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Animals 9 (11): 911 (2019)
University of British Columbia. Animal Welfare Program
Steiner, Aline R.
Axiak Flammer, Shannon
Beausoleil, Ngaio J.
Berg, Charlotte
Bettschart-Wolfensberger, Regula
García Pinillos, Rebeca
Golledge, Huw D.R.
Marahrens, Michael
Meyer, Robert
Schnitzer, Tobias
Toscano, Michael J.
Turner, Patricia V.
Weary, Daniel M.
Gent, Thomas C.
The use of carbon dioxide (CO₂) for stunning and killing animals is considered to compromise welfare due to air hunger, anxiety, fear, and pain. Despite decades of research, no alternatives have so far been found that provide a safe and reliable way to induce unconsciousness in groups of animals, and also cause less distress than CO₂. Here, we revisit the current and historical literature to identify key research questions that may lead to the identification and implementation of more humane alternatives to induce unconsciousness in mice, rats, poultry, and pigs. In addition to the evaluation of novel methods and agents, we identify the need to standardise the terminology and behavioural assays within the field. We further reason that more accurate measurements of consciousness state are needed and serve as a central component in the assessment of suffering. Therefore, we propose a roadmap toward improving animal welfare during end-of-life procedures.
animalsCommentaryHumanely Ending the Life of Animals: ResearchPriorities to Identify Alternatives to Carbon DioxideAline R. Steiner 1, Shannon Axiak Flammer 2 , Ngaio J. Beausoleil 3, Charlotte Berg 4 ,Regula Bettschart-Wolfensberger 1 , Rebeca García Pinillos 5, Huw D.R. Golledge 6 ,Michael Marahrens 7, Robert Meyer 8, Tobias Schnitzer 9, Michael J. Toscano 10,Patricia V. Turner 11 , Daniel M. Weary 12 and Thomas C. Gent 1,*1 Department of Clinical and Diagnostic Services, Section of Anaesthesiology, Vetsuisse Faculty, University ofZurich, Winterthurerstrasse 258c, 8057 Zurich, Switzerland; asteiner@vetclinics.uzh.ch (A.R.S.);rbettschart@vetclinics.uzh.ch (R.B.-W.)2 Department of Clinical Veterinary Medicine, Section of Anesthesia and Analgesia, Vetsuisse Faculty,University of Berne, Laenggassstrasse 124, 3012 Bern, Switzerland; shannon.axiak@vetsuisse.unibe.ch3 Animal Welfare Science and Bioethics Centre, School of Veterinary Science, Massey University,Palmerston North 4410, New Zealand; N.J.Beausoleil@massey.ac.nz4 Department of Animal Environment and Health, Swedish University of Agricultural Sciences, Box 234,SE-53223 Skara, Sweden; Lotta.Berg@slu.se5 Animal and Plant Health Agency and Department for Environment, Food and Rural Affairs, Nobel House,17 Smith Square, London SW1P 3JR, UK; rebeca.garcia@apha.gsi.gov.uk6 Universities Federation for Animal Welfare (UFAW), The Old School, Brewhouse Hill, Wheathampstead,Hertfordshire AL4 8AN, UK; golledge@ufaw.org.uk7 Institute of Animal Welfare and Animal Husbandry, Friedrich-Loeffler-Institut, Dörnbergstraße 25/27,29223 Celle, Germany; Michael.marahrens@fli.de8 Department of Clinical Sciences, College of Veterinary Medicine, Mississippi State University, MississippiState, MS 39762, USA; robert.meyer@msstate.edu9 Roche Pharma Research and Early Development, Pharmaceutical Sciences, Roche Innovation Center Basel, F.Hoffmann-La Roche Ltd., Grenzacherstrasse 124, 4070 Basel, Switzerland; tobias.schnitzer@roche.com10 Center for Proper Housing: Poultry and Rabbits (ZTHZ), Animal Welfare Division, VPH Institute,University of Bern, 3052 Zollikofen, Switzerland; michael.toscano@vetsuisse.unibe.ch11 Department of Pathobiology, University of Guelph, Guelph, ON N1G 2W1, Canada and Charles River,Wilmington, MA 01887, USA; Patricia.Turner@crl.com12 Animal Welfare Program, University of British Colombia, 2357 Main Mall, Vancouver, BC V6T 1Z4, Canada;dan.weary@ubc.ca* Correspondence: tgent@vetclinics.uzh.chReceived: 19 September 2019; Accepted: 25 October 2019; Published: 2 November 2019Simple Summary: Carbon dioxide is commonly used for stunning animals prior to killing. It allowsseveral animals to be killed at once, reduces the need for handling, and is a reliable method. However,research in laboratory rodents, poultry, and pigs has indicated that it causes considerable aversion atconcentrations above ambient conditions. Currently, there are no available alternatives with desirablecharacteristics. This manuscript describes a list of research priorities to find and implement the use ofalternative methods or agents to improve animal welfare.Abstract: The use of carbon dioxide (CO2) for stunning and killing animals is considered tocompromise welfare due to air hunger, anxiety, fear, and pain. Despite decades of research, noalternatives have so far been found that provide a safe and reliable way to induce unconsciousness ingroups of animals, and also cause less distress than CO2. Here, we revisit the current and historicalliterature to identify key research questions that may lead to the identification and implementation ofmore humane alternatives to induce unconsciousness in mice, rats, poultry, and pigs. In addition tothe evaluation of novel methods and agents, we identify the need to standardise the terminologyAnimals 2019, 9, 911; doi:10.3390/ani9110911 www.mdpi.com/journal/animalsAnimals 2019, 9, 911 2 of 25and behavioural assays within the field. We further reason that more accurate measurements ofconsciousness state are needed and serve as a central component in the assessment of suffering.Therefore, we propose a roadmap toward improving animal welfare during end-of-life procedures.Keywords: animal welfare; carbon dioxide; stunning; killing; euthanasia; rodents; poultry; pigs;aversion; air hunger1. DefinitionsAbnormal respiratory pattern—any deviation from what is considered the normal physiologicrange in that species in that environment (e.g., tachypnoea, apnoea, irregular rhythm, shallower ordeeper inhalation, dyspnoea). Refers to the physical respiratory activity that can be observed and notto associated experiences/sensations [1,2].Air hunger—the result of the discrepancy between the drive for respiratory and lung inflation [3].Reported as the most unpleasant component of dyspnoea by humans, and even at moderate levels ismore unpleasant than maximal respiratory effort [4].Anxiety—a negative emotional state characterised by anticipation of real or potential threats,typically enhances environmental scanning [5].Apnoea—absence of breathing [6].Aversion—“avoidance of a stimulus, situation or behaviour” [7]. Refers to the theoreticallybeneficial behaviours expressed to avoid or withdraw from potentially noxious stimuli or situationsrather than to the underlying mental or affective status. Aversion, per se, does not indicate that ananimal is suffering; however, suffering may result from inescapable exposure to aversive stimuli orsituations [7,8].CAS—controlled atmosphere stunning. Any method changing inspired gas concentrations orambient pressure in a way that leads to unconsciousness [9,10].Chest tightness—a sensation described by humans upon bronchoconstriction [3].Distress—unlike stress, distress is “an aversive, negative state in which coping and adaptationprocesses fail to return an organism to physiological and/or psychological homeostasis” [11].Dyspnoea—“laboured” breathing with observable respiratory effort [6] (intended use in thisdocument). In human medicine, the term dyspnoea describes the unpleasant sensation of air hungerand/or increased respiratory effort and/or chest tightness [3]. In humans, laboured breathing can occurwithout the sensation of dyspnoea and vice versa [3].Euthanasia—literally “good death”; defined as “ending the life of an individual animal in a waythat minimises or eliminates pain and/or distress” [12]. Controversy exists whether use of the term“euthanasia” additionally requires that animals are killed in accord with their own interests and/orbecause killing would relieve an unacceptable welfare state (e.g., [6] versus [12]). For this document,euthanasia is defined as killing that minimises welfare impacts and is in accord with an animal’sown interest; for example, the killing of sick animals or when a humane endpoint is reached in anexperiment. Euthanasia is not used to describe death solely for the purposes of meat consumption(slaughter), the depopulation of surplus healthy animals, or to harvest tissues from healthy animals fordata collection.Fear—a response to a present threat, typically initiates behaviours to negate the situation [5,13].Gasping—describes two different situations: Either, a breathing pattern caused by extremehypoxia [partial pressure of arterial oxygen (PaO2) <5 mmHg], that is potentially life-saving (restoresPaO2 to 30 mmHg [14], produces cardiac output during cardiac arrest [15], and increases survival inhumans with cardiac arrest [16]); otherwise, it is called agonal, and occurs under circumstances inwhich consciousness has typically already been lost [14,16]. Alternatively, it describes deep inhalationsAnimals 2019, 9, 911 3 of 25with or without open mouth/bill and extended neck that is observed in conscious animals exposed tocarbon dioxide (CO2) (i.e., before loss of posture) [17,18].Killing—general term for inducing death, regardless of the purpose and the method used [12,19].LAPS—low atmospheric pressure stunning. A CAS method gradually reducing ambient pressureand thereby partial pressure of oxygen, i.e., causing hypobaric hypoxia [9].Nociception—the detection, transmission, and processing of noxious (i.e., harmful) stimuli bynociceptors, to and within the central nervous system. Nociception refers to the physiologic basisrequired but not sufficient for perception of pain; however, it can occur in the absence of consciousnesswhere pain is absent [6].Pain—“an unpleasant sensory and emotional experience associated with actual or potential tissuedamage, or described in terms of such damage” [20].Respiratory effort—an experience relating to the amount of respiratory muscle force that iscurrently applied to generate ventilation. In humans, conscious awareness of respiratory effort ariseswhen the contribution of voluntary motor command to overall respiratory muscle drive increases [2].The sensation becomes unpleasant when the motor command needed to elicit a given level of ventilationis greater than normal, which may occur in animals with respiratory disease.Stress—“real or perceived perturbation to physiologic homeostasis or psychologic well-being”,effects can be “positive, negative, or inconsequential”; i.e., stress does not necessarily involve a negativeemotional component [11,21].Stunning—inducing unconsciousness prior to killing. The European Council uses the terms“simple stunning” when a loss of consciousness is reversible, and “stunning” when a loss ofconsciousness is irreversible [22]. However, in this document, this differentiation will not be made.Superficial/deep breathing—smaller/larger tidal volume than at resting baseline conditions.Tachypnoea—increased respiratory rate [6].Tonic Immobility Test—a test to determine fear levels in poultry. Birds are repeatedly restrainedmanually for a short period (typically 15 s) to simulate predation until they remain immobile afterrelease of restraint for at least 10 s, i.e., “feign death”. Number of attempts required to induce immobilityand total time of immobility are assessed; fewer attempts needed and a longer duration of immobilityare interpreted as higher fear levels [23].Welfare (state)—the state of welfare of an animal can be positive, negative, or neutral, dependingon the total of positive and negative affective states experienced within the relevant time period [24].2. IntroductionThis research strategy is an initiative of the Swiss Federal Food Safety and Veterinary Office (FSVO)aimed at determining research priorities to identify and implement alternative stunning methods forlive animals, specifically: mice, rats, pigs, and poultry. Whilst specific issues relating to carbon dioxideare discussed, it is not intended to be a comprehensive literature review on carbon dioxide stunning.Furthermore, this article is not an exhaustive list of research questions. Here, we focus on identifyinghumane methods to render animals unconscious prior to killing, rather than the methods of killing(Figure 1).An ideal stunning or euthanasia method should first reliably induce loss of consciousness withoutcausing distress and persist until death occurs (due either to the stunning method or to a secondarykilling method applied after loss of consciousness). Furthermore, the chosen stunning method shouldbe compatible with the purpose of the animal, (e.g., residual-free when animals are killed for meatconsumption) and also compatible with subsequent investigations when laboratory animals arekilled [12]. Of secondary importance, an ideal method should be both physically safe, easy to use andhave low psychological impact for the human operator [25]. Furthermore, it should be cost-effectiveand not environmentally damaging [12,26].Carbon dioxide (CO2) is one of the most commonly used agents for stunning prior to killingrodents [7,21], pigs [27], and poultry [28]. Controlled atmosphere (or non-physical) stunning (CAS)Animals 2019, 9, 911 4 of 25methods are attractive since they require a minimal physical restraint of animals, thereby reducingdirect interaction between animals and the operator, which may be a cause of stress. CO2 is cheap,and systems can be used without extensive training [21]. Furthermore, it is denser than ambient air,making it easier to contain in unsealed vessels. It reliably produces unconsciousness and death ina concentration-dependent manner. However, its use is controversial due to reports of impacts onanimal welfare, including aversion, which may reflect negative affective states such as fear, pain, andair hunger [7].CO2 exposure induces generalised intracellular acidosis, which depresses basal and evoked neuralactivity, and thereby induces unconsciousness [29]. Depending on CO2 concentrations, species, andthe method of assessment, loss of consciousness occurs 26 to 150 s after an onset of exposure (rats: 30to 150 s [30,31]; mice: 51 to 75 s [32]; pigs: 33 to 60 s [33,34]; poultry: 20 to 26 s [9,35]). CO2 exposurehas been shown to be aversive in rats [36–39], mice [32,40,41], pigs [42,43], and poultry [44–46], tocause abnormal breathing patterns commonly interpreted as air hunger (rats [18], mice [32], pigs [33],poultry [1], humans [47]) and to elicit anxiety [18,48], which is in line with reports of increased anxietyor even panic attacks in humans [49–51]. High concentrations of CO2 elicit pain in humans [52] andactivate nociception pathways in rats [53,54] and poultry [55]. Therefore, animals are highly likely toexperience pain and distress prior to loss of consciousness.Animals 2019, 9, x FOR PEER REVIEW 4 of 25 and systems can be used without extensive training [21]. Furthermore, it is denser than ambient air, making it easier to contain in unsealed vessels. It reliably produces unconsciousness and death in a concentration-dependent manner. However, its use is controversial due to reports of impacts on animal welfare, including aversion, which may reflect negative affective states such as fear, pain, and air hunger [7]. CO2 exposure induces generalised intracellular acidosis, which depresses basal and evoked neural activity, and thereby induces unconsciousness [29]. Depending on CO2 concentrations, species, and the method of assessment, loss of consciousness occurs 26 to 150 s after an onset of exposure (rats: 30 to 150 s [30,31]; mice: 51 to 75 s [32]; pigs: 33 to 60 s [33,34]; poultry: 20 to 26 s [9,35]). CO2 exposure has been shown to be aversive in rats [36–39], mice [32,40,41], pigs [42,43], and poultry [44–46], to cause abnormal breathing patterns commonly interpreted as air hunger (rats [18], mice [32], pigs [33], poultry [1], humans [47]) and to elicit anxiety [18,48], which is in line with reports of increased anxiety or even panic attacks in humans [49–51]. High concentrations of CO2 elicit pain in humans [52] and activate nociception pathways in rats [53,54] and poultry [55]. Therefore, animals are highly likely to experience pain and distress prior to loss of consciousness. Figure 1. Structural overview of research priorities. 3. General Aspects (All Species) Minimising the welfare impacts of rendering an animal unconscious prior to killing requires a great deal of species-specific consideration [7]. In this manuscript, we detail these considerations in sections devoted to each of the species groups, namely rodents, poultry, and pigs. However, we also identified that some considerations prevail across all species. In this first section, we will discuss these multi-species issues before moving on to discuss species specifics. 3.1. Standardised Terminology, Investigation of the Animal’s Experience, and Structured Welfare Assessment 3.1.1. Statement of the Problem A comparison of findings between studies in the same species is frequently hampered by variability in descriptions of behaviours and an inconsistent use of terms to describe them. This variability is problematic, as the description and measurement of indicator variables influences how they are interpreted as reflecting impacts on welfare. For example, CO2 commonly induces inspiration of an increased tidal volume in poultry, while the bill may be open, and the neck extended. This pattern is variably described as “gasping” [17,56–58], “deep breathing” [59], or “respiratory disruption” [1,44], which has variably been interpreted as distress. Definitions and the usage of more complex terms such as aversion, anxiety, and distress vary, as do techniques to measure them. For example, aversion has been assessed by place preference testing [39], escape behaviours [40], vocalisations [43,60], and physiological parameters during forced Figure 1. Structural overview of research priorities.3. General Aspects (Al Species)Minimising the welfare i pacts f re i s rior to ki ling requires agreat deal of species-specific consideratio [7]. il t se c siderations insections devoted to each of the species r , l i . o ever, e alsoidentified that some considerations prevail acros al s ecies. fi ti , e i l discu s thesemulti-species is ues before moving on to discuss species s ifi .3.1. Standardised Terminology, Investigation of the Ani al’s Experience, a d tr t lf Assessment3.1.1. Statement of the ProblemA comparison of findings between studies in the same species is frequently hampered by variabilityin descriptions of behaviours and an inconsistent use of terms to describe them. This variabilityis problematic, as the description and measurement of indicator variables influences how they areinterpreted as reflecting impacts on welfare. For example, CO2 commonly induces inspiration of anincreased tidal volume in poultry, while the bill may be open, and the n ck extended. This pattern isvariably described as “gasping” [17,56–58], “deep breathing” [59], or “respiratory disruption” [1,44],which h s variably been int rpreted s distress.Animals 2019, 9, 911 5 of 25Definitions and the usage of more complex terms such as aversion, anxiety, and distress vary,as do techniques to measure them. For example, aversion has been assessed by place preferencetesting [39], escape behaviours [40], vocalisations [43,60], and physiological parameters during forcedexposures [61]. Terms and definitions may be used interchangeably and sometimes incorrectly; forexample, stress and distress are often used to describe each other despite having different meanings(see definitions and ref: [62]). Aversion behaviours are often interpreted as signs of distress; however,they may not necessarily indicate distress if the animal can successfully escape the aversive agent [7].There are significant challenges associated with the use of both physiological and behaviouralmethods to infer the unpleasant affective experience of pain. Pain, or the absence of, is sometimesinferred from physiological parameters that are non-specific [63]. Although it cannot be proven thatanimals experience the negative emotional states associated with pain, it is generally assumed thatprocedures that are painful for humans are also painful for animals [8]. Based on this assumption,behavioural indicators of pain have been identified, and species-specific pain scales validated [64–66].A caveat with pain scales is that facial expressions and behaviours relying on motor function may beimpaired during stunning, and sensitivity may also be agent-specific, meaning that each pain scalemust be validated for each context, making comparisons between methods difficult [64]. Furthermore,most pain scales have been developed for assessing post-surgical pain, and are therefore unlikely tobe useful in association with stunning [67]. They are also notoriously inaccurate for very short-termpain expression. Other monitoring indicators of pain and distress commonly used in abattoirs includevocalisation and falling [68].The comparison of different stunning methods across studies requires a systematic and fullydocumented approach to assess the overall pain, suffering, and distress caused, as well as practicalaspects such as repeatability and operator-related aspects.3.1.2. Research Priorities1 Standardise the use of terminology by means of a consensus list of definitions and clarifyunderlying mechanisms, where required, for unambiguous definitions. Especially for poultry, athorough characterisation of respiratory patterns and behaviours is required to allow cross-studycomparison, as those are described, categorised, and interpreted differently across studies [9]. However,inconsistent use of terminology is an issue in all species covered by this strategy.2 Standardise behavioural and physiological testing for aversion. Aversion has been investigatedby observing spontaneous or conditioned behavioural, physiological reactions, or biochemicalparameter reactions to a stimulus. For both approaches, formulating hypotheses about the expectedresponse or behaviour increases the strength of evidence that can be obtained [8]. Whilst a battery ofbehavioural tests for aversion are validated in rodents (e.g., [69,70]), variable approaches have beenused to assess aversion in poultry [17,44,46] and pigs [27,42,43]. Therefore, robust aversion tests shouldbe developed and validated in these species.The overall experience of an animal may be tested using a reward that has to be relinquished inorder to escape an aversive condition, or by offering an aversive environment (e.g., a brightly lit spacein rodents) as an alternative into which to escape. Alternatively, the stimulus in question can be pairedwith a certain environment and conditioned place preference/aversion (compared to an alternativestimulus) measured in the absence of any stimuli [8].3 Formulate robust methods to identify specific negative affective experiences such as pain, fear,and anxiety, and to grade their intensity.Assessing the welfare impact of an intervention or condition ultimately depends on identifyingaffective states an animal experiences in the relevant period of time (e.g., [24,71]). In the absence ofclear evidence for the type of affective experience, observations are often interpreted on untestedassumptions (e.g., perception of an inactive animal as less stressed than an overactive one). Whilethe analogy argument is useful to posit the presence of felt emotions in animals, more sophisticatedmethods are required to identify the type and intensity of affective experience in a specific situation.Animals 2019, 9, 911 6 of 25To address contributions of specific affective states to the overall experience, several approachescan be used. The first would be to gain a comprehensive understanding of the effects of the stimulus orcondition on the physiology and neurophysiology of the animal. This, evaluated in conjunction withan understanding of the human physiology and neurophysiology and reported affective experience,informs what affective states (and thus observed responses) should be investigated in the behaviouraltests. Subsequently, behavioural tests developed to assess a specific component, e.g., anxiety, canbe applied during and/or immediately after exposure to the stimulus in question, such as the openfield test [23,72], light/dark test, and social interaction test in rodents [73,74], or tonic immobility inpoultry [23]. For rats, it was shown that thigmotaxis in the open field test was increased and socialinteraction reduced immediately after exposure to 20% CO2 [18], indicating increased anxiety, butsimilar testing was not performed for putative alternative methods. No known data exists in poultryor pigs investigating fear or anxiety induced by any of the stunning methods. Overall, the systematicinvestigation of anxiety caused by (elements of) different stunning methods is lacking. Additionally,pain scoring systems suitable for the assessment of acute pain should be developed and validated forall species. For example, it is challenging to assess whether low atmospheric pressure stunning (LAPS)causes colic pain in broiler chickens [9], when no validated pain scoring is available.A complementary approach to investigate the contribution of specific affective states duringstunning is to examine the modulating effects of drugs that are known to reduce anxiety, dyspnoea,or pain, on behavioural and physiological responses [8]. Likewise, using drugs known to induce aspecific negative affective state, e.g., anxiogenics, animals can be trained to perform specific behavioursin that state, expecting that they will generalise this behaviour when the same negative affective stateis caused by a different stimulus [8].An inherent challenge is that different stunning methods elicit different negative affectiveexperiences due to their different physiological effects. Therefore, behavioural indicators to assesscertain negative affective experiences cannot be used universally. For instance, behavioural tests usedduring controlled atmosphere stunning (CAS) are not applicable to physical methods due to differencesin exposure times.Correlating behavioural responses with physiological measurements of endocrine andneurophysiological parameters will validate the interpretation of behavioural responses and facilitatecomparisons of different stunning methods. However, physiological parameters should only bemeasured and interpreted in conjunction with behavioural evaluations of aversion because somemethods (e.g., CAS with CO2) can elicit physiological changes that may not reliably reflect consciousexperiences (e.g., measurably raised plasma cortisol levels or respiratory lesions may occur only afterloss of consciousness) [21].Integrating the different types of negative affective experiences caused by a specific stunningmethod into an overall assessment and comparing the welfare impact of different methods cannevertheless be challenging, and the following paragraph focuses on approaches to this challenge.4 Develop a structured, systematic welfare assessment specifically for stunning, taking intoaccount all potential sources of distress, including those occurring in a minority of animals, theirseverity, and duration. Distress is an umbrella term for strongly negative affective experiences thatarise when stress is continual or cannot be compensated for. In the context of stunning, different typesof stressors are involved and elicit different types and intensities of negative affective experiences(e.g., pain, air hunger) depending on the method or agent [9]. For a thorough comparison of methods,the type and intensity of negative affective experiences should be identified, as should the temporalprogression of the intensity, as well as the duration prior to the loss of consciousness on both anindividual and population level [75]. A descriptive ranking to assess killing methods has previouslybeen established [76]. Importantly, this system does not employ numerical scaling, and thereforenegates quantitative comparisons of severity. Detailing exactly how information about the quality,intensity, duration, and progression should be weighed to give an overall assessment is beyond thescope of this document, as is whether such a tool should directly compare one method with another,Animals 2019, 9, 911 7 of 25or simply classify it as “acceptable” versus “unacceptable”. Importantly, the terms ‘structured’ and‘systematic’ should not be understood as quantitative in this context, although quantifiable comparisonsmay be included in an assessment.3.2. Define Markers of (Un)Consciousness3.2.1. Statement of the ProblemFor an animal to experience distress, it must be conscious [7]; therefore, an accurate determinationof consciousness is required to assess welfare impacts. The exact point at which awareness to externalstimuli is abolished is unknown [77,78]. Current behavioural surrogates for loss of consciousness(e.g., loss of righting reflex, loss of posture, loss of motion, loss of response to a stimulus) are variablycorrelated with a supposed loss of consciousness [79]. They rely on a degree of responsiveness requiringa motor output that may be impaired or rendered absent by the procedure [80,81]. Unresponsiveness toexternal stimuli (e.g., loss of righting reflex) does not preclude awareness, and furthermore, responsesbased on spinal or cranial nerve reflexes, (e.g., pedal withdrawal or corneal reflex) can occur in theabsence of consciousness [82]. Therefore, such reflexes may not be reliable indicators of a consciousnessstate in the context of stunning. In the context of CAS, brain-wide electroencephalography (EEG)is used to infer loss of consciousness, often by the onset of slow-frequency high-amplitude waves,although these may also occur during wakefulness, as demonstrated in rodents [83] and humans [84].Furthermore, the EEG does not reveal the absence or presence of “information transfer” within thebrain that is required for awareness to stimuli [85]. Depth of anaesthesia monitors such as the bispectralindex (BIS) aggregate EEG and muscle activity parameters into a single score. Whilst these monitorsmay reliably indicate internal brain state in humans once anesthetised, they are notoriously inadequatefor determining loss of consciousness [86]. A complete absence of sensory evoked potentials (EPs)is definitive for the lack of awareness to external stimuli; however, they are highly modality-specific(e.g., auditory versus visual versus somatosensory), and EPs may be present in animals that are deeplyanesthetised [87]. In humans, EPs are correlated with awareness to a stimulus [88] or in response toverbal commands [89]; however, this cannot easily be performed in animals. Furthermore, anaestheticagents diminish EPs in a continuous rather than binary manner, and currently there is no definitionas to where in that continuum sensory awareness is lost. This is compounded by the observationthat certain non-executive sensory brain circuits remain electrically responsive to sensory stimulationduring deep anaesthesia [90]. Further to this, EPs are not informative about ongoing internal sensationssuch as pain and allodynia.The accurate assessment of state of consciousness is important both for the reporting of studiesaimed at refining stunning methods, as well as for the practical assessment of the welfare impact in thelaboratory or abattoir environments.3.2.2. Research Priorities1 Identify brain activity-based markers for loss and regain of consciousness. The European FoodSafety Authority panel’s “Guidance on the assessment criteria for applications for new or modifiedstunning methods regarding animal protection at the time of killing” [10] states that the “normalfunctioning of neurons in the thalamus and cerebral cortex or analogous structures” is a necessarycondition for consciousness and considers EEG as the gold standard to demonstrate a disruption ofnormal function. However, a more detailed characterisation of this “normal function” or of suitableEEG measures is not provided. Given that in human clinical anaesthesia, EEG-based monitoringis currently unable to detect the presence or absence of awareness [91], further search for brainactivity-based markers of consciousness is warranted, and the inclusion of additional methods toassess brain activity, e.g., functional MRI, may be required.2 Correlate these markers with behavioural and gross physiological endpoints. The assessmentof loss of consciousness during a general use of stunning methods is important to the development andAnimals 2019, 9, 911 8 of 25implementation of studies on stunning methods. Such validation methods as electrophysiology (e.g.,EEG, EMG, ECG, EPs) are unlikely to be of practical use in the laboratory or abattoir setting; therefore,reliable behavioural surrogates are required to allow successful monitoring.3 Determine which behavioural endpoints can be reliably used to report investigation findingsand assess animal welfare in a practical setting. The measurement of brain activity requires specialequipment and skills and may in some cases not be compatible with the stunning method underinvestigation. Establishing clinically accessible markers/proxies for brain activity-based markers of(un)consciousness would help to standardise the reporting of investigations of stunning methods,facilitate the assessment of distress caused by specific methods, and allow a more precise assessmentof animal welfare in practical settings.3.3. Validate Markers of Dyspnoea, Particularly Air Hunger, during Hypercapnic and Hypoxic Stunning3.3.1. Statement of the ProblemHumans exposed to hypercapnia and/or intense hypoxia often report a sensation of air hunger [3,92].In the context of CAS, air hunger might arise when an elevated drive to breathe cannot possibly besatisfied by increased respiration, even in healthy animals, i.e., the harder/faster the animal breathes,the more hypercapnic/hypoxic it becomes, and the greater the drive to breathe. In animals withpre-existing conditions that impair their ability to mount a respiratory response, the intensity of anyair hunger occurring during hypercapnic/hypoxic stunning will be exacerbated, i.e., lung inflation willbe further mismatched to drive. The experienced air hunger is consequently not necessarily linearlycorrelated to observed respiratory rates and depth.Abnormal respiratory patterns are described in rodents [21], pigs [27,43], and poultry [1] exposedto hypercapnic, hypoxic, as well as combined hypercapnic/hypoxic conditions. It remains uncertainat which point these behaviours change from indicators of adaptive compensation (i.e., increasedrespiration to satisfy enhanced drive) to indicators of distress (i.e., air hunger due to unsatisfiedrespiratory drive). Importantly, unpleasant respiratory sensations other than air hunger might alsooccur during exposure to higher than normal CO2 atmospheres; for example, humans report feelingsof choking and suffocation after a single breath, which cannot be ascribed to enhanced arterial CO2levels [93].Post-mortem examinations additionally demonstrate pulmonary lesions such as haemorrhageand oedema in rats [94] and mice [61,63] stunned with CO2 and inert gas methods. If these occur beforeloss of consciousness, such lesions could impair respiratory function, thereby causing or exacerbatingair hunger. Therefore, abnormal respiratory patterns and lung lesions are insufficient to assess thenature and degree of distress experienced by the animals.3.3.2. Research Priorities1 Correlate respiratory variables to neurophysiologic and endocrine measurements, aversiontesting, and utilise pharmacologic interventions to elucidate relations between observable respiratorypatterns and affective states. Respiratory patterns should be characterised during the course of exposureto identify changes occurring prior to loss of consciousness. It should also be determined at whichtimepoint lung lesions develop. Combining the measurement of the physiological stress response withaversion testing will help elucidate the effects of different gas concentrations and pressures, and aid theintegration of findings from previous studies measuring the physiological stress response to similarinterventions. Pharmacologic interventions with methods known to relieve air hunger in humans,such as inhaled furosemide [95] or anxiolytics [3], may elucidate the components of dyspnoea thatcause distress.2 Investigate air hunger caused by pure hypoxia and strategies to minimise the duration anddegree of associated distress. Hypoxia-inducing CAS methods are prominently proposed alternatives toCO2 [41,96]. Although ultimately air hunger comes from a mismatch between the drive for respiratoryAnimals 2019, 9, 911 9 of 25motor activity and lung inflation, hypercapnic and hypoxic responses involve different chemoreceptorsin ascending pathways [2]. The correlation between respiratory patterns and associated experiencesmay consequently be specific for each condition. For example, gasping is commonly reported withboth CO2 and inert gas stunning in mice and rats [21], poultry [45,56], and pigs [33,97]; however,whilst gasping due to hypercapnia commonly occurs prior to a loss of posture, gasping due to inertgas hypoxia is often considered agonal (see definitions). Therefore, the extent of air hunger causedspecifically by hypoxic CAS methods should be characterised, and flow as well as decompression ratesshould be optimised to minimise the distress caused by air hunger.3.4. Refinement of Handling and Environment3.4.1. Statement of the ProblemHandling and environmental factors prior to and during stunning add to the overall experienceand affect how much stress animals experience during the stunning process. Pigs and poultry arrivingat the abattoir are typically food deprived, removed from their familiar environment, and transported,and may therefore already experience a degree of short-term stress [98,99]. In rodents, pre-existingcumulative stress from procedures or experimentation may be more variable (e.g., animals at the end ofan experiment versus surplus animals). Despite species-specific differences, most animals experiencestress resulting from light, noise, the grouping of animals, and handling by humans.3.4.2. Research Priorities1 Determine the influence of lighting conditions on stress and aversion in general and on time tounconsciousness for all CAS methods. Rats and mice find bright light aversive [73,100,101], and miceshow significantly less anxious behaviours in a darkened chamber compared to a brightly lit chamberduring CO2 killing [102]. In poultry, illumination promotes active behaviours, whereas darknesspromotes sleep [103]. Accordingly, when LAPS is applied in the dark, time to loss of consciousnessis shorter, and time to deep inhalation, a suggested indicator for air hunger, is longer in poultrybroilers [103]. Pigs tend to move from darker to brighter areas [104], which may facilitate handlingprior to stunning; however, how lighting affects the stunning is unknown.Investigations of variable lighting conditions, including intensity and wavelength, are needed todetermine stress reduction regardless of the stunning method. Additionally, investigations of whetherlow light may reduce the time to unconsciousness for CAS methods are needed. Optimal lightingconditions, which minimise stress whilst providing safe and efficient working conditions, should beidentified for each species.2 Develop equipment and optimise processes to minimise environmental noise as well as noisecaused by stunning equipment. Noise causes a stress response in rodents [101]; additionally, poultryexhibit increased plasma cortisone levels [105], and pigs show increased grouping behaviours [99]when exposed to pre-recorded abattoir noise. Therefore, strategies should be developed/adopted toreduce noise in the lairage and stunning areas of abattoirs as well as noise associated with the stunningmethod per se. A special consideration with rodents is that rats and mice are able to hear frequenciesin the ultrasonic range [106]. Therefore, equipment should be designed to operate without producingultrasonic noise. Further potential refinements include auditory environmental enrichment, which isconsidered to reduce stress in multiple species [107].3 Optimise grouping and handling procedures prior to and during killing. Rodents, poultry,and pigs are social species, and the presence of conspecifics ameliorates stress and anxiety in socialanimals [108]. These species are usually unacclimatised to regular handling, and therefore, thesepractices should generally be considered as a stressor.Stunning practices are most variable in rodents, and largely depend on the situation in whichthey are used, including experimental requirements, local regulations and practices, and populationsize [7]. Mice killed with CO2 in groups had lower plasma catecholamine concentrations than miceAnimals 2019, 9, 911 10 of 25killed individually [109]. Single rats killed with CO2, showed no significant difference in physiologicalvariables, and there were only minor behavioural differences between animals killed in their homecage and animals killed in an induction chamber [110]. The stress-alleviating effect of killing animalsin groups remains to be confirmed for rats and for other CAS methods. Whether killing in the homecage generally does not provide a measurable stress reduction or whether the stress of CO2 killingmasked beneficial effects of home cage killing also remain to be determined.Poultry are typically stunned in groups when CO2 is used or remain in sight of other birds whenelectrical stunning is used. In the presence of other birds, animals are more alert during exposure toCO2, argon (Ar), or a combination of these [1]. Nevertheless, several behaviours indicative of stresshave longer latencies to first occurrence or shorter durations and/or less bouts in birds undergoingLAPS in groups, indicating a beneficial effect of group stunning [1,111]. Only latencies for slow wingflapping (i.e., prior to loss of posture) were shorter in groups, which was possibly because birds weredisturbed by wing-flapping neighbours [111]. Optimal stocking densities for CAS methods should bedetermined and electrical stunning processes adapted to minimise the stress associated with handling.Little data exists in pigs to demonstrate the benefits of single versus group stunning. Electricalstunning is sometimes performed in a race that animals enter one at a time; however, the use ofelectrical prods adds to the aversion of this method [112]. Social isolation in pigs is considered stressful,and the optimisation of group sizes for stunning for CAS methods and the feasibility of electricalstunning in group pens should be investigated.4. Rodents4.1. Rodents-IntroductionKilling by gradual fill (10%–30% chamber volume per minute) CO2 exposure is the currentlyrecommended practice [12]. This method is commonly used at the end of experimentation, for sick andinjured animals, for surplus animals, and for feed animals in zoos [7]. Recommended and practicallyused chamber replacement rates vary widely [21]. It is thought that slow displacement rates induceunconsciousness at CO2 concentrations below the threshold for pain [113], although the absence ofpain in this regard has not been proven.Mice and rats are sensitive to detecting low O2 or high CO2 concentrations [37,114–116]. Bothmice [115] and rats [36,37,39,117] will chose to escape from chambers at 12%–18% CO2 either despitethe presence of a food reward or if the alternative is a brightly lit open space, which they wouldnormally avoid, thus indicating the degree of aversion [118,119]. Animals unable to escape will showa range of avoidance behaviours including rearing, pushing the lid of the chamber, jumping, freezing,and face grooming, and show physiological indicators of stress [18,40,41,61,109,117,120,121]. Ratsshow gasping at around 10% CO2 [18], and laboured breathing occurs in mice after only 14 s of CO2exposure at flow rates as low as 20% chamber replacement per minute [32]. Whether gasping is merelya physiological response to CO2 or associated with an experience of air hunger as reported by humansat similar concentrations of CO2 (8% and above) is unknown. A freezing response, which is acceptedas the manifestation of fear in rodents, is elicited upon exposure to 10% CO2 [18,48], which is mediated(among others; see [122]) by a number of mechanisms. Among these are the acid-sensitive ion channelsin the amygdala, which form a likely component of the aversion to CO2. When exposure is continued,seizures occur in some animals, soon after the loss of motion in some mice, i.e., at a timepoint whereloss of consciousness is uncertain, as demonstrated by EEG and EMG [96]. Nociceptor density in thenasal mucosa is similar to humans who experience discomfort at CO2 levels around 30%–35% [52],and it is therefore presumed to be similarly painful.Inert gases, notably Ar and N2, as well as inhalant anaesthetics have been studied as alternativesto CO2 [37,39,41], but controversy exists about the relative degree of distress caused by each agent.Currently, LAPS is under investigation as an alternative CAS method. Other inert gases (e.g., helium,xenon) as well as carbon monoxide and nitrous oxide, which have been investigated as potentialAnimals 2019, 9, 911 11 of 25alternatives to CO2 [96], will not be covered by this research strategy due to concerns with operatorsafety (nitrous oxide, carbon monoxide) or because gases are expensive/not easily accessible, andsubstantial engineering would be required to create cost-effective systems (helium, xenon). Therefore,it is unlikely that systems would be implemented even if a potential animal welfare advantage overCO2 could be demonstrated.Additional to CAS methods, barbiturate overdose and physical methods are currently the mostcommon methods [7]. These are suitable for single animal use, depending on the experimental regime,but their refinement will not be discussed, as this strategy focuses on methods that are suitable forlarger numbers of animals.An important consideration for the development and evaluation of stunning and killing methodsis that in both mice and rats, strain and sex differences are reported for sensitivity to generalanaesthetics [123], the avoidance of perceived risks [124], and CO2 stunning [40,109]. Any potentialalternative method consequently needs to be tested in different strains and both sexes. Additionally,stunning and killing methods must be validated for specific age groups, as physiological responses innew-born animals may differ considerably from adult animals (e.g., higher resistance to hypoxia andhypercapnia [125–128]).4.2. Rodents-Strategy4.2.1. Identifying Less Aversive Controlled Atmosphere Stunning MethodsStatement of the ProblemConceptually, CAS techniques are attractive because they allow stunning multiple animals at atime without restraint and with minimal direct interaction, which is aversive to mice [129,130]. Theyalso kill animals without gross anatomical damage (unlike some physical methods), which may beimportant for some scientific purposes; however, physiological disturbances may reduce tissue viabilityand yields [131]. The majority of CAS alternatives have focused on inert gases which, unlike CO2,do not have narcotic properties at normobaric conditions (the notable exception being xenon [132]).Hypoxia per se may be aversive to rodents [37,115], and the duration and extent of hypoxia beforeloss of consciousness in mice exposed to inert gases is significantly increased compared to CO2 [41];therefore, the overall difference in distress caused by the two methods is unknown.Research Priorities1 Assess the welfare impact of hypoxia-inducing stunning methods.Argon: When concentrations below those required for stunning are included, studies inmice and rats suggest that Ar is less aversive than CO2 based on voluntarily tolerated exposuretimes [38,120]. However, rats and mice consistently avoid Ar exposure in approach–avoidance testingwhen residual oxygen concentration is below 7% (rats) or 9% (mice), i.e., higher than that required forstunning [37,39,115]. On one hand, gasping and seizure-like behaviour was observed in rats prior torecumbency [94], and hyperreflexia was observed when the animals became recumbent [31]. On theother hand, seizures occurred in mice significantly after recumbency near the point of isoelectricEEG. Furthermore, EEG patterns prior to loss of consciousness were indicative of cortical depression(rather than arousal as with CO2 [96]). Whether this represents a species difference or variability isnot clear. It further remains unclear whether the observed aversion is due to Ar per se, or if it is aresponse to all hypoxic environments. The level of distress experienced during stunning with Arrelative to CO2 remains to be fully characterised in both species, and comparison with N2 is also likelyto be informative.Nitrogen: Studies in mice suggest that N2 is less aversive than CO2 based on locomotor activity,jumping and freezing behaviour, and EEG patterns indicative of cortical depression rather thanarousal [41,96]. While seizures were only observed in mice at the onset of isoelectric EEG, hyperreflexiawas reported in rats at the timepoint of loss of movement [31]. It is currently unknown whether thishyperreflexia represents neuromuscular dysfunction due to hypoxia or if it is a sign of aversion leadingAnimals 2019, 9, 911 12 of 25to distress. There is a general lack of data on aversion to N2, and suitability is generally inferredfrom studies investigating Ar, assuming that the degree of hypoxia is predictive for the degree ofdistress experienced. However, Ar has a higher density than N2, and rats survive the same levels ofhypoxia longer when air is replaced by N2 rather than Ar, which is supposedly due to the lower workof breathing during hypoxia-induced hyperventilation [133]. In rats exposed to N2, heart rates werelower than when exposed to Ar (but not CO2) suggesting different physiological effects, although it isnot clear if this indicates a different degree of stress [31].Low Atmospheric Pressure Stunning: LAPS is currently being investigated in pilot studies inrodents, and has been approved for the stunning of poultry [9]. Humans are frequently unaware of theonset of hypobaric hypoxia occurring over 2–3 min and require specific training to recognise it [134].Rodents may respond differently to hypobaric hypoxia than humans, especially given their increasedsensitivity to hypoxia [135]. Aversion caused by LAPS should be determined for a thorough assessmentof the welfare impact. Optimal rates and timing of decompression also have to be determined.The development of reliable, affordable, easy-to-use systems for routine use would be a prerequisitefor the broader implementation of LAPS.2 Systematically assess the welfare impact of inhalant anaesthetics for stunning in previouslyexposed animals. Rodents frequently undergo general anaesthesia for experimental procedures.Increased aversion to secondary exposure to isoflurane occurs in mice [119,123] and rats [136,137],and increased aversion to secondary exposure was also demonstrated for sevoflurane in rats [136].Importantly, rats were able to generalise a learned aversion to one inhalant anaesthetic comparedto a novel inhalant anaesthetic [138]. Therefore, aversion to stunning with isoflurane or sevofluranedepends on prior experience. Stunning with isoflurane or sevoflurane may involve a period ofbehavioural excitation not observed with CO2; however, post-mortem catecholamine levels are lowerthan with CO2 [61,123], indicating reduced stress. Thus, the advantages of volatile anaesthetic use forstunning is debated.Differences in aversion caused by sevoflurane and isoflurane in rats and mice is controversial, withsome studies finding less aversion with sevoflurane [139,140], but others finding similar levels [61,136].The effect of the interval between exposures on the development of increased levels of aversion and theduration of the increased sensitivity is unknown. Similarly, whether aversion continues to increase ifthe number of exposures exceeds two is unknown. This should be investigated in both species for bothisoflurane and sevoflurane in order to quantify the expected distress caused by inhalant anaesthetics.Furthermore, modulating effects of the flow rates, carrier gases (O2 versus air), and N2O admixtureon time to loss of consciousness and displayed aversive behaviours remain to be fully characterised.Strategies to reduce the development of aversion should be investigated, not only for the potentialuse of inhalant anaesthetics for euthanasia, but also to reduce stress in animals undergoing repeatedinhalant anaesthesia for experimentation. In all applications, the correct scavenging of inhalantanaesthetics is important, because halogenated ethers are pollutants, and exposure to operators carrieshealth risks.3 Optimise flow rates and gas mixtures. While some work has been done to optimise CO2 flowrates (gradual fill CO2 at 30% is recommended in rats [141], whereas a faster fill rate may be preferablein mice based on behavioural measurements [32,41,63]), little or no data is currently available foroptimising flow rates with inert gases [41] or inhalant agents, nor for optimising decompression ratesfor LAPS.4.2.2. Identify Suitable Stunning Methods for Neonatal RodentsStatement of the ProblemAnimals under 2 weeks of age are more resistant to hypercapnia and hypoxia than older animals,and therefore cannot be reliably stunned and killed by CAS methods. In Switzerland, the decapitationof conscious animals and immersion in liquid nitrogen of mice up to 2 days of age are both currentlyallowed. Both techniques kill animals without stunning and may be painful. The consciousness andAnimals 2019, 9, 911 13 of 25ability of neonatal rodents to experience pain are debated, but European legislation assumes thatmammals can experience suffering from the last trimester of gestation [7]. Therefore, methods toreliably stun neonatal animals prior to killing should be identified.Research Priorities1 Investigate the suitability of inhalant anaesthetics for stunning neonatal rodents.Anaesthesia has been successfully induced with halothane, isoflurane, and sevoflurane in neonatalrodents, although higher concentrations are required than in adult animals [142]. The level of aversionto volatile agents in neonates should be determined.4.2.3. Develop Reliable, Restraint-Free Physical MethodsStatement of the ProblemConventional physical methods, i.e., cervical dislocation and decapitation, require restraint of theanimal, have high failure rates (around 20% for cervical dislocation [143]), and face ongoing controversyabout consciousness after force application, as the brain is not directly targeted [7,12,144]. In manyEuropean countries, cervical dislocation and decapitation are prohibited in unanaesthetised animalswith the exception of decapitation in rats and mice younger than 2 weeks [22]. For the stunningof larger animals, e.g., cattle, captive bolts are a widely accepted stunning method (which inducesunconsciousness by concussion [145]), and an automated captive bolt trap to kill wild rats and mice iscommercially available (e.g., the Goodnature trap [146]). However, to date, no feasible application forkilling several animals within a short time—all while minimising restraint—is available.Research PrioritiesAdapt captive bolt for use in the laboratory. Automated captive bolt units have been used toattract wild rats and mice using food. When animals rear to eat food in the device, a captive bolt isautomatically released, and the animal is killed. While the exact timing doesn’t matter when killingwild rodents, the ability to schedule killing is a prerequisite for any method used in the laboratory.There should be a strong incentive for the rodent to enter the automated device, preferably not requiringprior conditioning. Another factor to be considered is that other animals should not observe whathappens upon interaction with the device as rats are able to learn from observations [147], and arecapable of causal reasoning [148]. Additionally, alarm calls [149] or pheromones [150,151] emitted uponstunning may cause stress in subsequent animals. Several practical questions remain regarding theimplementation of a captive bolt in the laboratory setting to minimise welfare impact, including failurerates. In contrast to decapitation, which leaves the brain (physically) intact, captive bolt techniques havea direct impact on the brain, and are not suitable for experiments investigating the brain post-mortem.5. Poultry5.1. Poultry-IntroductionCO2 stunning and electrical water-bath stunning are the two most commonly used methods forrendering broilers unconscious prior to slaughter. Additionally, mobile CO2 containers are used tokill low economic value laying hens on-farm at the end of the laying period, or in cases of diseaseoutbreaks [152]. CAS methods were originally introduced with the intention of improving animalwelfare by avoiding the shortcomings of electrical water bath stunning, notably the shackling of livebirds, the risk of painful electrical shocks, and ineffective stunning in a proportion of animals [28], whichmay also affect meat quality [153]. However, with CO2 stunning headshaking [17,44,56–59], wingflapping [1,58], mandibulation [1,154], and abnormal respiratory patterns [17,44,56–59] are commonlyobserved prior to loss of consciousness. Although the reliability of those behaviours as indicators ofdistress is debated for individual parameters, their summation is generally interpreted as a relevantlevel of distress being present.Inert gas stunning with Ar or N2 is permitted, but whilst there is equipment in the marketavailable, it is currently not commercially used in Europe. Low atmospheric pressure stunning (LAPS)Animals 2019, 9, 911 14 of 25was recently approved for the stunning of broilers up to 4 kg in the European Union, as it was judged as“at least equivalent, in terms of animal welfare outcomes, to currently available stunning methods” andappears to result in acceptable meat quality [9,155]. In parallel, high-throughput head-only electricalstunning is being developed as an alternative to conventional multi-bird water bath stunning toprevent pre-stun shocks and insufficient current delivery as well as to improve meat quality [156,157].For the on-farm killing of larger groups, rapidly expanding gas-filled foam has been verified as aquick and effective method to kill birds directly in the barn [158]. Practical issues with its application(i.e., the over-production of water and logistics of delivering the foam) need further refining to make itcommercially viable.Poultry kept for the production of meat or eggs vary in body size, age, physiology, and behaviourwhen they are stunned (e.g., laying hens versus broilers versus turkeys). Therefore, the level of aversioncaused by a specific stimulus and the behavioural response to a comparable level of experiencedaversion may vary [157]. Additionally, differences in age as well as body size and composition arerelevant for the effectiveness of some stunning techniques; for example, chicks are more resistantto CO2 than older animals, and require lower pressure to be killed by LAPS [159], and maximaltimes to motionlessness were higher in LAPS studies investigating Ross broilers than in studiesinvestigating Cobb broilers, whether it was due to genotype, bodyweight, or the age of the differentstudy populations [9]. Therefore, the suitability of a stunning method needs to be assessed for eachproduction type, age group, strain, and breed separately, keeping in mind that genetics are constantlyevolving within the breeding industry.5.2. Poultry-Strategy5.2.1. Identify Less Aversive Controlled Atmosphere Stunning MethodsStatement of the ProblemCAS techniques are attractive because they can be performed with birds remaining in thetransport cage, and avoid the shackling of birds prior to stunning, which is stressful and may bepainful [160]. However, CO2 is aversive to poultry [44–46,161], activates nociceptors in the nasalmucosa of chickens [55], and leads to behaviours that are generally interpreted as signs of distressor air hunger [1,56,59,162]. Different CAS methods should be identified to reduce distress, and toolsshould be established to allow comparisons between methods/techniques.Research Priorities1 Systematically assess the welfare impact of hypoxia-inducing stunning methods at optimalflow or decompression rates. CAS methods induce unconsciousness by inducing hypoxia. Studiesgenerally indicate that Ar or N2 are less aversive than CO2 [44] and delay the time to onset of aversivebehaviours [35,56], as well as that birds stunned with Ar show less abnormal respiratory patterns thanwith CO2 [1,17,56,59]. The meat quality of broilers stunned with 90% Ar was similar to that of broilersstunned with biphasic CO2 exposure [1].However, comparisons of CO2 and LAPS as well as N2 and LAPS have only been performed inday-old male chicks so far [159]. Cortisone and serotonin levels did not significantly differ between thethree stunning methods, but headshaking and gasping prior to loss of consciousness were significantlyless common with N2 and LAPS than with CO2, with no significant difference between N2 and LAPS.With both hypoxia methods, time to loss of posture was significantly longer than with gradual fill CO2,and time to loss of posture with gradual fill of N2 was significantly longer than with LAPS. As chicksare more resistant to CO2 than older animals and require lower pressures to be killed by LAPS [159], itremains to be determined whether differences exist between hypoxic stunning methods and if LAPSresults in reduced distress compared to CO2 in older birds.Optimal rates of decompression have been determined for LAPS in broilers up to 4 kg, but thesuitability of the method for other production types and species remains to be investigated, as well asoptimal decompression rates.Animals 2019, 9, 911 15 of 25Due to their respective densities, Ar and N2 are suitable for immersion and gradual fill designs,respectively. However, there is no data available regarding whether immediate exposure to targetconcentrations or gradual fill with the same gas elicit different levels of aversive behaviours andabnormal respiratory patterns prior to loss of posture.2 Develop alternatives for large-scale on-farm killing. As an alternative to currently usedmobile CO2 containers, the development of mobile N2 or Ar containers [163] or LAPS units should beinvestigated. However, mobile units require that animals are caught and carried out of the barn priorto stunning, which is a major source of stress.Whole-house gas-killing methods would avoid the stress of capture and restraint. Anoxic gasmixtures could be delivered by high-expansion foam filled with N2 (or other gases), which—due tolarge bubble size—do not occlude the airways in an experimental setting (in contrast to less expansivefoams), but locally creates an atmosphere containing only 1–2% O2 [164]. Further research is needed totest their feasibility in commercial settings.5.2.2. Improve Electrical Stunning Techniques and Investigate Alternatives Directly Targetingthe BrainStatement of the ProblemElectrical stunning traditionally induces unconsciousness by causing generalised seizures.Conventional multi-bird electrical water bath stunning has several shortcomings, such as the need forshackling, the risk of painful shocks, and the risk of ineffective stunning resulting in consciousnessduring neck cutting [28,165]. However, electrical stunning does have the advantage of providing almostimmediate unconsciousness (within a second) when sufficiently high currents are delivered [166].Attempts have been made to increase the efficacy of the stunning with a head-cloaca stunning system,which ensures that the full current is delivered to each bird once contact with the water bath ismade [167], and individual head-only stunning [156]. For the head-only stunning, the shackling of livebirds is replaced with upside-down restraint in a plastic cone, which allows movement of the headand results in birds voluntarily exposing their head in the required position as they try to place ithorizontally. Birds were effectively stunned, and meat quality was superior compared to conventionalwater-bath stunning [156]. Systems are now commercially available (e.g., [168]). However, the durationof unconsciousness induced by head-only stunning may be insufficient [157].Research Priorities1 Develop electrical stunning techniques that immediately and reliably stun birds of all sizes,strains, and ages, while minimising handling and restraint. Even with the head-only stunning, birdsstill need to be handled, restrained, and turned upside-down in conventional cones. This could beimproved by the use of an inclined rather than inverted cone, as suggested by Gregory and Gibson [157].For shackling, it has been shown that the duration of shackling positively correlates with cortisonelevels and stress levels, as indicated by the tonic inhibition test [169]. Restraint systems and set-ups forcurrent delivery should be further refined to minimise handling and restraint, including the durationof restraint, while still reliably stunning all types of birds. Furthermore, alternative voltage and currentsettings should be investigated, especially for head-only stunning, to ensure that the duration of thestun lasts until death. For example, a single-pulse ultra-high current (SPUC) uses a substantiallyhigher current than conventional electrical water-bath or head-only stunning and successfully inducedunconsciousness for up to four minutes in cattle without causing seizure activity [145].2 Compare aversion between optimised electrical and CAS stunning techniques. Electricalstunning methods and CAS differ in the type of aversive stimulus; electrical stunning requires handlingand—to date—restraint in an upside-down position, whereas CAS requires an extended period ofexposure to unphysiological ambient conditions. The differences in distress resulting from restraintcompared to pre-stun CAS periods are unknown, and should be investigated.Animals 2019, 9, 911 16 of 256. Pigs6.1. Pigs-IntroductionThe two most common methods of stunning pigs prior to slaughter are electricity and CO2 [170].The development of CO2 stunning was introduced to reduce stress related to social isolation andthe use of electrical prods to separate animals, which is traditionally required for electrical stunning.Physiological variables (plasma lactate) have been correlated to behaviours indicative of aversion(vocalisation, falls, rearing, and backing up), and concluded that group pens were less aversive thansingle file races [171,172]. Such data is complicated by the subsequent stunning paradigm as bloodis collected post mortem. Furthermore, neither physiological variables nor the cited behaviours arevalidated measures of distress in pigs.There are several reports of aversive behaviours in pigs exposed to various concentrations of CO2,as low as 15% [27,42,43,171,173]. The inert gas Ar used at 90% reduced aversive behaviours duringstunning; however, it still resulted in some aversion [43]. No studies of high concentration N2 stunninghave been published in pigs, despite suggestions from data in rats that it may result in reduced stressresponses compared to Ar [31].6.2. Pigs-Strategy6.2.1. Determine Aversiveness of Inert Gas HypoxiaStatement of the ProblemHypoxia resulting from displacement by 90% Ar results in less aversive behaviour than either15% or 30% CO2 with N2 as a filler gas [43]. This suggests that inert gas hypoxia may offer a welfareimprovement over CO2 containing mixtures, but is not non-aversive. The stunning of animals byhypoxia can be achieved by three primary methods: gas displacement, immersion into gas, and lowatmosphere pressure stunning (LAPS), all of which lower the partial pressure of O2 in the chamberuntil it is insufficient to support consciousness. Times required to ensure death in pigs immersed in N2and Ar are around 7 min compared to 3 min for CO2, suggesting a longer period of conscious exposure.The degree of aversion and distress during this period is unknown. There are currently no publishedstudies on aversion to LAPS in pigs. Recurring behavioural findings of inert gas displacement hypoxiaacross species are “air hunger” and “gasping”, which are characterised by heavy open mouth breathingwith exaggerated thoracic movement. This is frequently assumed to indicate distress; however, nodirect evidence exists to support this.Research Priorities1 Compare LAPS to N2, Ar, and CO2 to determine if hypoxic aversion is reduced. Inert gashypoxia has been shown to be less aversive than CO2 exposure [43]; however, there is currently nodata on aversiveness to LAPS-induced hypoxia. Given the financial investments required to establishCAS methods for pigs, establishing which method is least aversive is important.2 Improve process engineering to consistently produce and maintain an anoxic (<2% volumeO2) atmosphere in commercial stunning and euthanasia equipment. Effective inert gas stunningrequires sustained ambient O2 concentrations under 2%. Increases to 6% may result in spontaneousrecovery [42,173]. Since N2 is not sufficiently dense to use in dip-lift systems, sealed chambers are requiredto achieve hypoxic conditions. However, Ar mixtures may be used in dip-lift systems [43]. Furthermore,such systems should be capable of inducing anoxia in an appropriate timeframe, which is yet to bedetermined. Insufficient filling rates are likely to result in prolonged stress before loss of consciousness.6.2.2. Comparison of Physical Stunning MethodsStatement of the ProblemElectrical stunning is considered to render pigs unconscious immediately and is a commonlyused technique prior to slaughter [174]. The main problems with this stunning technique are thatAnimals 2019, 9, 911 17 of 25seizures may occur, which can pose a danger to operators; unconsciousness may be short lived, riskingthe chance of regaining consciousness before bleeding has been completed; and pigs are commonlyseparated from their group to load into a stunning pen. This latter point poses several welfare issues,namely that electrical prods may be used to load the animals, social isolation may be distressful [175],and the nature of the stunning pen may be aversive to the pigs. One form of restraining devicefor electrical stunning has been suggested to be equally aversive as exposure to 90% CO2, and theelectrical prodder required for loading into the restrainer was more aversive [112]. The issue of isolationcan be overcome by using group stunning pens. Then, the electrical stun is applied by an operatorusing hand-held tongues. This can induce more variability in the stun efficacy and lengthen the timefrom stunning to exsanguination, therefore increasing the risk of regaining consciousness [176]. Onealternative to conventional electrical stunning is the application of single-pulse ultra-high current(SPUC) stunning, which has been demonstrated to increase times of unconsciousness compared toconventional stunning in cows [145]. Furthermore, no epileptiform activity is reported with this method.Group stun pens may increase anxiety spreading between individuals; therefore, the advantage oversingle-chute systems is unclear. A further refinement to group stun pens could be to use alternativestunning methods that allow pigs to be stunned together, in the same way as with gas. Exploringwhether microwave radiation would be effective could be an option.This method rapidly heats the brain to induce insensibility in rats and unconsciousness incattle [177]. This may be applied using a hand-held device and only requires one point of contact,unlike electrical stunning tongues.Research Priorities1 Evaluate distress in group pen and isolation stunning paradigms. Pigs suffer from isolationstress; however, a single animal exhibiting aversion-related behaviours may result in such behavioursbeing exhibited by others in the group. The proposed benefits of electrical stunning in groups remainsinconclusive since they are kept in groups, they are individually restrained at the point of stunning,and stunning is staggered within the group in sight of other pigs.2 Develop techniques for more accurate electrical stunning without the need for restraint.Electrical stunning may be performed in group pens, although this relies on partial individual restraintat the point of stunning and the manual placement of stunning tongues by an operator. This mayresult in less accurate placement and therefore inadequate stunning, and also exposes animals tounfamiliar humans, which is considered a stressor. One refinement to this might be the use of SPUC toinduce a more rapid and longer duration loss of consciousness [145], which should be tested in pigs.The feasibility of using SPUC in a group pen situation would also be of value.7. ConclusionsHere, we suggest a series of research priorities that are intended to identify humane alternativesfor the stunning of mice, rats, poultry, and pigs. The aims detailed in this document are not anexhaustive list of avenues to explore, but rather the methods that offer the most promising opportunityfor practical implementation. Our proposal is not based on any political or legislative landscape, and assuch, opens an avenue for international co-operation toward improving animal welfare by humanelyending the life of animals.Author Contributions: All authors wrote and edited the manuscript. T.C.G. supervised the writing of themanuscript. Conceptualisation: A.R.S., R.B.-W., T.C.G. Writing original draft: A.R.S., S.A.F., N.J.B., C.B., R.B.-W.,R.G.P., H.D.R.G., M.M., R.M., T.S., M.J.T., P.V.T., D.M.W., T.C.G. Writing revisions: A.R.S., T.C.G.Funding: This work was funded by the Swiss Federal Food Safety and Veterinary Office (FSVO), grant number714. 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Vancouver : University of British Columbia Library
Multidisciplinary Digital Publishing Institute
CC BY 4.0
Faculty
animal welfare
carbon dioxide
stunning
killing
euthanasia
rodents
poultry
pigs
aversion
air hunger
Humanely Ending the Life of Animals : Research Priorities to Identify Alternatives to Carbon Dioxide
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