stunning is humane when properly performed, and (2) it enables collection of blood and other tissues without chemical contamination. Disadvantages. (1) Stunning is inhumane if improperly performed; (2) it is impossible to ensure constancy of performance by personnel; (3) stunning may be aesthetically displeasing for personnel performing or observing the procedure; and (4) it must be followed by other means to ensure the death of the unconscious animal. Recommendations. Stunning of rabbits by a sharp blow to the head as a method of euthanasia is not encouraged because of the inherent risk of not rendering animals immediately unconscious. Stunning must be followed by other means to ensure death of the unconscious animal.
CERVICAL DISLOCATION Cervical dislocation is used to euthanatize poultry and rabbits. For immature rabbits, the head is held in one hand and the hind limbs in the other. The animal is stretched and the neck is hyperextended and dorsally twisted to separate the cervical vertebrae from the skull. 24 Advantages. (1) Cervical dislocation is a technique that may induce immediate unconsciousness; (2) it does not chemically contaminate tissues; and (3) it is rapidly accomplished. Disadvantages. (1) It may be aesthetically displeasing to personnel, and (2) its use is limited to poultry and immature rabbits. Recommendations. When properly executed, cervical dislocation is a humane technique to euthanatize poultry and for rabbits that weigh less then 1 kg. In heavier rabbits, the greater muscle mass in the cervical region makes cervical dislocation physically more difficult and, accordingly, it should not be performed. Because unconsciousness may not occur immediately, it is preferable to anesthetize lightly or sedate the animal before cervical dislocation.
ELECTROCUTION Electrocution as a form of euthanasia or stunning has been employed for years in species such as dogs, cattle, sheep, and hogs. 8 , 11, 19, 30, 37, 38, 42, 43 Experiments in dogs have shown the necessity of directing the electric current through the brain in order to produce instant stunning with loss of consciousness. In the dog, when electricity passes between fore- and hindlimbs or neck and feet, it causes the heart to fibrillate promptly but does not induce unconsciousness. The dog does not lose consciousness for at least 12 seconds after ventricular fibrillation, which causes cerebral hypoxemia. These data dictate against use of methods that direct current through the heart and not directly through the brain. An apparatus that applies electrodes
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to opposite sides of the head, or in another way directs electric current immediately through the brain, is necessary to induce immediate unconsciousness. The signs of an effective electric stun are extension of the limbs, opisthotonos, downward rotation of the eyeballs, and a tonic spasm changing into a clonic spasm, with eventual muscle flaccidity. This effect should be followed promptly by electrically induced fibrillation of the heart, exsanguination, or other appropriate methods to ensure death. Advantages. (1) Electrocution is humane if current is directed through the brain; (2) it does not chemically contaminate tissues; and (3) it is economical. Disadvantages. (1) Electrocution is hazardous to personnel; (2) it is not a useful method for mass euthanasia because considerable time is required per animal; (3) it is not a useful method for a vicious, intractable animal; (4) because violent extension and stiffening of the limbs, head, and neck occur, electrocution is aesthetically objectionable; and (5) in small animals, electrocution may not result in death because ventricular fibrillation and circulatory collapse do not always persist after cessation of current flow. Recommendations. Electrocution for euthanasia and preslaughter stunning requires special skills and equipment to assure passage of sufficient current through the brain to produce unconsciousness followed by electrically induced fibrillation of the heart. Although the method is acceptable if the above requirements are met, the disadvantages far outweigh its advantages in most applications.
PRECAUTIONS CONCERNING USE OF EUTHANATIZING AGENTS IN ANIMALS INTENDED FOR HUMAN FOOD In euthanasia of animals for human food, agents cannot be used that lead to tissue residues, unless approved by the U.S. Food and Drug Administration. 5 Carbon dioxide is the only chemical currently used in euthanasia of animals (primarily swine) for human food that does not lead to tissue residue. 25 When euthanasia of poikilothermic (cold-blooded) animals and aquatic animals is performed, the differences in their metabolism, respiration, and tolerance to cerebral hypoxemia may preclude methods that are acceptable in terrestrial mammals.
Postface This panel's report summarizes contemporary scientific knowledge on euthanasia in animals and calls attention to the lack of scientific reports assessing pain and discomfort in animals undergoing euthanasia. Many reports on various methods of euthanasia are either anecdotal or testimonial narratives and are, therefore, not cited here. The panel unanimously endorses the need for well-designed experiments to determine more fully the extent to which each procedure meets the criteria used for judging the methods of euthanasia.
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EUTHANASIA OF FOOD ANIMALS
The Panel on Euthanasia is fully committed to the concept that whenever it becomes necessary to kill any animal for any reason whatsoever, death should be induced as painlessly as possible. It has been our charge to develop workable guidelines for addressing this need, and it is our sincere desire that these guidelines be conscientiously used by all who exercise stewardship over animals on earth.
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