For Animals in Research, Euthanasia Should Not Add Suffering

The conclusion of a study almost always means euthanasia (from the Greek for “easy death”) for animals in research. As with all phases of research, there are moral, regulatory, and scientific imperatives to use the least painful and stressful method possible. These imperatives have led to much debate, including a recent symposium sponsored by the American Veterinary Medical Association (AVMA) that focused on improving animal euthanasia methods, due to growing realization that one of the most commonly used methods is inhumane, causing both pain and distress.

Much of the discussion has focused on which methods should be used for rats and mice, given the vast numbers killed following experimentation. The most common method to euthanize them is suffocation with carbon dioxide (CO2) gas, due to its low cost, relatively rapid action, and ease and long history of use. However, as evidence mounts from decades of studies that CO2 is aversive and painful to rodents, its use is increasingly questioned. Studies have demonstrated CO2-induced distress, likely caused by dyspnea (“air hunger”), including increased rearing, escape behaviors, and vocalizations. A 1958 study documented seizures. A 2009 study found that rats choose to escape chambers with CO2, even when they contain a sweet food reward. At a concentration just one-third the level required to produce unconsciousness, CO2 stimulates parts of the brain associated with fear behaviors. At higher concentrations, CO2 turns to carbonic acid upon contact with mucous membranes, eliciting significant pain. Just the sound of the gas whistling into the euthanasia chamber produces substantial stress responses in the animals. Even when the animals look like they are not stressed, they may be displaying a fear response, such as lying still to avoid pain or as a natural behavior by a prey species to evade detection.

CO2 euthanasia not only causes animal suffering, it can also result in skewed research data. One study examined the potential influence of CO2 euthanasia in rodents by testing various drugs; the authors found plasma concentration differences to be so significant, they suggest that reevaluation of data generated from research using CO2 may be warranted. Other studies have shown that pain during euthanasia can significantly affect brain chemistry, as well as blood and tissue composition.

Thus, more humane methods are needed when animals are to be killed. Specifically for CO2 euthanasia, studies suggest that using an anesthetic gas first (Wong, 2013) or in combination with CO2 (Thomas, 2012) may significantly reduce the stress and pain associated with using CO2 alone.

Institutions must examine all euthanasia methods carefully, as they have a regulatory obligation to use humane euthanasia techniques. The USDA mandates methods that produce death “without evidence of pain or distress.” The National Institutes of Health (NIH) ultimately defers to the AVMA Guidelines on Euthanasia, which state that CO2 “is acceptable, with conditions, for euthanasia in those species where aversion or distress can be minimized,” but notes that there are many disadvantages to its use (AVMA, 2013). Yet, most US institutions are not changing their practices because the data are not unequivocal in favor of anesthesia. While some data may suggest that some anesthetic methods are aversive to rodents, the data are compelling that CO2 is aversive and should not be used alone. Even if CO2 is conditionally acceptable, watching animals gasp for breath as they suffocate is an emotionally draining experience.

This dithering by US institutions and oversight bodies is also at odds with the findings of the Canadian Council on Animal Care, which states that CO2 alone should not be used when other methods are practical, and recommends anesthetizing animals prior to its use. A 2005 report commissioned by the European Union regarding the welfare of animals in research included a recommendation that CO2 should never be used alone in any species unless the animal has first been rendered unconscious, and that it should be phased out “as soon as possible.” As Dr. Jim Gourdon, director of the Comparative Medicine and Animal Resources Centre at McGill University (quoted in The Scientist) put it: even if questions remain, “‘in doubt, … let’s lean toward animal welfare.’”

AWI’s Policy on Research and Testing with Animals addresses these scientific and regulatory issues, but also confronts the moral obligations of those who must euthanize animals by stating: “Euthanasia must be considered a major responsibility. Staff carrying out euthanasia must be well trained, efficient in performing the procedure, and empathetic to the animals. The primary concern must be the animals.”

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