Science - November 24, 2016

How do you know an animal is unconscious?

Albert Sikkema

Animals which lack rhythmic breathing, eyelid reflexes and corneal reflexes are unconscious. Slaughterhouse workers can use these three indicators to check whether the animals have been successfully stunned, says PhD candidate Merel Verhoeven in her thesis.

<The presence of the eyelid reflect is one of three indicators for successful stunning of animals in the slaughterhouse, photo Merel Verhoeven>

Large numbers of cattle, pigs, sheep and chickens destined for human consumption end their lives in the slaughterhouse. After some time in a holding pen they go into an stunning machine or space, where they are knocked unconscious with CO2, an electric shock or a bolt stunner. Their throats are then cut so they bleed to death.

Verhoeven investigated how you can assess whether the animals really are unconscious after being stunned, and for how long. This is important because it is the subject of much discussion. The researcher measured the brain activity of animals on the slaughter belt which had been knocked out with CO2 or a bolt stunner. What emerged was that stunning with the bolt stunner rendered the animals unconscious instantly, whereas pigs that were knocked out with CO2 lost consciousness in 30 to 60 seconds, depending on the concentration. If the stunning is carried out correctly and effectively, the animal suffers no pain during the rest of the slaughter process, says Verhoeven.

She looked into which animal behaviour is the best indicator of whether they are really unconscious, as the slaughterhouse workers do not have time to measure the brain activity of all the animals. She came up with three indicators: rhythmic breathing, eyelid reflex and corneal reflex. Verhoeven: ‘A conscious animal blinks when you come close to its eye. If you don’t get a reaction the animal is unconscious.’ She advises slaughterhouses to use these three indicators to check whether the animals really are unconscious.

Her research is not really news for slaughterhouses, she thinks. ‘My impression is that a lot of slaughterhouses already use these indicators to check whether animals on the slaughter line are conscious. The slaughterhouses I’ve seen have strict rules on slaughter and they really do keep to them.’

Merel Verhoeven received her PhD on 11 November from Bas Kemp, professor of Adaptation Physiology.