A Two-Step Process of Nitrous Oxide before Carbon Dioxide for Humanely Euthanizing Piglets: On-Farm Trials

Refinement of water-based foam depopulation procedures for finisher pigs during field conditions: Welfare implications and logistical aspects

Water-based foam (WBF) depopulation is currently being researched as an alternative for rapid destruction of swine populations under emergency circumstances. Appropriate guidelines are needed to maintain method reliability and depopulation efficacy while minimizing animal distress under field conditions. Finisher pigs were depopulated using WBF with a 7.5-minute dwell time in two trials to evaluate the effect of; trial 1) foam fill level (1.5, 1.75, or 2.0 times the pig's head height) and trial 2) foam fill rate (slow, medium, or fast) on aversive pig responses (surface breaks, vocalization, and escape attempts) and time to cessation of cardiac activity. Activity and cardiac activity were recorded using subcutaneous bio-loggers for swine in trial 2. The average time to cessation of movement (COM) from the start of foam filling was compared for the foam fill rate groups using a generalized linear mixed effect model under Poisson distribution. Foam rate group was used as an independent variable, and replicates as a random effect. For trial 1, the average (mm:ss ± SD) time to fill completion was 01:18 ± 00:00, 00:47 ± 00:05, and 00:54 ± 00:05, for 1.5, 1.75, and 2.0 times the pig's head height, respectively. For trial 2, the average time to fill completion was 03:57 ± 00:32, 01:14 ± 00:23 and 00:44 ± 00:03, and the average time (mm:ss ± SE) to COM was 05:22 ± 00:21, 03:32 ± 00:14, and 03:11 ± 00:13 for slow, medium, and fast fill rate groups, respectively. A higher number of aversive pig responses were observed for the lowest foam fill level and slowest foam fill rate compared to increased fill levels and faster fill rates. For trial 2 the median (mm:ss ± IQR) time to fatal arrhythmia was 09:53 ± 02:48, 11:19 ± 04:04, and 10:57 ± 00:47 post-foam initiation for fast, medium, and slow foam rate groups, respectively. Time to cessation of cardiac activity was significantly shorter for the fast foam rate group compared to medium and slow foam rates groups (P = 0.04). For both trials, vocalizations were absent, and all pigs were unconscious following the 7.5-minute dwell time and no pigs needed a secondary euthanasia method. This WBF study showed that slower fill rates and low foam fill levels may extend the time until cessation of cardiac activity in swine during depopulation. A conservative recommendation with consideration of swine welfare during an emergency scenario would be a minimum foam fill level twice the pig's head height and a foam fill rate capable of covering pigs in foam within 60 s to minimize aversive responses and expedite cessation of cardiac activity.

On-farm culling methods used for pigs

The culling of injured and non-viable pigs (Sus scrofa) (neonate to breeding stock) is a routine and necessary procedure on most farms. Usually, pigs are culled using one of the following methods: blunt-force trauma (manual and mechanical), captive-bolt stunners, electrical stunning and electrocution or carbon dioxide. Manual blunt-force trauma is one of the most widely used methods due to its low or absent operational and investment costs. However, as a method, it has serious limitations, which include the risk of incomplete concussion, pain, and distress. Manual blunt-force trauma is also aesthetically unpleasant to operators and wider society. To address these issues there has been significant recent research into the development of alternatives to manual blunt-force trauma, these include: captive-bolt stunners, on-farm, gas-based controlled atmosphere systems, low atmospheric pressure systems and electrical stunning. Some of these are currently in commercial use while others are still in the developmental phase. This review brings together the relevant research in this field, evaluating the methods in terms of mechanism of action (mechanical and physiological), effectiveness and animal welfare.

Review: Potential alternatives to high-concentration carbon dioxide stunning of pigs at slaughter

Using carbon dioxide (CO₂) for stunning pigs at slaughter is common in Europe. The use of group stunning is a major advantage with CO₂, which is done without restraining the pigs and with minimized human contact. However, high concentrations of CO₂ have been known for decades to cause pain, fear and distress in pigs before loss of consciousness, and the stunning method is clearly associated with animal welfare concerns. This study reviewed the scientific literature to find recent developments or evaluations of alternative methods that could lead to the replacement of CO₂ for stunning pigs at slaughter. Potential alternative methods found in the literature were described and then assessed to identify specific research and development needs for their further development. Only 15 empirical studies were found in the search of peer-reviewed literature since 2004, which is less than one per year. Furthermore, half of the studies focused on evaluating methods to improve high-concentration CO₂ stunning rather than an alternative to CO₂. Since no clear alternative has emerged, nor a method to improve CO₂ stunning, there is obviously a strong need to focus research and development to find solutions for improving animal welfare when stunning pigs at slaughter.

A Systematic Literature Review on Depopulation Methods for Swine

Swine mass depopulation refers to the destruction of large numbers of pigs and may include not only animals affected with a disease but also healthy pigs in a facility or surrounding areas. Emerging applications of mass depopulation include reducing welfare issues associated with slaughter delays, which was observed in the United States in 2020 as a result of the Coronavirus disease (COVID-19) pandemic. The objectives of this review were to summarize the available literature on swine depopulation methods and to highlight critical gaps in knowledge. Peer-reviewed articles were identified through a systematic search in electronic databases including Web of Science, MEDLINE, and PubMed. A total of 68 publications were assessed. Gaseous carbon dioxide inhalation was the most commonly reported depopulation method for both small- and large-scale trials. Measurements of consciousness state, which serves to assess suffering and humaneness, appeared to be lacking in a high proportion of the studies. None of the published studies demonstrated an ideally reliable and safe way to induce rapid unconsciousness in large groups of pigs. Development of rapid mass depopulation methods applicable to large groups of pigs is necessary to provide industry partners with suitable and low-cost emergency preparedness procedures while adhering to personnel safety and animal welfare standards. Lastly, there is an urgent need to standardize comprehensive reporting guidelines for depopulation studies.

Wireless 'under the skull' epidural EEG and behavior in piglets during nitrous oxide or carbon dioxide gas euthanasia

Consciousness is central to animal welfare concerns. Its assessment is most often conducted based on behavior, with a poor understanding of the correspondence between behavior and the neurobiological processes that underlie the subjective experience of consciousness. Recording of brain electrical activity using electrodes placed under the skull improves EEG recording by minimizing artifacts from muscular or cardiac activities, and it can now be combined with wireless recording in free-moving animals. This experiment investigated the correspondence between wireless 'under the skull' epidural EEG and the behavior of 18 five-week-old female piglets undergoing nitrous oxide (N₂O) or carbon dioxide (CO₂) gradual fill gas euthanasia at 25% replacement rate per minute of the chamber volume. Piglets exposed to CO₂ had a peak in EEG total power ('Ptot') during the flailing stage, whereas piglets exposed to N₂O had a higher EEG 95% spectral edge frequency ('F95') during their initial explorative behavior phase and a drop in EEG median frequency ('F50') after loss of posture. Loss of posture without righting attempt, as the last behavioral state observed during euthanasia, preceded the onset of transitional EEG on average by 0.9 and 3.1 min (for CO₂ and N₂O treatments, respectively), and the onset of isoelectric EEG by 4.5 and 6.2 min (for CO₂ and N₂O treatments, respectively). Paddling movements occurred shortly before and during transitional EEG but never during isoelectric EEG, whereas gasps persisted after the EEG had become isoelectric. The dynamics of EEG spectral changes were complex to interpret in relation to the degree of consciousness, but isoelectric EEG as an unequivocal indicator of unconsciousness appeared several minutes after loss of posture with no righting attempt. This leaves a window of uncertainty in regards to the potential for consciousness after loss of posture during gradual fill gas euthanasia in piglets.

Welfare of pigs during killing for purposes other than slaughter

Pigs at different stages of the production cycle may have to be killed on-farm for purposes other than slaughter (where slaughter is defined as killing for human consumption) either individually (e.g. severely injured pigs) or on a large scale (e.g. unproductive animals or for disease control reasons). This opinion assessed the risks associated with the on-farm killing of pigs and included two phases: 1) handling and moving of pigs and 2) killing methods (including restraint). The killing methods were subdivided into four categories: electrical methods, mechanical methods, gas mixture methods and lethal injection. Four welfare consequences to which pigs can be exposed to during on-farm killing were identified: pain, fear, impeded movement and respiratory distress. Welfare consequences and relevant animal-based measures were described. In total, 28 hazards were associated with the welfare consequences; majority of the hazards (24) are related to Phase 2 (killing). The main hazards are associated with lack of staff skills and training, and poor-designed and constructed facilities. Staff was identified as an origin of all hazards, either due to lack of skills needed to perform appropriate killing or due to fatigue. Corrective measures were identified for 25 hazards. Outcome tables linking hazards, welfare consequences, animal-based measures, hazard origins, preventive and corrective measures were developed and mitigation measures proposed.

Behavioral Response of Weaned Pigs during Gas Euthanasia with CO2, CO2 with Butorphanol, or Nitrous Oxide

Simple Summary

Pig farmers are forced to euthanize a significant number of pigs due to injuries, hernias, or unthriftiness. The majority of pigs are euthanized using carbon dioxide gas asphyxiation. However, the humaneness of carbon dioxide is being increasingly questioned. An alternative is the use of nitrous oxide gas. We conducted this study to compare the euthanasia of young pigs using nitrous oxide or carbon dioxide. In addition, we tested the administration of a pain relief drug prior to carbon dioxide exposure to determine if we could eliminate behaviors indicative of pain. Pigs became unable to control their muscle movement, breathed heavily, and lost posture at the same time regardless of treatment. Pigs exposed to both gases showed heavy breathing and open-mouth breathing prior to losing posture. However, pigs exposed to carbon dioxide made more escape attempts but fewer squeals than pigs exposed to nitrous oxide. Administration of pain relief prior to exposure to carbon dioxide did not alter behaviors indicative of pain. The findings are inconclusive as to whether using nitrous oxide is significantly better than using carbon dioxide, but the results show that its use is just as effective, and possibly more humane.

Abstract

The swine industry is often forced to euthanize pigs in the first few weeks of life due to injuries, hernias, or unthriftiness. The majority of pigs are euthanized using carbon dioxide (CO₂) gas asphyxiation but concerns as to the humaneness of CO₂ are increasing. This study compared the euthanasia of weaned pigs using N₂O (N₂O; n = 9) or CO₂ (n = 9), at 50% and 25% min⁻¹ exchange rate, respectively. In addition, we administered an analgesic prior to euthanasia with CO₂ (CO₂B) exposure as a third treatment (n = 9) to elucidate behaviors indicative of pain. Pigs in the CO₂ and N₂O treatments lost posture at similar times (latency of 145.0 ± 17.3 and 162.6 ± 7.0 s respectively, p > 0.10), while the CO₂B treatment pigs lost posture the soonest (101.2 ± 4.7 s, p < 0.01). The pigs in the CO₂B treatment made more escape attempts than the CO₂ or N₂O pigs (16.4 ± 4.2, 4.7 ± 1.6, 0.3 ± 0.2, respectively; p < 0.0004). However, pigs in N₂O squealed more often than either the CO₂ or CO₂B pigs (9.0 ± 1.6, 2.8 ± 1.2, 1.3 ± 0.6, respectively, p < 0.001). Given the similar time to loss of posture and shorter time displaying open mouth breathing, N₂O may cause less stress to pigs; however, the greater number of squeals performed by these pigs suggests the opposite. It was not apparent that any behavior measured was indicative of pain. In conclusion, N₂O applied at a 50% min⁻¹ flow rate can be an alternative to CO₂ for pig euthanasia.