Consciousness, unconsciousness and death in the context of slaughter. Part II. Evaluation methods

Abstract

This second review describes indicators of consciousness and unconsciousness that can be used in the abattoir. These indicators evaluate different aspects of cerebral functioning, but only indirectly. It is therefore necessary to monitor several indicators. Animals are considered unconscious if signs of consciousness are absent, and signs of unconsciousness are present. Given that the unconscious state may be reversible it is further necessary to monitor these indicators until the end of bleeding. The techniques used to diagnose brain death in humans cannot be used in the slaughterhouse. Under field conditions, at the end of bleeding, the absence of breathing and of brainstem reflexes and the adequacy of the exsanguination are verified. If these three aspects are confirmed, in the context of the slaughterhouse and at this stage of the slaughter process the loss of vital functions is irreversible and the animal can be considered dead.

Assessing unconsciousness in livestock at slaughter

“…The results from this study support the hypothesis that a positive corneal-or eyelid reflex alone does not necessarily indicate consciousness, since positive brain stem reflexes might occur on the basis of residual brain stem activity and do not distinguish clearly between consciousness and unconsciousness (Anil, 1991). Consequently, a negative corneal-or eyelid reflex is considered a general indication of unconsciousness (Laureys, 2005;Terlouw et al, 2016b).…”

“…The eyelid reflex is known to be present after an animal is considered unconscious (Dugdale, 2010). The order in which the eyelid-and corneal reflex cease may differ, providing uncertainty which reflex is more resistant to the effects of anoxia (Bourguet et al, 2011;Terlouw et al, 2016b). The results from this study support the hypothesis that a positive corneal-or eyelid reflex alone does not necessarily indicate consciousness, since positive brain stem reflexes might occur on the basis of residual brain stem activity and do not distinguish clearly between consciousness and unconsciousness (Anil, 1991).…”

“…Bleeding of the animal will result in unconsciousness, eventually followed by death. Consciousness in sheep and poultry is generally lost around 15 s and 20-35 s, respectively after the neck cut, with little variation between animals (Gregory and Wotton, 1984;Devine et al, 1986;Gregory and Wotton, 1986;Terlouw et al, 2016b). Results from studies on non-stunned slaughter of cattle show considerable more variation in time to loss of consciousness compared to sheep and poultry.…”

“…Cognitive responses and voluntary motor activity, such as coordinated walking movements, should be absent after stunning, since effective stunning induces a general dysfunction of the brain (Terlouw et al, 2016b). Vocalisation is also a conscious response, often following a stressful event and should therefore be absent after stunning (Grandin, 1998).…”

“…The general opinion, however, is that the initial phase of CO 2 stunning is aversive to pigs (Nowak et al, 2007). While changes in breathing pattern are generally associated with aversion, there is little consensus concerning the interpretation of the occurrence of convulsions or (involuntary) muscle contractions (Terlouw et al, 2016b). These muscle contractions have been observed both before (Velarde et al, 2007;Rodriguez et al, 2008) and after loss of consciousness (Hoenderken, 1983;Forslid, 1987;Velarde et al, 2007).…”