Candyflipz
New member
I just wanted to express my frustration with so frequently seeing the wide misconception that using CO2 is an inhumane method of euthanizing feeder mice because of uncomfortable suffocation. I used CO2 quite frequently in my labs to anesthetize animals we were performing genetic crossing experiments on and never witnessed anything that gives me reason to believe it is harmful to the animals.
The misconception I've seen most often is the thought that if you place mice/rats into a CO2 chamber, it pushes the oxygen out of the chamber the rodents are contained in, and basically suffocates them. There was also a thread I found that someone insisted the CO2 closes airways, and therefore causes asphyxiation, with no mention of blood levels or brain activity.
So I decided to do my own research and not use any sort of online resource other than published scientific journal articles on experiments done by licensed and trained professional scientists. In this way there is minimal bias, and raw data to compare and make my own conclusions.
I encourage everyone who is interested and/or worried about the subject to do the same! There are dozens of science journals online and just as many search databases!
From my searches I found a few sources that performed experiments examining the aversion witnessed by rodents when exposed to CO2. They also did similar experiments on humans. The widely accepted ethical protocol on humane scientific practice on animals is that if the same procedure is performed on a human and it is painful and/or adverse, then it is generally not to be used in the laboratory on animals (K.M.Conlee et. al.). CO2 euthanization has been widely used and accepted in the past because of its rapid and painless qualities. It has only been in the recent years that any question has been asked that this method may not be as humane as once believed.
Before beginning this search I was certain this method was perfectly harmless, painless, and pleasant. I was initially performing the search to prove my point to all the non-believers. What I found, however, was experimental, statistical, and scientifically sound evidence that it may not be as humane as once thought.
Studies have been done on both rodent and human to test the physical effects experience during CO2 asphyxiation. In one study (Measurement of aversion to determine humane methods of anesthesia and euthanasia ) CO2 was examined as a method of euthanization and possible aversion exhibited by the rodents was measured. The conclusion? CO2 administration to mice and rats were found to be aversive agents (if you'd like to read how their experiment was performed, follow the link). A previous study was published with inconclusive results as whether reactions were observed before or after unconciousness; but found to cause many adverse reactions in rodents, increasingly so with LOWER concentrations of CO2 over LONG PERIODS OF TIME.
In another study, human subjects were asked to inhale a deep breath of various concentrations of CO2. In lower concentrations subjects reported feeling "irritation of the nose, palpitation, faintness, ‘generally uncomfortable,’ muscle tremor, and substernal pain." In higher concentrations "Many described 100% CO2 as piercing, stabbing, painful or causing the eyes to burn or water." This can be found in "Carbon dioxide for euthanasia: concerns regarding pain and distress" published in Laboratory Animals Volume 39 Issue 2 Pages 137-161 (I have a PDF version of the report if you'd like to see it)
There were studies that found that it was one of the most effective substances for euthanization rapidly. It is noninvasive as you do not have to inject the substance directly, along with many other positive qualities (not to mention readily available and cheap). Check out Euthanasia of mouse fetuses and neonates published in Contemporary Topics in Laboratory Animal Science Volume 43 Issue 5 Pages 29-34. From much of my reading I have seen much of the adverse reactions occurring AFTER the loss of conciousness...and therefore minimally unpleasant for the animal. Most of the studies done used methods for measuring adverse effects of the gas while the subject was alert or administered concentrations of CO2 to alert subjects for extended periods of time. From my experience of euthanization, the animal is unconscious much sooner than the experimentally reported times of adverse reactions in rodents.
So my conclusion: CO2 Does give an unpleasant experience to euthanization subjects when breathing high concentrations for long periods of time and remaining alert. However, in the common home-euthanized feeder rodents' case, they generally loose consciousness far before severe discomfort is experienced. Should this mean the practice should be regulated in at-home administration? I don't like the idea of it because if I ever do decide to breed mice, I'd rather use CO2 than put the mouse in a bag and whacking it's head against the wall/floor or smashing it as I cannot imagine how that is a consistent means of humane euthanization. I guess it is up to the individual what your opinion/practices are.
The misconception I've seen most often is the thought that if you place mice/rats into a CO2 chamber, it pushes the oxygen out of the chamber the rodents are contained in, and basically suffocates them. There was also a thread I found that someone insisted the CO2 closes airways, and therefore causes asphyxiation, with no mention of blood levels or brain activity.
So I decided to do my own research and not use any sort of online resource other than published scientific journal articles on experiments done by licensed and trained professional scientists. In this way there is minimal bias, and raw data to compare and make my own conclusions.
I encourage everyone who is interested and/or worried about the subject to do the same! There are dozens of science journals online and just as many search databases!
From my searches I found a few sources that performed experiments examining the aversion witnessed by rodents when exposed to CO2. They also did similar experiments on humans. The widely accepted ethical protocol on humane scientific practice on animals is that if the same procedure is performed on a human and it is painful and/or adverse, then it is generally not to be used in the laboratory on animals (K.M.Conlee et. al.). CO2 euthanization has been widely used and accepted in the past because of its rapid and painless qualities. It has only been in the recent years that any question has been asked that this method may not be as humane as once believed.
Before beginning this search I was certain this method was perfectly harmless, painless, and pleasant. I was initially performing the search to prove my point to all the non-believers. What I found, however, was experimental, statistical, and scientifically sound evidence that it may not be as humane as once thought.
Studies have been done on both rodent and human to test the physical effects experience during CO2 asphyxiation. In one study (Measurement of aversion to determine humane methods of anesthesia and euthanasia ) CO2 was examined as a method of euthanization and possible aversion exhibited by the rodents was measured. The conclusion? CO2 administration to mice and rats were found to be aversive agents (if you'd like to read how their experiment was performed, follow the link). A previous study was published with inconclusive results as whether reactions were observed before or after unconciousness; but found to cause many adverse reactions in rodents, increasingly so with LOWER concentrations of CO2 over LONG PERIODS OF TIME.
In another study, human subjects were asked to inhale a deep breath of various concentrations of CO2. In lower concentrations subjects reported feeling "irritation of the nose, palpitation, faintness, ‘generally uncomfortable,’ muscle tremor, and substernal pain." In higher concentrations "Many described 100% CO2 as piercing, stabbing, painful or causing the eyes to burn or water." This can be found in "Carbon dioxide for euthanasia: concerns regarding pain and distress" published in Laboratory Animals Volume 39 Issue 2 Pages 137-161 (I have a PDF version of the report if you'd like to see it)
There were studies that found that it was one of the most effective substances for euthanization rapidly. It is noninvasive as you do not have to inject the substance directly, along with many other positive qualities (not to mention readily available and cheap). Check out Euthanasia of mouse fetuses and neonates published in Contemporary Topics in Laboratory Animal Science Volume 43 Issue 5 Pages 29-34. From much of my reading I have seen much of the adverse reactions occurring AFTER the loss of conciousness...and therefore minimally unpleasant for the animal. Most of the studies done used methods for measuring adverse effects of the gas while the subject was alert or administered concentrations of CO2 to alert subjects for extended periods of time. From my experience of euthanization, the animal is unconscious much sooner than the experimentally reported times of adverse reactions in rodents.
So my conclusion: CO2 Does give an unpleasant experience to euthanization subjects when breathing high concentrations for long periods of time and remaining alert. However, in the common home-euthanized feeder rodents' case, they generally loose consciousness far before severe discomfort is experienced. Should this mean the practice should be regulated in at-home administration? I don't like the idea of it because if I ever do decide to breed mice, I'd rather use CO2 than put the mouse in a bag and whacking it's head against the wall/floor or smashing it as I cannot imagine how that is a consistent means of humane euthanization. I guess it is up to the individual what your opinion/practices are.
