Assessment of unconsciousness in pigs during exposure to nitrogen and carbon dioxide mixtures

Abstract

The aim of this study was to assess unconsciousness in pigs during and after the exposure to gas mixtures of 70% nitrogen (N2) and 30% carbon dioxide (CO2) (70N30C), 80% N2 and 20% CO2 (80N20C) and 85% N2 and 15% CO2 (85N15C) compared with 90% CO2 in air (90C) by means of the Index of Consciousness®(IoC), their behaviour and the absence of brain stem reflexes. The experiment included three trials of 24 pigs divided into four groups according to the number of treatments. Half of the group was exposed for a short time and the other half for a long time (3 and 5 min for the N2/CO2 mixtures exposure and 2 and 3 min in 90C exposure, respectively). During exposure, the IoC and the electroencephalography suppression rate (ESR) were assessed, as well as the time to onset and percentage of gasping, loss of balance, vocalizations, muscular excitation and gagging. At the end of the exposure, the corneal reflex, rhythmic breathing and sensitivity to pain were each assessed at 10 s intervals for 5 min. Brain activity decreased significantly (P < 0.05) 37.60 s after the start of the exposure to 90% CO2, which was significantly earlier than in 70N30C, 80N20C and 85N15C exposure, (45.18 s, 46.92 s and 43.27 s, respectively). Before brain activity decreased, all pigs experienced gasping and loss of balance and a 98% muscular excitation. The duration of the muscular excitation was longer in animals exposed to 70N30C, 80N20C and 85N15C than 90C (P < 0.01). After a long exposure time, all animals exposed to 90C died, whereas the 30.4% of animals exposed to N2/CO2 gas mixtures survived. Pigs exposed to 85N15C recovered corneal reflex and sensitivity to pain significantly earlier than when exposed to 90C. Exposure to 90C causes a higher aversive reaction but a quicker loss of consciousness than N2/CO2 gas mixtures. Exposure to N2/CO2 gas mixtures causes a lower percentage of deaths and an earlier recovery of the brain stem activity than 90C, whereas the time to recover the cortical activity is similar. In conclusion, the inhalation of N2/CO2 gas mixtures reduces the aversion compared with high concentrations of CO2; however, the period of exposure for inducing unconsciousness may be longer in N2/CO2 gas mixtures, and the signs of recovery appear earlier, compared to CO2.

Use of high concentrations of carbon dioxide for stunning rabbits reared for meat production

“…In the present study, we observed that during the recovery phase rhythmic breathing was the first reflex to reappear, in comparison to corneal reflex or righting reflex ( Figure 6) and, in consequence, as well before the recovery of a basal EEG signal. This outcome agrees with Llonch et al (2013), who found animals with rhythmic breathing and corneal reflex with brain activity below the basal. However, as Anil and McKinstry (1991) suggested, some symptoms commonly considered to be relevant for the assessment of consciousness are indicative of brain stem activity only and do not relate to cortical function.…”

“…In addition, the time for the corneal reflex to appear was also lower (P<0.0001) at 70% than in the other three treatments and higher (P<0.0001) at 98 compared to 80 and 90%. Therefore, the duration of consciousness is related with the CO 2 concentration, as has previously been described (Raj et al, 1999;LLonch et al, 2013).…”

“…Gagging has been defined as low-frequency inhalations with the neck towards the front legs and occasional emitting of sounds similar to snoring in pigs (Rodriguez et al, 2008) and it is considered an indicator of deep unconsciousness (EFSA, 2004). According to Llonch et al (2013), gagging is a rudimentary brain stem response occurring in association with the process of exhalation in pigs that takes place when general anaesthesia is achieved in gas exposures. However, it is possible than in rabbits movements and expression are more similar to and difficult to distinguish from a normal respiratory movement.…”

Hot-water spraying is a sensitive test for signs of life before dressing and scalding in pig abattoirs with carbon dioxide (CO2) stunning

“…However, dwell times and CO 2 concentrations were relatively low in this abattoir, whereas stun-to-stick intervals were rather long. These variables are known to be of crucial importance in preventing SOL (Atkinson et al, 2012;Von Wenzlawowicz et al, 2012;Llonch et al, 2013). In addition, the stick-to-hot water interval was relatively short in this abattoir, possibly increasing the likelihood of SOL at the time of HWS.…”

“…Animal welfare is compromised when consciousness is present at any time in the period between the end of the stunning intervention and death. Known risk factors include insufficient stunning effectiveness as well as delayed, ineffective or omitted sticking (Rodríguez et al, 2008;Von Wenzlawowicz et al, 2012;Llonch et al, 2013). The affected pigs run the risk of being exposed to further processing steps (usually scalding) if no additional stun/kill intervention is performed.…”