Rats and Mice are the most common rodents used in research facilities. “Approximately 3 million mice are currently used per year in the United Kingdom for scientific research” (Thomas, Flecknell, & Golledge 1). After completion of the purpose of the study or research, all the used mice and rats are euthanized. Euthanasia entails that every rat or mice deserve a good death that minimizes pain and distress to the animal and, therefore, the process requires a careful assessment of the best and appropriate method.
The most popular method of euthanasia in research rodents is the use of carbon dioxide (CO2). The use of carbon dioxide (CO2) is not an optimal euthanasia agent because it causes aversion, dyspnea, and a high concentration of pain in animals while the addition or replacement of gases used for euthanasia are required to ensure the painless and quick death of mice and rats due to them deserving this.
For many years, scientists have opted for the use of carbon dioxide (CO2) as a euthanasia agent.
According to Marquardt et al. the Annex IV of the Directive 2010/63/EU details that the gradual filling of carbon dioxide (CO2) is considered as a sufficient technique for killing laboratory rodents (2). This is because it quickly causes loss of consciousness in the animals with minimal safety concerns to the personnel inducing the euthanasia. The use of carbon dioxide (CO2) as a method of euthanasia does not necessitate exceedingly particular apparatus or significant training for the staff to carry out the process effectively. Also, research shows that the use of carbon dioxide (CO2) does not result in drug residue that could negatively impact predators or scavengers should the carcass be consumed.
From these reasons, the use of carbon dioxide (CO2) has been considered the most appropriate euthanasia agent under certain circumstances.
Recent studies have caused scientists to question the use of carbon dioxide (CO2) as a euthanasia agent. Marquardt et al. further elucidate that scientists have determined that the exposure to carbon dioxide (CO2) causes aversion in rats and mice as demonstrated by various behavioral tests, such as the aversion-avoidance test, the preference test, and the approach-avoidance test (2). Human being intake of carbon dioxide (CO2) concentration of more than 50% is considered as unpleasant and painful as it provokes a feeling of breathlessness (2). This is because of the quality and quantity of carbon dioxide (CO2) concentration associates entirely to the incomplete pressure of carbon dioxide (CO2) in the blood. Also, the inhaled carbon dioxide (CO2) disseminates into the mucosal cells of the respiratory tract and declines the intercellular Ph by responding with water which may selectively stimulate primary afferent nociceptors. Therefore, after a high concentration of carbon dioxide (CO2) inhaled by an animal, its reaction causes the animal to suffer and eventually die.
Exposure to significantly high concentrations of carbon dioxide (CO2) causes dyspnea in animals. According to Moody, Chua, and Weary, the American Thoracic Society describes feelings of dyspnea as, “a subjective experience of breathing discomfort that consists of qualitatively distinct sensations that vary in intensity’ (298). The authors further explain that human beings express that the feeling is extremely stressful. The perception of dyspnea has been used to cause terror and fright among human beings using a concentration of carbon dioxide (CO2) between 7.5% and 35% (Moody, Chua, & Weary 298). The description of the different feelings of dyspnea is feelings of pathological breathlessness, which includes starvation, tension, and work for air. Studies have shown that the response from human beings on dyspnea corresponds to that from mice and rats as concentrations of carbon dioxide (CO2) in mice and rats between 3%-20% are aversive and 10%-35% causes fear (298). The pathological breathlessness caused by carbon dioxide (CO2) is known as labored breathing in rats and mice, and it is a distressing feeling that causes suffering. Hence, mice and rats do not deserve suffering so that alternatives have to be found in order to ensure their peaceful death.
Research shows that the use of carbon dioxide (CO2) as a euthanasia agent causes a high concentration of pain in animals during the process. Therefore, the associated institutions formulated guidelines that direct the personnel when using carbon dioxide (CO2) to carry out the process of euthanasia. Current guidelines stipulate that the compartment should be filled progressively using the flow speed between 10% and 30% compartment vol/min of carbon dioxide (CO2) (Moody, Chua & Weary 298). This means that the flow speed of less than the 30% compartment vol/min is known to minimize the probability that the carbon dioxide (CO2) concentration in the compartment will surpass painful stages before insensibility is reached. The notion is that a higher level of carbon dioxide (CO2) when administering the process of euthanasia causes less pain as the animal dies instantly is wrong. Therefore, the set guidelines are essential as they ensure that the research animals are well taken care of and deserve a peaceful and painless death.
Evidence of distress, pain, and aversion to significant concentrations of carbon dioxide (CO2) in rodents cause researchers to change the procedure of euthanasia when using carbon dioxide (CO2). Thomas, Flecknel, and Golledge explain that the mentioned negative impacts cause researchers to shorten the duration and exposure to carbon dioxide (CO2) when undertaking euthanasia processes (2). Reducing the period of exposure is desirable when the concentration of carbon dioxide (CO2) to which the animals are exposed is decreased and the levels of distress that the animals’ experience is also decreased. The authors further explain that the best shortening method of exposure is the addition of Nitrous Oxide (N2O) to the euthanasia gas mixture (Thomas, Flecknel, & Golledge 2). The study by Thomas, Flecknel, and Golledge on the role of Nitrous Oxide (N2O) as a second gas in the euthanasia gas mixture illustrate that it shortens the conscious experience of any probable suffering by the rodents during the euthanasia procedure (2). This conclusion shows that the use of Nitrous Oxide (N2O) as a second gas is evidence of animal welfare refinement. However, more research has to be conducted on the use of gases for quicker and painless death of mice and rats.
Researchers have come up with various viable options to replace carbon dioxide (CO2) as a euthanasia agent. For instance, Marquardt et al. conducted a study to evaluate whether Isoflurane and Sevoflurane were better alternatives to carbon dioxide (CO2) as euthanasia agents in mice (1). The results of the study showed that all the three gasses were sensed aversive by the animals in the research (29). However, the researchers explain that the agitated behavior in the mice evoked by inhaling of isoflurane was an excitement phase. On the other hand, the silent behavior by the mice elicited by inhaling carbon dioxide (CO2) was an indication of high-stress levels marked by an increase in adrenaline and noradrenaline. The observations could be very confusing so that interpretations through evaluations of other elements are needed, such as stress hormones in the animals. The researchers of the study concluded that the results of the examination could not determine whether the use of Isoflurane and Sevoflurane were better alternatives to carbon dioxide (CO2) as euthanasia agents (Marquardt et al. 29). Therefore, they urged other researchers to conduct further examinations of the concept with the aim of identifying the better euthanasia agent that would give research animals peaceful and non-painful deaths. Unfortunately, little attention is paid to the current problem of inhumane euthanasia, but further research might change the issue for better.
In conclusion, the use of carbon dioxide (CO2) is not an optimal euthanasia agent because it causes aversion, dyspnea, and a high concentration of pain in animals. Studies have shown that the use of carbon dioxide (CO2) has been used for a long time because it quickly causes loss of consciousness in the animals with minimal safety concerns to the personnel inducing the euthanasia among other reasons. Shortening of the duration and exposure to carbon dioxide (CO2) is a viable option, and it is desirable only when there is an addition of Nitrous Oxide (N2O) to the euthanasia gas mixture. This is done with the aim of providing the research animals with peaceful and non-painful deaths. Other research concentrated on other gases but were unsuccessful to find a better solution to the problem so that the hope remains that future researchers will find the way to grant rats and mice a better alternative.