Pigs exposed to nitrogen-, argon- or carbon dioxide-filled high-expansion foam: assessment of unconsciousness and death.

Different gases, such as nitrogen (N2), argon (Ar) or carbon dioxide (CO2), can be used to create anoxic environments for the stunning and killing of pigs. High-expansion gas-filled foam is effective at quickly displacing the air in a container, creating an anoxic environment that will stun and kill exposed animals. However, like conventional controlled atmosphere stunning methods, the choice of gases used to create gas-filled foam may have different effects on animal welfare and the induction of unconsciousness. In total, 36 pigs (12 per gas treatment) were included, and 26 pigs with usable Electro-encephalogram (EEG) recordings were included in the EEG-based loss of consciousness analyses. The aim of this study was to compare the effectiveness of stunning pigs with high-expansion foam created with three different gases, N2, Ar or CO2. Brain activity, measured using two surface electrodes and a reference electrode on the skull, was used to assess the relationship between loss of posture and EEG-related onset of unconsciousness. There was no clear difference between the three gases in heart activity (beats per minute) before and during exposure to gas-filled foam. Exposure to CO2-filled foam resulted in earlier EEG suppression indicative of unconsciousness (55.5 s) compared to N2- or Ar-filled foam (74.5 and 80.0 s, respectively). However, because CO2 foam filled the container faster, this difference cannot be attributed to gas composition alone. Loss of posture occurred on average 5-6 s before unconsciousness was detected based on EEG suppression. However, the EEG recordings were disturbed by severe movement artefacts and as soon as the artefacts paused, a strongly suppressed EEG, indicative of unconsciousness, was detected. This indicates that detecting unconsciousness based on EEG may be delayed due to movement artefacts on the signal. Since movement artefacts obscure the exact moment of strong suppression of the EEG, loss of posture can at least be considered an early behavioural indicator that the animal is transitioning to loss of consciousness.