Humanely Ending the Life of Animals: Research Priorities to Identify Alternatives to Carbon Dioxide

Abstract

Simple SummaryCarbon dioxide is commonly used for stunning animals prior to killing. It allows several animals to be killed at once, reduces the need for handling, and is a reliable method. However, research in laboratory rodents, poultry, and pigs has indicated that it causes considerable aversion at concentrations above ambient conditions. Currently, there are no available alternatives with desirable characteristics. This manuscript describes a list of research priorities to find and implement the use of alternative methods or agents to improve animal welfare.AbstractThe use of carbon dioxide (CO2) for stunning and killing animals is considered to compromise welfare due to air hunger, anxiety, fear, and pain. Despite decades of research, no alternatives have so far been found that provide a safe and reliable way to induce unconsciousness in groups of animals, and also cause less distress than CO2. Here, we revisit the current and historical literature to identify key research questions that may lead to the identification and implementation of more humane alternatives to induce unconsciousness in mice, rats, poultry, and pigs. In addition to the evaluation of novel methods and agents, we identify the need to standardise the terminology and behavioural assays within the field. We further reason that more accurate measurements of consciousness state are needed and serve as a central component in the assessment of suffering. Therefore, we propose a roadmap toward improving animal welfare during end-of-life procedures.

Aversion to Desflurane and Isoflurane in Sprague-Dawley Rats (Rattus norvegicus)

“…Inhalant anesthetics are commonly administered to induce general anesthesia for procedures, such as surgery, as well as for euthanasia. It is well-established that rats find CO 2 aversive and potentially painful, leading to calls for refinements and alternative killing methods [3,[7][8][9][13][14][15][16][17][18][19][20][21]. Nevertheless, it remains popular and is used more frequently for killing than inhalant anesthetics because it is practical, easy to use, relatively safe for personnel and is a common procedure with established methods [4,14].…”

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

“…Carbon dioxide is widely used in the swine industry with automated gas chambers designed for on-farm use, typically using a fill flow rate of 25% of the chamber volume per minute (half-life 2:46). However, CO 2 is aversive to pigs [5,6] and is a welfare concern [7]. Research in our laboratory [6] found that pigs exposed to CO 2 squealed and flailed when concentrations of CO 2 reached about 20%, which was interpreted as an aversive response.…”

I Am a Compassionate Conservation Welfare Scientist: Considering the Theoretical and Practical Differences Between Compassionate Conservation and Conservation Welfare

“…Within the Conservation Welfare paradigm, progressing more peaceful coexistence also involves: prioritizing research that aims to find methods with fewer welfare impacts, including strategic funding of such research [34,92,93]; and systematically exploring the relative harms associated with existing approaches to facilitate application of those that cause least harm [40,87,91]. Importantly, the harms and benefits of novel approaches to controlling unwanted animals should be systematically evaluated before approval and implementation to determine whether they are genuinely more humane than existing methods (see Responding to new technologies for Conservation below).…”

“…In contrast, many Conservation Welfare scientists accept some such harm if the outcomes facilitate convincing arguments for the need to protect animals in the future from situations that cause those unpleasant experiences. As an example, collation of historical research on the ability of non-human mammals to experience unpleasant breathlessness reviewed by [115]) now allows us to strongly advocate against the use of stunning methods that cause such experiences [92,93]. Animal welfare scientists and others also work towards minimizing the harms experienced by animals subjected to such research [45,120,121] and we should continue to do so and to push for stronger requirements to safeguard the welfare of research animals [44].…”