Challenges of animal welfare assessment for controlled atmosphere killing methods

When assessing the animal welfare impacts of a killing method, two fundamental questions must be answered: (1) how long does it take for the animal to lose consciousness; and (2) what affective states are likely to be experienced until this point? These questions are particularly relevant for killing methods which involve a gradual transition to unconsciousness such as exposure to modified environments, where the animal has the potential to experience negative affective states such as pain and anxiety during induction (as opposed to rapid-acting methods such as electronarcosis). Time to loss of consciousness (LOC) is often measured indirectly by loss of posture and interpretation of spontaneous behaviour has been the basis of assessment of the animal welfare impacts of various killing and slaughter methods. Such behavioural indicators are non-invasive, accessible, and practical in challenging environments such as slaughter plants. They are also largely transferrable for immediate application by range of users (e.g., veterinarians and technicians). However, their limitations relate primarily to difficultly of interpretation relating to the lack of understanding of underlying motivational drive and assumptions about the conscious state of the animal. The use of electroencephalography (EEG) provides a more direct measure of vigilance state, and has evolved from purely visual interpretation of EEG trace to complex spectral analysis providing objective parameters to describe and characterise brain state during the killing process. While this approach has aided identification of time to LOC, EEG outputs must still be interpreted, and care is required as different agents influence electrical brain activity in different ways. Nevertheless, when used in unison with spontaneous behaviour, EEG measures can validate behavioural indicators of LOC, as well as providing insight into the sliding scale of cognitive impairment prior to LOC, improving our ability to interpret the spontaneous behaviours observed. Additionally, a detailed, mechanistic approach to ethogram construction (avoiding consequential descriptions) may be useful, promoting objectivity and avoiding confounds arising from an observer’s interpretation of motivational drive (e.g. an escape attempt versus a vigorous righting reflex). The use of pharmacological interventions adds further clarity, by identifying behaviours which may be supressed by the use of analgesic or anxiolytic agents, inferring the experience of pain and anxiety. Their value is highly dependent on rigorous validation of the agents in the study species, as well as considering their wider physiological and behavioural effects. Finally, recent attempts to answer question two (above) have involved the implementation of innovative and theoretically sophisticated behavioural paradigms. These challenging approaches aim to directly assess the subjective mental experiences of animals during killing, using as operant exit and conditioned aversion paradigms. They require animal training and may be affected by cognitive impairment, but hold promise to provide uniquely animal-centric data. Together, the approaches described here represent continuous efforts to improve our capacity to more fully and objectively understand the end of life experiences of animals, a crucial endeavour to protect welfare in a wide range of animal use contexts.