Thomas Passler Department of Clinical Sciences, College of Veterinary Medicine, Auburn University, Auburn, AL, USA As implied by the Greek terms “eu” meaning good and “thanatos” meaning death, the process of euthanasia should end the life of an animal by procedures that eliminate or minimize pain and distress [1]. When a decision to euthanize an animal has been made, three phases of the process must be considered in an effort to achieve this ideal balance: (i) appropriate animal handling prior to euthanasia to minimize pain and distress, (ii) appropriate technique of euthanasia, and (iii) legal and safe disposal of the carcass. The decision to euthanize a farm animal is often not exclusively influenced by the desire to relieve animal suffering, as factors such as financial costs and practicality of continued therapy have to be considered; however, it is the veterinarians’ and caregivers’ responsibility to utilize a method of euthanasia that provides the ideal balance of minimal pain and distress while considering the environment in which the euthanasia is performed. Veterinary guidance is critical in forming the decision to euthanize a farm animal, as the prognoses for life and function based on the veterinary examination can establish whether euthanasia is the most appropriate option, or whether salvage by slaughter of a chronically ill animal is a morally and financially sound option. While euthanasia is easily recognized as the only viable option in certain conditions such as fractures of the spine or severe painful trauma that cannot be relieved by treatment, the necessity for euthanasia may not be as easily appreciable with other disorders, especially by farm managers and caregivers [2]. An increasing body of research has evaluated physiological and behavioral indicators of pain in farm animals (e.g. [3–6]), but the accurate and objective assessment of pain and resulting physical suffering remain difficult. In surveys assessing perceptions of bovine veterinarians on pain in cattle, significant variation existed among individuals about the perceived amount of pain associated with disease or veterinary procedures, and the utilized methods of pain control also differed significantly among veterinarians [7–9]. Veterinarians’ opinions also vary on the topic of which health conditions necessitate immediate euthanasia, continued therapy, or salvage by slaughter [10]. Therefore, it is reasonable to infer that for animal caregivers, who have varying degrees of knowledge and experience, the accurate assessment of pain and need for euthanasia can be even more challenging [11]. Some diseases may be more visually striking but less painful (e.g. rectal prolapse), while others are more painful but inconspicuous (e.g. chronic arthritis) [12], and misinterpretation of clinical signs could result in inappropriate administration of euthanasia procedures. When immediate and direct veterinary advice is not available, and the decision to euthanize is made by animal caregivers, creation of clear criteria for euthanasia in the form of standardized euthanasia protocols can help to alleviate employee stress associated with euthanasia and improve the overall welfare of a herd [12, 13]. Figure 13.1 Example of a decision algorithm that can be implemented as part of an on‐farm euthanasia plan to provide farm personnel with clear direction for the handling of sick and debilitated animals. Source: adapted from [14] . On‐farm euthanasia plans should be part of the overall herd‐health management program and should include specific planning for the three phases of euthanasia. A decision tree can be used to determine the approach to a sick or debilitated animal (Figure 13.1). This decision tree can be modified to include treatment modalities, duration of attempted therapy before deciding on euthanasia, and methods of euthanasia as part of a farm‐specific herd management plan. Developing clear criteria for sick animals can ensure consistent decisions among farm personnel and timely implementation of euthanasia procedures [14]. General guidelines have been proposed to aid in the decision‐making process on whether an animal should be euthanized [14–16]. These guidelines commonly include specific endpoint criteria that form indications for euthanasia such as: In adaption for euthanasia guidelines for small animals, generalized guidelines for use in swine operations have been proposed [12]: As part of an on‐farm euthanasia plan, procedures for handling debilitated animals prior to euthanasia should be reviewed. The prevention of pain and distress during handling before euthanasia can often be achieved by simple environmental changes, and animal welfare recommendations for cattle transported and handled for slaughter apply [17–20]. Separation of the animal to be euthanized from healthy conspecifics is generally advisable to prevent distress and safety concerns, such as from a missed or ricocheting gunshot. Stress responses including vocalization, fearful behavior, and olfactory stress signals can communicate anxiety and apprehension among animals [21–23], and it is often easiest to remove all healthy animals from the area in which euthanasia will be performed. While ambulatory animals may be moved for short distances to a location where euthanasia procedures and carcass removal can be more effectively performed, fractious or recumbent animals may best be euthanized at their present location. Ensuring the prevention of pain and distress, animals that tolerate human contact can be moved using appropriate carts or sleds. As most methods of euthanasia require close contact to the animal, animals must be restrained by a halter or chute. The required restraint should be chosen based on the disposition of the animal and the method of euthanasia. Restraint by a halter can be adequate for small and large farm animals accustomed to human handling and allow intravenous administration of barbiturates or accurate placement of a penetrating captive bolt. Premedication with sedatives or tranquilizers should be considered to ensure safety of personnel and reduce distress when euthanizing fractious animals. Premedication can be administered in a chute if possible with minimal pain and distress, or can be administered by blow‐dart or dart‐gun. For some animals, gunshot may be the most appropriate form of euthanasia, as it requires the least amount of human contact. Any appropriate method of euthanasia is aimed at inducing unconsciousness as rapidly as possible. Unconsciousness is defined as the loss of individual awareness and occurs when the brain’s ability to integrate information is blocked or disrupted [1, 24]. The underlying mechanisms that achieve unconsciousness in anesthetized individuals are still incompletely understood, but most anesthetic agents appear to induce an unconscious state by deactivation and disintegration of the posterior corticothalamic complex [25]. In animals under general anesthesia, movement can be observed despite the presence of a state of unconsciousness and amnesia, which is a result of continued spinal cord activity, as memory and awareness are abolished at anesthetic concentrations which are less than half of those needed to abolish movement [26]. While much more pronounced deactivation of cortical function and rapid loss of consciousness are expected with methods of euthanasia, it is important to ensure that all activities observed in an animal to which a method of physical or chemical euthanasia has been administered occur subsequent to unconsciousness. In animals, the loss of unconsciousness is defined as loss of the righting reflex [24]. By means of electroencephalography (EEG), various studies have evaluated brain function in animals at slaughter and although data obtained with EEG are limited in their ability to precisely determine the onset of unconsciousness (e.g. [27–30]), indicators of successful stunning in slaughtered animals also apply to euthanasia. While weakly present in rare cases, the corneal reflex is expected to be absent after successful stunning by captive bolt [27, 31]. After stunning, immediate collapse, followed by a period of tetanic spasticity and then clonic slow hind limb movements with increasing frequency occur [24]. In cattle and small ruminants, loss of righting reflex, loss of rhythmic breathing, protrusion of tongue, absence of corneal reflex, and absence of nystagmus indicate successful stunning by captive bolt [31, 32]. During euthanasia, loss of residual muscle movement and cardiac and respiratory arrest occur subsequent to loss of unconsciousness. Before movement and disposal of the carcass, death must be confirmed. A combination of criteria is most reliable to confirm death, which includes absence of pulse and corneal reflexes, failure to respond to strong pinches to the nose or interdigital space, inability to auscultate respiratory and heart sound by stethoscope, graying of mucous membranes, and rigor mortis. Only the presence rigor mortis can, by itself, serve as confirmation of death, and other criteria must be verified in combination [24]. The American Veterinary Medical Association (AVMA) Panel on Euthanasia categorizes methods of euthanasia as being acceptable, conditionally acceptable, or unacceptable [24]. Additionally, the panel provides species‐specific adjunctive methods that can be used in conjunction with other methods to produce a humane death and ensure death in an unconscious animal. An acceptable method of euthanasia is one that consistently produces a humane death when used as the only means of euthanasia. For all farm animals, euthanasia by injection of barbiturates and barbituric acid derivatives is categorized as an acceptable method. Methods that are considered conditionally accepted vary by species and age, but for all farm animals, including cattle, small ruminants, and swine, physical methods such as gunshot and penetrating captive bolt are conditionally accepted. Properly administered, conditionally accepted methods ensure a humane death, but because of the greater potential for operator error or safety hazards, they might not do so as consistently as acceptable methods. Additionally, conditionally acceptable methods include those not well documented in the literature [24]. In addition to barbiturates and barbituric acid derivatives, the AVMA Guidelines on Euthanasia discuss other injectable euthanizing agents, including tributame, T‐61, and ultrapotent opioids. Unavailable in the USA, T‐61 was evaluated in German cattle and induced death less rapidly and with more excitatory events than a sodium pentobarbital‐containing euthanasia solution [33]. Availability and practicality make barbiturate‐containing solutions the most commonly used injectable method of euthanasia in farm animals. Barbiturates rapidly induce unconsciousness by depressing the function of the cerebral cortex and result in collapse of the animal. In a step‐wise fashion, other central nervous system (CNS) functions are subsequently depressed, with reduction in blood pressure, depression of the medullary respiratory center and apnea, cerebral death, and subsequently cardiac arrest. Various manufacturers currently provide commercially available euthanasia solutions containing sodium pentobarbital at similar concentrations and sometimes in combination with local anesthetics and other CNS depressants such as phenytoin sodium. Additionally, euthanasia solutions contain a dye to ensure ease of identification. Unlike sodium pentobarbital, which is a Drug Enforcement Administration (DEA) schedule II drug, many commercially available euthanasia solutions are categorized as schedule III drugs, enabling less strict regulations concerning their availability and storage. Most euthanasia solutions are labeled only for dogs or dogs and other small animals, and dosing information for farm animals is extrapolated. Product information from a euthanasia solution labeled for large animals recommends a dose of 1 ml/4.5 kg (10 lb) of body weight, to a maximum of 100 ml, in accordance with its small animal dosage and label information for other products (40 mg of sodium pentobarbital per kg [2.2 lb] of body weight) [33, 34]. Barbiturate solutions should always be administered by intravenous route, which ensures rapid distribution to the CNS. However, intravenous injection can be complicated in fractious or aggressive animals. In these cases, tranquilization or identification of alternative methods of restraint prior to euthanasia should be considered. Alternatively, intraperitoneal administration of nonirritating formulations of sodium pentobarbital may be a substitute to intravenous injection and is considered acceptable by the AVMA Panel on Euthanasia; however, it is not practical for many large farm animals [24]. For euthanasia by intraperitoneal route, larger quantities of barbiturate solution should be administered, which can be cost prohibitive. Intraperitoneal injection of euthanasia solutions results in a significantly greater duration until loss of consciousness as compared to intravenous administration [35]. While some solutions without an added local anesthetic are labeled for intraperitoneal injection, this route of administration may result in pain, which can be reduced by addition of lidocaine to sodium pentobarbital formulation (e.g. 10 mg of lidocaine per ml of sodium pentobarbital) [36]. Failure of injecting the peritoneal cavity was reported in 6.7% and 58% of intraperitoneally injected piglets and adult cats, respectively, and subsequent necropsies demonstrated that the euthanasia solution was delivered into the urinary bladder, gastrointestinal tract, or spleen [35, 37]. While nonvascular administration of injectable euthanasia agents, including subcutaneous, intramuscular, or intrathoracic, is not acceptable, in unconscious animals injection into organs such as intracardiac injection is considered conditionally acceptable. Unacceptable as the sole method of euthanasia, saturated solutions of potassium chloride, magnesium chloride, or magnesium sulfate are categorized as adjunctive methods of euthanasia for large and small ruminants [24]. These can be used in anesthetized or unconscious animals and induce cardiac or respiratory arrest following intravenous or intracardiac administration. Prior to administration, the surgical plane of anesthesia must be confirmed [24]. While injectable anesthetic protocols such a ketamine/xylazine are able to induce unconsciousness, sole use of alpha‐2 agonists such as xylazine prior to potassium chloride or magnesium sulfate administration comprises an unacceptable method of euthanasia [24, 38]. The AVMA Panel on Euthanasia categorizes euthanasia by gunshot and penetrating captive bolt as conditionally acceptable methods for cattle, small ruminants, and all age groups of swine, except suckling pigs [24]. Firearms are widely available and gunshot is often the most practical method of euthanasia, especially when veterinary assistance is unavailable. On dairy farms, on‐farm euthanasia is most commonly performed by gunshot (85–90%), and only a minority of farms utilized barbiturates or captive bolt for euthanasia [39, 40]. On dairy farms, decisions regarding euthanasia were made in absence of veterinary consultation in 67.7% of surveyed farms, and larger farms were less likely to seek veterinary assistance for euthanasia [40]. The absence of veterinary supervision, safety concerns for personnel that perform euthanasia, and the emotional effects of the procedure on farm personnel are challenges of on‐farm euthanasia by physical methods [12]. These challenges should be addressed by providing adequate training by veterinarians and farm managers because physical methods of euthanasia may be best suited to ensure rapid relief of pain and suffering in different situations [12, 41]. The goal of euthanasia by gunshot or captive bolt is the immediate loss of consciousness by inducing massive damage to the cerebral hemispheres and vital regions of the brainstem [42]. To reliably achieve this goal, accurate placement of the shot or captive bolt on the animal’s head is critical, and has been reviewed [24
13
Euthanasia of Farm Animals
13.1 Considerations Prior to Euthanasia
13.2 The Process of Euthanasia
13.3 Methods of Euthanasia
13.3.1 Injectable Euthanasia Agents
13.3.2 Physical Methods
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