During the course of research, rodents are commonly euthanized for tissue collection, to end suffering or to terminate surplus animals. Several methods of euthanasia are possible, and out of these, CO2 euthanasia is most preferred.
An overview of the types of euthanasia available:
Cervical dislocation describes a method in which the head is separated from the spine. If performed correctly, it can be a quick death. However, it is technically demanding and any errors would result in suffering by the rodent. Due to it’s technical nature and the potential for error, it is not a commonly used technique.
Decapitation describes the physical separation of the head from the body. Some investigators may pursue this method of euthanasia because it does not contaminate the blood or the tissues, which may be of importance in some studies. While the death itself is quick, again, it relies on the skill of the technician to perform the euthanasia quickly and flawlessly. Additionally, the handling and restraint required to decapitate a rodent may cause unnecessary stress prior to death.
Chemical methods of euthanasia are generally brought about through the use of lethal injections, typically barbituates. These are generally administered through a subQ injection. Chemical euthanasia does not require a lot of handling nor technical skill, but some chemicals may be controlled substances or be considered too costly and time consuming to use on large numbers of animals.
Finally, CO2 euthanasia is a form of gaseous euthanasia. It is the most common form used in laboratory animal science. It is cheap, requires little to no technical skills, and has some anesthetic properties. In this form of euthanasia, carbon dioxide is allowed to gradually fill a chamber containing one or more rodents. As the oxygen levels decline, the animals are rendered unconscious, followed by death from asphyxiation.
While CO2 euthanasia has been the go-to method in recent years, a 2006 thesis by a UBC Animal Welfare Program grad suggests that CO2 euthanasia may cause distress* in rodents due to the dyspnea (the sensation of “breathlessness”) that precedes unconsciousness. The idea that CO2 euthanasia causes distress in rodents has been explored in other studies as well.
I will note that the UBC Animal Welfare Program does not harm animals used in it’s research nor does it breed animals for the purposes of research. All animals used are “surplus” from other labs which would have otherwise been euthanized. I will also note that all information provided here is freely available from the above mentioned papers, Wikipedia, and other online sites.
KM Conlee, ML Stephens, AN Rowan and LA King (2005) Carbon dioxide for euthanasia: concerns regarding pain and distress, with special reference to mice and rats. Lab Anim 39:137-161.
Lee Niel Ph.D. (2006) “Assessment of distress associated with carbon dioxide euthanasia of laboratory rats” (thesis)
* “Distress” is one of those nebulous, poorly defined words used in animal research. Much like “ethics”. ;) For the purposes of this post, I use “distress” as defined by the author: “… an umbrella term that encompasses negative affect associated with more specific negative states such as pain, discomfort and fear”.