Nitrous oxide for piglet gas euthanasia

The neonatal stage is a critical time in the life of a pig, when they are prone to become sick or weak. This is the stage at which most euthanasia procedures are required for pigs. The search for humane methods to euthanize piglets is critical to address public concern that current methods are not optimal. Blunt force trauma is humane but aesthetically unpleasant. Carbon dioxide (CO₂) gas is used but aversive to piglets (Rault et al. 2013). This previous research suggested that nitrous oxide (N₂O; ‘laughing gas’) is less aversive than CO₂ for piglets. This research sought to: evaluate the aversiveness of inhaling N₂O using an approach-avoidance test relying on the piglet’s perspective; and validate its humaneness to induce loss of consciousness by electroencephalography (EEG), a neurobiological technique that provides insight into brain processes and state of consciousness (Murrell and Johnson 2006). It was hypothesised that exposure to N₂O is less aversive to piglets than exposure to CO₂.

The gas mixtures tested were: N₂O and air (90%:10%; ‘90N’); N₂O, oxygen and air (60%:30%:10%; ‘60N’); and, for experiments 2 and 3, CO₂ and air (90%:10%; ‘90C’) as a control. All piglets were the progeny of Yorkshire × Landrace dams bred to Duroc × Hampshire sires. Data were analysed using mixed models or Kruskal-Wallis tests (SAS^®; USA). Experiment 1 allowed 16, 2-week-old female piglets to walk freely between one chamber filled with air and another prefilled with 60N or 90N, using a previously validated behavioural paradigm (Rault et al. 2013). All piglets exposed to 60N finished the 10 min test whereas all piglets exposed to 90N had to be removed within 5 min (mean ± SE: 255.4 ± 65.5 sec) because they fell recumbent and non-responsive and then started to flail. Hence, N₂O could be used as a sedative agent for piglets. Experiment 2 performed the same test using 24 female piglets except the gas chamber held N₂O prefilled at 25%, 50%, or 75%; or CO₂ prefilled at 7%, 14%, or 21%. The test was shorter at higher concentrations (P < 0.001). Time spent disoriented was greater in the middle concentration gradients (P < 0.002). Flailing behaviour (e.g. erratic movements, jumps) tended to correlate with increasing concentrations of CO₂ (r = 0.40, P = 0.06), but not N₂O (r = 0.28, P = 0.19). Overall, these data supported our hypothesis that exposure to N₂O is less aversive to piglets than exposure to CO₂. Experiment 3 used the minimal anaesthesia model (Murrell and Johnson 2006) on 15, 10-day-old male piglets to record EEG. Both 90N and 90C induced isoelectric EEG (Table 1), equivalent to brain death, but not 60N over 15 min, which then had to be euthanised using 90C for ethical reasons.

Table 1.  Latency (sec) and range of latency (sec; in brackets) to the onset of transitional and isoelectric EEG in Experiment 3. Values are means ± SE

The EEG results supported the behavioural findings by demonstrating differences in terms of effects on the brain. This means that the behavioural changes seen reflect differences in the piglet’s perceptive experience of the treatments rather than, for example, alterations in motor function. The EEG data strengthen the link between the behavioural results and the implications for animal welfare, namely that N₂O is less aversive than CO₂, taking 13 s longer to induce full loss of consciousness in our settings. This project also demonstrated that 90% N₂O can kill piglets.