Wireless epidural EEG and behavior in pigs during nitrous oxide or carbon dioxide gas euthanasia

Authors: RAULT, JEAN-LOUP - University Of Veterinary Medicine; LAI, ALAN - University Of Melbourne; HEMSWORTH, LAUREN - University Of Melbourne; LE CHEVOIR, MATTHIAS - University Of Melbourne; BAUQUIER, SEBASTIEN - University Of Melbourne; GATES, RICHARD - University Of Illinois; Lay Jr, Donald

Submitted to: Physiology & Behavior
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/17/2020
Publication Date: 8/18/2020
Publication URL: https://handle.nal.usda.gov/10113/7080936
Citation: Rault, J., Lai, A., Hemsworth, L., Le Chevoir, M., Bauquier, S., Gates, R., Lay Jr, D.C. 2020. Wireless epidural EEG and behavior in pigs during nitrous oxide or carbon dioxide gas euthanasia. Physiology and Behavior. https://doi.org/10.1016/j.physbeh.2020.113142.
DOI: https://doi.org/10.1016/j.physbeh.2020.113142

Interpretive Summary: Consciousness remains ‘the hard problem’ for science. Assessing consciousness is central to animal welfare concerns, especially for evaluating procedures aimed at inducing unconsciousness such as euthanasia and slaughter. Neurophysiological approaches such as those based on electroencephalography can improve the reliability of the assessment of consciousness. Electroencephalographic data can provide information on the state of consciousness in a range of animal species as well as evidence of pain perception. This experiment investigated the correspondence between wireless electroencephalography and behavior to assess loss of consciousness in pigs undergoing gas euthanasia. We compared exposure of pigs to nitrous oxide gas to the most commonly used carbon dioxide gas to assess their animal welfare implications for humane euthanasia. At the flow rate used in this study, exposure to nitrous oxide took twice as long to induce transitional and isoelectric electroencephalography (indicators of unconsciousness and brain death) compared to carbon dioxide. Transitional electroencephalography appeared 73.7% of the way to isoelectric for nitrous oxide and 53.5% of the way for carbon dioxide in the present study. The behavioral response and its time course was visibly more variable in pigs exposed to nitrous oxide compared to pigs exposed to carbon dioxide; pigs appeared to be affected by nitrous oxide (change in breathing patterns) while still alternating with exploratory (locomotion, rooting) or inactive behaviors, in contrast to the relatively time-consistent progression in response to carbon dioxide with more clearly defined stages from normal to distress and finally panic (flailing) prior to loss of posture. Chemically inert gases such as nitrous oxide possess anesthetic properties without imparting any sense of breathlessness. Exposure to high concentrations of inert gases such as nitrous oxide, through deprivation of oxygen, appears to circumvent the distress response seen in pigs with other gases which inducing high levels of carbon dioxide. In the present study, pigs exposed to nitrous oxide showed respiratory distress to a lesser extent but for a longer duration, which was interrupted by bouts of exploration and inactive behavior. Both of these responses would support the interpretation that pigs were experiencing less distress in nitrous oxide than carbon dioxide. The behavioral response of pigs exposed to nitrous oxide could be interpreted as the gas causing confusion or discomfort rather than the panic or distress shown by pigs exposed to carbon dioxide just prior to unreversed loss of posture. This method of euthanasia provides a more humane approach when having to euthanize pigs and should be considered in both a research and farm setting.

Technical Abstract: Consciousness is central to animal welfare concerns. Its assessment is most often conducted based on behavior, with a poor understanding of the correspondence between behavior and the neurobiological processes that underlie the subjective experience of consciousness. Recording of brain electrical activity using electrodes placed under the skull improves EEG recording by minimizing artefacts from muscular or cardiac activities, and it can now be combined with wireless recording in free-moving animals. This experiment investigated the correspondence between wireless epidural EEG and the behavior of 18 five-week-old pigs undergoing nitrous oxide (N2O) or carbon dioxide (CO2) gas euthanasia. Loss of posture with no righting attempt, as the last behavioral state observed during euthanasia, preceded the onset of transitional EEG by 0.9 to 3.1 min (for CO2 and N2O treatments, respectively), and the onset of isoelectric EEG by 4.5 to 6.2 min (for CO2 and N2O treatments, respectively), hence pigs are unlikely to be unconscious after losing posture. Paddling movements occurred shortly before and during transitional EEG but never during isoelectric EEG, whereas gasps persisted after the EEG had become isoelectric. Latency to loss of posture correlated with the onset of isoelectric EEG in pigs exposed to CO2 but not N2O, and not with the onset of transitional EEG in either gases. The lack of temporal correspondence between epidural EEG and the behavioral signs recorded in this study reflects that behavioral assessment based on loss of posture likely precedes loss of consciousness in pigs during gas euthanasia.