Journal American Veterinary Medical Association, 1994 Vol 204, pp 1354-1360.
Euthanasia is defined as a humane death that occurs without pain and distress1. Physical methods of euthanasia are used in slaughter plants and in clinical practice. These methods are unpleasant to watch, but humane when used properly. In some cases, the most humane euthanasia method for an injured cow or horse is by gunshot, because fear and anxiety are minimized.
During the past 20 years, the author has worked in more than 1OO slaughter plants as a consultant for improving handling techniques and equipment used for slaughter in an effort to improve the humaneness of slaughter. When slaughter is performed properly, it is euthanasia; however, when it is performed improperly, animal suffering may result. There is a tremendous need for veterinarians to increase their knowledge of physical methods of euthanasia and humane slaughter. Observations by the author at slaughter plants indicate that some veterinarians lack such practical and scientific knowledge. It is the responsibility of veterinarians to enforce the Humane Methods of Slaughter Act of 19782. The purpose of this report is to provide veterinarians with practical and scientific information on the use of captive bolt guns, electrical stunning, CO2 anesthesia, and ritual slaughter.
Figure 1: Correct position for euthanasia of livestock by use of a captive-bolt gun or firearm
A captive-bolt gun kills the animal by concussive force and penetration of the bolt4,5. A nonpenetrating, mushroom—head captive bolt only stuns the animal, thus, it cannot be used as the sole method of euthanasia1. A nonpenetrating captive bolt must be followed by an adjunctive method, such as exsanguination1. The gun must be carefully cleaned and maintained to achieve maximal hitting power. Observations by the author in slaughter plants indicate that poor gun maintenance is a major cause of poor stunning, in which more than one shot is required. In large, high-speed plants, maintenance personnel must be dedicated to servicing captive-bolt guns6. Many large slaughter plants use pneumatic powered captive-bolt guns. To obtain maximal hitting power, the gun must be supplied with sufficient air pressure and air volume per the manufacturer’s recommendations.
When a standing animal is shot, it should instantly drop to the floor. In cattle, the neck contracts in a spasm for 5 to 10 seconds. Hogs have violent convulsions. Observations by the author indicate that this can occur even when the brain and part of the spinal cord have been destroyed. These are normal reactions. Rhythmic breathing must be absent and the animal must not moan, bellow, or squeal7. All eye reflexes should be absent7. Gasping or gagging reflexes are permissible because they are signs of a dying brain7. Within 10 seconds, the neck and head should be completely relaxed. ln a clinical situation and in a slaughter plant, the animal‘s limbs may make uncoordinated movements for several minutes.
Properly applied electrical stunning in a slaughter plant induces instantaneous unconsciousness, and is approved under the Humane Methods of Slaughter Act of 19782 Sufficient amperage must be applied through the brain to induce a grand-mal epileptic seizure9,10. The animal’s brain must be in the current path between the two electrodes.
Hogs in small locker plants or meat science laboratories often are stunned with head—only reversible electric stunning. To prevent retum to sensibility, the animal must be exsanguinated within 30 seconds10, and some researchers recommend 10 to 15 seconds11. The author has observed that delays between reversible head-only stunning and exsanguination are common in small locker plants because the hoist is too slow. lnadequate equipment causes severe welfare problems.
In large pork and sheep slaughter plants, animals are held in a conveyor restrainer, and stunning that induces cardiac arrest (cardiac-arrest stunning) is used. The interval between stunning to exsanguination is less critical. One electrode is placed on the forehead or in the hollow behind the ears, and the second electrode is placed on the back, side of the body, or forelimb. In pigs and sheep, this method simultaneously induces instantaneous insensibility and cardiac arrest10,12. The head electrode must not be placed on the neck, because failure to induce an epileptic seizure causes suffering12. Properly and poorly stunned animals look similar, because cardiac arrest masks the clinical signs of the grand-mal seizure12. The animal may be conscious, but appears to be properly stunned.
When cardiac-arrest stunning is used, electrode positions and electrical settings must be verified by measures of electrical or neurotransmitter activity in the brain9,13. Only scientifically verified electrode positions and electrical settings should be used. Visual assessment must not be used to verify new settings for cardiac-arrest stunning. New settings must be verified by scientific research to ensure that instantaneous insensibility occurs.
For large (108 kg) market-weight hogs, a minimum of 1.25 A at 300 V for one second should be used10. For slightly smaller pigs, the voltage can be dropped to 250 V7. For sheep, a minimum of 1 A at 375 V for three seconds should be used14. In New Zealand, electrical stunning is being successfully used in cattle15-17. Electrical stunning of cattle requires the use of a restraint device to hold the head17. Unlike pigs or sheep, cattle must have a stunning current (2.5 A) passed through the brain before the head-to-body cardiac-arrest current is applied17. In one study15, a single, 400 V 1.5 A current passed from the neck to the brisket failed to induce epileptic form changes in the electroencephalographic recordings. Equipment manufacturers have found that a minimum of 400 to 450 V is required to achieve insensibility. Practical experi- ence indicates that greater amperages and voltages are required to achieve insensibility than to achieve cardiac arrest.
Amperage (current) is the most important factor in inducing unconsciousness9. Modern stunning circuits use a constant amperage power source, and voltage (electrical potential) varies with animal resistance. Some slaughter plants attempt to reduce meat quality defects by lowering the amperage or using high frequencies. This must not be permitted. Petechial hemorrhages can be minimized by the use of a constant amperage power supply18. Electrical frequencies of over 200 Hz must not be used unless they are scientifically verified9. In one report, a frequency of 500 Hz failed to induce unconsciousness. Most electrical stunning devices operate at 50 to 60 Hz.
In animals that have been shot with a captive bolt or stunned electrically, the limbs may move. Random limb movement should be ignored, but a limb that responds vigorously in response to a stimulus is a possible sign of return to sensibility. After the animal is hung on the overhead rail, the head must hang straight down and the neck should be limp. The tongue should hang out and the ears should droop down. Gasping and gagging reflexes are permissible7, but rhythmic breathing and vocalization must be absent. The animals must not have an arched-back righting reflex. Fully conscious animals suspended upside-down arch their backs in an attempt to lift their heads.
To reduce excitation during anesthetic induction, pigs should be rapidly exposed to 80 to 90% CO221. Veterinarians at slaughter plants should monitor CO2 concentrations because plant management may be tempted to lower concentrations to save money. Observations in the field have indicated that pigs that walk quietly into the CO2 chamber have a milder excitation phase than agitated, excited pigs. A new CO2 stunning system in Denmark appeared to greatly reduce excitement and squealing during handling because groups of five pigs were moved into the chamber at one time. The author observed that these pigs had little reaction when they first contacted the gas, and the motorific excitation phase appeared to occur after they became unconscious. One possible explanation for this observation is that Denmark has a low prevalence of pigs sensitive to halothane.
Findings in studies28-30, have indicated that cortisol concentrations can double or triple when cattle slip on slick floors, are restrained in poorly designed equipment, or are over-prodded. When this occurs, cortisol concentrations may greatly exceed on-farm handling concentrations. Epinephrine and norepinephrine are of limited value for evaluating preslaughter stress, because electric and captive-bolt stunning trigger massive release of these substances31,32. When stunning is performed correctly, the animal does not feel any discomfort because it is unconscious when the hormones are released.
ln large (1,000 head/h) pork-slaughter plants, it is likely that hogs experience more stress than from on-farm handling because they squeal and jam together as they move through the single-file chute. To improve conditions for the hogs' welfare and for pork quality, two restrainer systems may be required in high—speed plants. Providing confinement hogs with rubber hoses to chew on and additional contact with people during finishing results in calmer hogs that are easier to handle. Indiscriminate genetic selection for leanness and rapid growth tends to produce nervous, excitable hogs33.
Figure 2: Diagram of an animal’s flight zone (personal space). To move the animal forward, the handler must stand behind the point of balance at the shoulder and should work on the edge of the flight zone. Moving to position B causes the animal to move forward, whereas moving to position A usually causes the animal to stop.
In new and old facilities, distractions that cause animals to balk must be eliminated. The author has observed that distractions, such as shadows, puddles, light reflections on the floor, and visible people ahead, can ruin the performance of a well-designed system. Moving an overhead lamp to eliminate a water reflection or installing a shield to prevent approaching animals from seeing a person ahead facilitates animal movement. Veterinarians need to look up chutes and see what the animals see. Lamps can be used to attract animals into dark chutes and restrainers. The light must not glare in the eyes of approaching animals or cause reflections off the floor or chute walls. Equipment should be designed to minimize noise. High pitched motor sounds, air hissing, and metal clanging and banging are more likely to cause excitement than the low rumble of a conveyor. Ventilation systems must not blow slaughter or rendering smells into the faces of approaching animals. The novelty of a smell or shadow causes animals to balk. Novelty is highly stressful when livestock are being handled in an unfamiliar environment. The author has observed that in a familiar environment, such as a feedlot pen, animals initially fear a novel stimulus, such as a loader used for pen cleaning. After they learn that it will not hurt them, it becomes environmental enrichment and the animals may approach and lick a parked machine. A spot of blood on the floor of the chute sometimes impedes animal movement. This appears to be attributable to visual contrast. The author has observed that throwing a piece of paper in a chute has the same effect.
Possibly, the substance that the cattle are smelling is cortisol or some other substance that is secreted in conjunction with cortisol. Cortisol is present in the blood and saliva of cattle.39 Cortisol is a time-dependent measure, up to 20 minutes is required to reach peak values.40 The time course of cortisol secretion fits the author’s observations. If an animal is stressed for only a few seconds by an electric prod, the next animal usually remains calm and walks into the restrainer. The most serious balking and refusals to enter occur after an animal has become seriously stressed by becoming jammed in a piece of equipment. The other cattle often balk and refuse to enter for several hours.
Large slaughter plants use a variety of restraint devices for holding animals during stunning and slaughter35,42,43. Proper operation is essential for good animal welfare. The best equipment causes stress and suffering if it is operated roughly and animals are poked repeatedly with electric prods.
It is beyond the scope of this article to discuss equipment design in detail, but some basic principles should be mentioned. Solid sides should be installed on chutes, crowd pens, and restraint devices34. Solid sides keep animals calmer because they block outside distractions and prevent animals from seeing people deep inside their flight zone. Solid sides also make an animal feel more secure because there is a solid barrier between it and a threatening person. A basic principle is that an animal remains calmer in a restraint device if its vision is blocked until it has the feeling of restraint43,44. On conveyor restrainers, the solid hold-down over the entrance must be long enough to block the animal’s vision until its rear feet are off the entrance ramp and it is completely settled down on the conveyor42.
A second basic principle is that sudden jerky motions of equipment or people excite animals, and slow steady movements have a calming effect. A third principle is the concept of optimal pressure. A restraint device should apply enough pressure to provide a feeling of restraint, but excessive pressure, which would cause pain, must be avoided. If an animal struggles because of excessive pressure, the pressure should be slowly released. When head restraint equipment is used, the animal should be stunned or ritually slaughtered immediately after the head is restrained. On restraint devices with moving parts that press against the animal, pressure limiting valves must be installed to prevent discomfort. A pressure limiting valve automatically prevents a careless operator from applying excessive pressure.
Because ritual slaughter is exempt, some plants use cruel methods of restraint, such as suspending a conscious animal by a chain wrapped around one hind limb. In other plants, the animal is held in a restrainer that holds it in an upright position43,45,45. Whether or not ritual slaughter conforms to the requirements of euthanasia is a controversial question. When ritual slaughter is being evaluated, the variable of restraint method must be separated from the act of throat cutting without prior stunning. Distressful restraint methods mask the animals’ reactions to the cut.
The author designed and operated four state-of-the-art restraint devices that hold cattle and calves in a comfortable upright position during kosher (Jewish; Fig 3) slaughter35,42,43,46. To determine whether cattle feel the throat cut, at one plant the author deliberately applied the head restrainer so lightly that the animals could pull their heads out. None of the 10 cattle moved or attempted to pull their heads out. Observations of hundreds of cattle and calves during kosher slaughter indicated that there was a slight quiver when the knife first contacted the throat. Invasion of the cattle’s flight zone by touching its head caused a bigger reaction. In another informal experiment, mature bulls and Holstein cows were gently restrained in a head holder with no body restraint. All of them stood still during the cut and did not appear to feel it. Disturbing the edges of the incision or bumping it against the equipment, however, is likely to cause pain. Observations by the author also indicated that the head must be restrained in such a manner that the incision does not close back over the knife. Cattle and sheep struggle violently if the edges of the incision touch during the cut.
Figure 3: Head-holding device for ritual slaughter mounted on a double-rail (center-track) restrainer. The chin lift and forehead brachet should be equipped with pressure-limiting valves. The 15 cm wide forehead brachet with a 7.5 cm round pipe behind the animal’s poll holds the animal's head securely with little pressure. The pipe prevents the animal from pulling its head out. The pipe and forehead bracket are covered with rubber belting.
The design of the knife and the cutting technique appeared to be critical in preventing the animal from reacting to the cut. In kosher slaughter, a straight, razor-sharp knife that is twice the width of the throat is required, and the cut must be made in a single continuous motion. For halal (Muslim) slaughter, there are no knife-design requirements. Halal slaughter performed with short knives and multiple hacking cuts resulted in a vigorous reaction from cattle. Fortunately, many Muslim religious authorities accept preslaughter stunning. Muslims should be encouraged to stun the cattle or use long, straight, razor-sharp knives that are similar to the ones used for kosher slaughter.
Investigators agree that kosher slaughter does not induce instantaneous unconsciousness47,48,b. In some cattle, consciousness is prolonged for over 60 seconds. Observations by the author indicated that near immediate collapse can be induced in over 95% of cattle if the ritual slaughterer makes a rapid, deep cut close to the jawbone45. Further observations indicated that calm cows and bulls lose sensibility and collapse more quickly than cattle with visible signs of agitation. The author has observed that cattle that fight restraint are more likely to have prolonged sensibility. Gentle operation of restraint devices facilitates rapid loss of sensibility.
Cattle do not appear distressed even when the onset of unconsciousness is delayed. Pain and distress cannot be determined by measurements such as an electroencephalogram. Behavioral observations, however, are valid measures for assessing pain49. The author has observed that cattle appear unaware that their throat is cut. Investigators in New Zealand have made similar observations50. Immediately after the cut, the head holder should be loosened slightly to allow the animal to relax. The author also has observed that after the head restraint is released, the animal collapses almost immediately or stands and looks around like a normal, alert animal. Within 5 to 60 seconds, cattle go into a hypoxic spasm and sensibility appears to be lost. The spasms are similar to those that occur when cattle become unconscious in a headgate that is used for restraint in feedlots. Practical experience has shown that pressure on the carotid arteries and surrounding areas of the neck from a V-shaped headgate stanchion can kill cattle within 30 seconds.
Even though exsanguination is not an approved method of euthanasia by the AVMA1, the author has observed that kosher slaughter performed with the long, straight, razor-sharp knife does not appear to be painful. This is an area that needs further research. One can conclude that it is probably less distressful than poorly performed captive—bolt or electrical stunning methods, which release large amounts of epinephrine31,32.
Welfare can be greatly improved by use of a device that restrains the animal in a comfortable upright position. For cattle and calves, a conveyor restrainer or an upright restraint pen can be used42,43,46. In small plants, sheep or goats can be held by a person. If an upright pen is used, vertical travel of the lift under the animal’s belly should be restricted to 71 cm to prevent the animal from being lifted off the floor. A pressure limiting valve must be installed on the head holder and rear pusher gate43,45. Many existing upright restraint boxes apply excessive pressure. To prevent excessive bending of the neck, the head holder should position the animal's forehead parallel to the floor. Equipping the head holder with a 15-cm wide, rubber covered forehead bracket will make the head holder more comfortable (Fig 3). The animal should stand in the box with its back level. An arched back is a sign of excessive pusher-gate pressure. In some plants, animals are removed from the restrainer before they become unconscious. Discomfort to the animal can be minimized by allowing it to lapse into unconsciousness before it is removed from the restrainer.
During the past five years, many large kosher slaughter plants for cattle have replaced shackling and hoisting with upright restraint. Large numbers of veal calves and sheep, however, are still shackled and hoisted. Progressive plant owners have installed upright restraint equipment, but unfortunately there are some plant owners who still refuse to install humane restraint equipment because they are not legally required to do so. Animal handling guidelines published by the American Meat institute recommend the use of upright restraint6.
a Koch Supplies, Kansas City, MO
b Nangeroni LL, Kennett PD. Department of Physiology, Cornell University, lthaca, NY: Unpublished data, 1963.
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