Assessment of aversion and unconsciousness during exposure to carbon dioxide at high concentration in lambs

Abstract

The most widely used stunning method in sheep is electrical. However, in lambs, this method leads to rupture of the blood vessels, provoking ecchymoses. In pigs (Sus scrofa), the use of CO2-stunning systems has increased in popularity due to positive effects on meat quality and animal welfare (movement of animals in groups). The aim here was to assess the effectiveness of a progressive exposure to 90% carbon dioxide (CO2) in inducing unconsciousness in lambs (Ovis aries) through changes in the middle latency auditory-evoked potentials (MLAEP) of the central nervous system (CNS), blood parameters (pH, carbon dioxide partial pressure (pCO2), oxygen partial pressure (pO2), oxygen saturation (SatO2) and bicarbonate (HCO3J, behaviour (head-shaking, sneezing, gasping and gagging) and physiological reflexes (corneal reflex, breathing and sensitivity to pain). Fourteen male lambs of the Ripollesa breed, weighing between 19 and 25 kg were progressively exposed to an atmosphere of 90% CO2 over 66 s. All blood parameters changed between 23 and 43 s after the onset of the immersion. The MLAEP did not decrease significantly until after 48 s exposure to CO2, suggesting an absence of auditory-evoked brain activity. Before that, lambs exhibited head-shaking and sneezing starting at 10.6 (± 0.77) s, and gasping starting at 20.6 (± 1.36) s. After exposure, all animals showed absence of breathing and sensibility to pain, and 36% of them absence of corneal reflex. The pH and pCO2 recovered basal values at 90 and 120 s, respectively, after the end of the exposure. The burst suppression index (BS%) and the A-Line ARX index (AAI) recovered basal levels at 116 and 159 s, respectively. Exposure to CO2 at high concentration induces effective stunning in sheep for a period of 124 s. However, during exposure, the animals exhibited signs of aversion and breathlessness.

Time to Loss of Consciousness and Its Relation to Behavior in Slaughter Pigs during Stunning with 80 or 95% Carbon Dioxide

“…Some of the previous studies simulated commercial conditions [e.g., Ref. ( 9 , 32 )] where pigs were immersed gradually to the required concentration at the bottom of a well. Part of the descending time was, therefore, taken up with transit through the interface between air and CO 2 , which is expected to prolong the conscious period.…”

“…Raj ( 34 ) observed loss of posture 17 ± 3 s after pigs were exposed to the highest (80–90%) CO 2 concentration. Rodriguez et al ( 9 ), on the other hand, monitored brain activity using auditory evoked potentials and concluded time to loss of consciousness to be 60 s after being submersed into a well pre-filled with 90% CO 2 . It should be noted, however, that the use of absence of evoked responses may provide more conservative times to loss of consciousness compared to loss of spontaneous EEG activity ( 35 , 36 ).…”

“…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 ( 13 ). These muscle contractions have been observed both before ( 3 , 9 ) and after loss of consciousness ( 3 , 23 , 24 ). The objective of the current study was to assess the relationship between behavioral measurements and onset of unconsciousness as identified by EEG activity during 80% CO 2 (80C) or 95% CO 2 (95C) stunning in pigs.…”

“…Time to loss of posture, as the first indicator of the onset of unconsciousness, was reported at 25, 17, 22, and 15 s after immersion into 60, 70, 80, and 90% CO 2 , respectively ( 7 ). Studies that examine brain activity, presented in an electroencephalogram (EEG), reported loss of consciousness 14–60 s after initial exposure to 80–90% CO 2 ( 9 , 10 ). Pigs do not need to be individually restrained and can be stunned in groups during CO 2 stunning, which are considered to be advantages in terms of animal welfare in comparison to other stunning methods ( 10 , 11 ).…”

I Am a Compassionate Conservation Welfare Scientist: Considering the Theoretical and Practical Differences Between Compassionate Conservation and Conservation Welfare

“…For example, despite current scientific understanding that fetal mammals are not conscious before birth and are thus not capable of experiencing harms during that stage of development, they do make physical responses to disturbance in utero [108,109]. Similarly, unconscious animals can still show some behavioural and physiological responses after stunning [110,111]. Those responsible for, or witnessing, invasive or aesthetically unpleasant acts on such animals, that would cause harm if the animal was conscious, may still feel that these acts are inconsistent with their desire to be compassionate, and they may suffer accordingly.…”