Gas alternatives to carbon dioxide for euthanasia: A piglet perspective

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 if the pig is judged unable to recover. Any euthanasia method should be humane, practical, economical and socially acceptable to be universally accepted.

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 piglet’s own perspective (Experiments 1 and 2), and 2) test different gas mixtures to determine if they are effective and humane for neonatal piglet euthanasia, using a two-step anesthesia-euthanasia procedure (Experiment 3). The investigators hypothesized that nitrous oxide, and alternatively argon and nitrogen, were less aversive than carbon dioxide and would induce anesthesia prior to using carbon dioxide to complete the euthanasia procedure.

Pigs were allowed to walk freely between one chamber filled with air and another chamber either gradually filled with gas (Experiment 1) or pre-filled with gas (Experiment 2). Experiment 1 tested carbon dioxide, CO₂ (90%) and air (10%); nitrous oxide, N₂O (60%) and CO₂ (30%); argon, Ar (60%) and CO₂ (30%); and nitrogen, N₂ (60%) and CO2 (30%). Since piglets had to be removed when they became panicked, the test was shortest (P < 0.01) for the pigs in the CO₂ treatment compared to pigs in the N₂O/CO₂, Ar/CO₂, and N₂/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 Experiment 2, piglets were allowed to enter a chamber pre-filled with N₂/CO₂ or N₂O/CO₂ (both 60%/30%). Pigs exposed to the pre-fill chambers entered a state of panic in less than 20 s, much faster in comparison to the gradual fill method, which support than this method was more aversive. In Experiment 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, N₂/CO₂, N₂O/CO₂ or N₂O/O₂ (the last three mixtures at 60%/30%) followed by placement into a 90% CO₂ pre-fill chamber when the pigs became panicked or were anesthetized. All three gas treatments that contained CO₂ killed pigs more quickly than N₂O/O₂ (P < 0.05). However, N₂O/O₂ was the only treatment that anesthetized the pigs instead of causing squeals or panic, although requiring about 12 min longer. Although longer, a 2-step procedure in which pigs are anesthetized with a mixture of N₂O and O₂ prior to being euthanized by immersion in CO₂ may prove to be more humane than CO₂ alone.

They found that nitrous oxide in oxygen appeared to be less aversive than nitrous oxide, nitrogen, or argon all combined with low (30%) concentrations of carbon dioxide or 90% carbon dioxide by itself. This study is the first to investigate the use of nitrous oxide at sufficiently high concentrations to cause anesthesia. Nitrous oxide, commonly referred to as laughing gas, has been widely used in human surgery and dental offices for its pain-relieving, sedative and anxiolytic effects. It is cheap, non-flammable, non-explosive, legally accessible and not classified as a drug in the U.S., and already commonly used in the food industry as a propellant for food products. Development of its use into an automated procedure will allow producers to implement it with little effort. Thus its use as an anesthetic/euthanasia agent may prove to be affordable, feasible and more humane than other alternatives.

J.-L. Rault, K. McMunn, J. Marchant-Forde, D. Lay. Gas alternatives to carbon dioxide for euthanasia: A piglet perspective. ARS-USDA.