Gas alternatives to carbon dioxide for euthanasia: A piglet perspective

The search for alternative methods to euthanize piglets is critical to address public concern that current methods are not optimal. Scientific evidence supports that blunt force trauma is humane when performed correctly, but most people find it visually difficult to accept. The use of CO2 is often recommended; at the same time, it is criticized as being aversive to pigs. This research sought to 1) identify a method of scientifically determining if piglets find a gas aversive, using an approach-avoidance test, which relies on the perspective of the piglet, and 2) test different gas mixtures to determine if they are effective and humane for neonatal piglet euthanasia. Pigs were allowed to walk freely between 1 chamber filled with air and another chamber either gradually filled with gas mixtures (Exp. 1) or prefilled with gas mixtures (Exp. 2). Experiment 1 tested CO2 (90%) and air (10%), N2O (60%) and CO2 (30%), Ar (60%) and CO2 (30%), and N-2 (60%) and CO2 (30%). Because piglets had to be removed when they started to flail, the test was shortest (P < 0.01) for the pigs in the CO2 treatment compared with pigs in the N2O/CO2, Ar/CO2, and N-2/CO2 treatments, 3.1 +/- 0.2, 8.5 +/- 0.6, 9.6 +/- 0.4, and 9.9 +/- 0.1 min, respectively. Nonetheless, all gas mixtures adversely affected the pigs, causing the pigs to leave the test chamber. In Exp. 2, piglets were allowed to enter a chamber prefilled with N-2/CO2 or N2O/CO2 (both 60/30%). Pigs exposed to the prefill chambers started to flail in fewer than 20 s, much faster in comparison with the gradual fill method, which supports that this method was more aversive. In Exp. 3, piglets were euthanized using a 2-step procedure. Pigs were first placed in a gradual fill chamber with 1 of 4 gas mixtures: 90% CO2, N-2/CO2, N2O/CO2, or N2O/O-2 (the last 3 mixtures at 60/30%) followed by placement into a 90% CO2 prefill chamber when the pigs started to flail or were anesthetized. All 3 gas treatments that contained CO2 killed pigs more quickly than N2O/O-2 (P < 0.05). However, N2O/O-2 was the only treatment that anesthetized the pigs instead of causing squeals or flailing although requiring about 12 min longer. Although longer, a 2-step procedure in which pigs are anesthetized with a mixture of N2O and O-2 before being euthanized by immersion in CO2 may prove to be more humane than CO2 alone.