Indicators used in livestock to assess unconsciousness after stunning: a review

Abstract

Assessing unconsciousness is important to safeguard animal welfare shortly after stunning at the slaughter plant. Indicators that can be visually evaluated are most often used when assessing unconsciousness, as they can be easily applied in slaughter plants. These indicators include reflexes originating from the brain stem (e.g. eye reflexes) or from the spinal cord (e.g. pedal reflex) and behavioural indicators such as loss of posture, vocalisations and rhythmic breathing. When physically stunning an animal, for example, captive bolt, most important indicators looked at are posture, righting reflex, rhythmic breathing and the corneal or palpebral reflex that should all be absent if the animal is unconscious. Spinal reflexes are difficult as a measure of unconsciousness with this type of stunning, as they may occur more vigorous. For stunning methods that do not physically destroy the brain, for example, electrical and gas stunning, most important indicators looked at are posture, righting reflex, natural blinking response, rhythmic breathing, vocalisations and focused eye movement that should all be absent if the animal is unconscious. Brain stem reflexes such as the cornea reflex are difficult as measures of unconsciousness in electrically stunned animals, as they may reflect residual brain stem activity and not necessarily consciousness. Under commercial conditions, none of the indicators mentioned above should be used as a single indicator to determine unconsciousness after stunning. Multiple indicators should be used to determine unconsciousness and sufficient time should be left for the animal to die following exsanguination before starting invasive dressing procedures such as scalding or skinning. The recording and subsequent assessment of brain activity, as presented in an electroencephalogram (EEG), is considered the most objective way to assess unconsciousness compared with reflexes and behavioural indicators, but is only applied in experimental set-ups. Studies performed in an experimental set-up have often looked at either the EEG or reflexes and behavioural indicators and there is a scarcity of studies that correlate these different readout parameters. It is recommended to study these correlations in more detail to investigate the validity of reflexes and behavioural indicators and to accurately determine the point in time at which the animal loses consciousness.

Behavioural, brain and cardiac responses to hypobaric hypoxia in broiler chickens

“…While time to loss of posture may be affected by a range of factors including ambient temperature, there is a need for brain and behavioural measures in the same bird to allow a more robust assessment of the welfare impact of the process and corroborate indicators of loss of consciousness [51].…”

“…In this way, we ensured good visibility of birds for observations while controlling for social isolation which might otherwise affect the behaviour of birds killed individually, while maximising the size of the behavioural data set. To interpret EEG responses in relation to loss of consciousness, we applied a range of methods including spectral analysis which has been recently widely adopted [7,20,25,50,51]. In particular, we determined latencies to recently validated species-specific thresholds for different clinical states of consciousness [32,33,47].…”

“…The occurrence of vocalisations during LAPS raises questions about the conscious state of the birds and also the welfare implications of the process. In general, vocalisation after stunning is thought to indicate consciousness and possibly distress [51], though vocalisations have been reported in pigs during stunning after loss of posture [22] and in dogs during anaesthesia when apparently unconscious [e.g. 3].…”

“…The baseline heart rate of birds killed at temperature setting 4 was higher than those killed at setting 3 -these birds were killed earlier in the day and underwent a holding period in crate in cold conditions. The effect of temperature on heart rate is well established; thermoregulatory effort increases heart rate and oxygen consumption [21,51] and there is evidence that modern broilers have compromised thermoregulatory capacity compared to laying hens [28]. These factors suggest that responses to LAPS will be related to ambient temperature in a non-linear way, since thermoregulatory effort and potential alterations in metabolic rate and respiratory control outside the thermally neutral zone will be altered in both hot and cold conditions.…”

“…Together, they provide a loss of consciousness estimate of around 60 s [30], which is not identical to values reported previously. More conservative measures of loss of consciousness would be onset of clonic convulsions [11,12,26,51], or time for the EEG signal to have less than 10% of the total power of baseline [45,46]. On this basis, the data suggest that mean estimates of potential consciousness during LAPS are 75 s and 114 s, respectively.…”