Animal consciousness and euthanasia: chicken euthanasia in the two-phase stunning system.

A form of consciousness cannot be excluded for animals and this has important ethical implications for acceptable slaughter methods, which should result in minimal signs of agitation and distress in the period before unconsciousness is reached. Simultaneous EEG, ECG and behavioural responses were recorded in birds exposed to either two-phase (40% CO2, 30% O2, 30% N2 for 1 min followed by 80% CO2, 5% O2, 15% N2) or single-phase (30% CO2, 60% N2 with <2% O2) controlled atmosphere stunning in a pilot scale system. The EEG traces of birds in the carbon dioxide/nitrogen condition showed many more movement artefacts reflecting wing flaps and strong body movements such as convulsions, and this was confirmed by behavioural observations. These occurred during the period that consciousness could not be excluded; the time till unequivocal unconsciousness (expressed in an iso-electric EEG pattern) was estimated to be 20-50 seconds in the carbon dioxide/nitrogen atmosphere and 50-60 seconds in the carbon dioxide/oxygen/nitrogen condition. In addition, intense and long lasting artefacts in the ECG traces were visible in the carbon dioxide/nitrogen condition but were completely absent in the two-phase condition. These are thought to indicate strong isometric muscle contractions which may be painful and distressing. Time to death was established as heart rate under 180 beats per minute and was around 80 seconds in the carbon dioxide/nitrogen condition and 190 seconds in the carbon dioxide/oxygen/nitrogen condition. The chickens reached unconsciousness slower and died significantly later in the two-phase condition, as judged by the EEG, heart rate and behaviour. However, in the absence of oxygen leading to a faster death, the birds showed strong signs of agitation and distress. We are convinced that a milder death, which takes longer, is to be preferred to a quicker but more distressing death.