Home: Euthanasia

Euthanasia – The Carbon Dioxide Debate

As you probably know, there are lots of ways of putting an animal to sleep, of which a select few are humane, quick and painless. These methods have been in debate for years. As you would expect, different animal – different answer. But I personally believe that no matter how big or small the animal, they all deserve to die as comfortably as possible in the prevention of further suffering. The following incorporates an online discussion about euthanasia in mice and the use of CO2 in particular. Before we begin I would like to thank Joe Willmore for his participation and large input into this discussion.


I believe that if we assume ownership of a pet, we take on a guardian responsibility that involves us having to make some decisions about their welfare and the quality of their life - thus euthanasia is an issue that responsible pet owners have to confront and resolve for themselves where they stand on this issue.

Let me offer some background information about the use of CO2 (carbon dioxide). I understand the concerns about the use of CO2 (carbon dioxide) as an agent for euthanasia in mice. And let me say at the outset that (a) when it comes to pets, there are no easy decisions and I understand it's a difficult choice for each of us when it comes to those we love dearly and respect and (b) this issue (euthanasia) is fraught with a number of moral and ethical dilemmas as well as conflicting research and data. So I offer these comments only in the form of information and with respect to individual beliefs and situations. Ultimately, each of us must do what we feel is best and what we can ethically and morally live with.

Before we start to talk about euthanasia using carbon dioxide, for the sake of clarity let's distinguish between CO2 (carbon dioxide) and CO (carbon monoxide). CO is what you get when you run your car engine in an enclosed space. It also produces unconsciousness and eventually death. I'm not aware of any details on using CO as a means of euthanasia for mice. My understanding is that (according to what I've read), CO2 as a form of euthanasia is relatively common with vets. However, that has not been the case with the vets in my immediate area (most of whom do not even treat ‘exotic small mammals’, let alone do euthanasia for them).

Typically I haven't used vets for euthanasia because (a) in my neck of the woods, the vets and animal hospitals have very little expertise with mice (there is an animal hospital about 50 minutes away that has a very good reputation with rats though); and (b) typically, decisions about euthanasia have often been ones I've had to make very quickly (a mom who delivered a litter of stillborn pups and then went into distress, a mouse with a tumour that suddenly became obvious was pressing against her vocal chords and, I assume, impairing her ability to eat and breathe). I do not cull (and have tried at times to foster orphans or runts that some parents have taken on themselves to cull - never with any success but that won't stop me from trying!). I've been very fortunate in that with all the mice I've bred (300+? over several years), I've had to euthanise no more than ten adults (all singly). If I owned only one or two mice, I'd probably just go to the extra effort to use a vet (rather than acquire the necessary apparatus).

I had a discussion at one point on a mouse and rat listserv (which consisted of mostly rat owners in the USA) about euthanasia and their position seemed to boil down to the idea of the ‘two shots’ method (one to induce sleep, the second into the heart). Obviously, other groups may take their own positions - it's probably a mistake for me to imply that any group (rat owners, whomever) has a consensus on a preferred means of euthanasia.

First, the issue of injections for mice is one that some of us have problems with because it's a needle (and the injection for the mouse is likely to be very threatening and also very painful). A larger animal (cat, dog) is, in my limited experience, less likely to find the shot traumatic. Second, I've heard different schools of thought on this issue (of shots). I am not a vet and have little recent experience with dogs and none (realistically) with cats. But traditionally (unless different chemicals are used), I've heard that two shots are usually used: one to produce unconsciousness and another to stop the heart and lungs from working.

I've also heard from a number of rat owners/breeders who argue the humane method is a double injection that CO2 is thus inhumane. The problem with this standard (as it was explained to me by both one vet and also someone who did animal studies at a University) is that a rat's heart is pretty easy to find (and thus inject into). Even vets who are used to making such injections with rats will find that the mouse isn't just a smaller version of a rat (as all of us mouse owners know). The chances of the vet either missing the heart or having to make repeated injections is high. Of course, I’m sure there are a number of vets who would insist this is not the case. However, in my experience, very few vets actually have any practical experience making such injections for mice. Consequently, I’m not sure it's accurate or fair to argue that if rat owners/breeders prefer a particular method (double injection) because it works best for rats that a similar process would be appropriate for mice. I mean no disrespect to rat owners, I just think that what some of them recommend (because I've also had this same debate with rat owners/breeders) may be guilty of ethnocentrism - what works for rats must ‘obviously’ be appropriate for other rodents.

Having said that, I am not saying that CO2 is the perfect method of euthanasia for mice. It is of the utmost importance that it is done the right way. For example, here is a question I was asked in the past: ‘I have not had to euthanise any of our mice, but if I had to, I also had thought that CO2 was humane since people who have been subjected to too much CO2 do not seem to suffer. I suppose a mouse in a rather small space airtight would generate its own excess CO2 fairly quickly - I guess adding CO2 just speeds up the process for the mouse?’

I'd argue that just letting the CO2 build up naturally would be a very traumatic experience. Mice don't produce that much CO2 - it would take a very long time. In the meantime they'd produce a tremendous amount of moisture. Mice don't respond well to such enclosed spaces (panic or uncertainty) and they'd soon be wet from all the moisture, and you still wouldn’t have reached a CO2 level sufficient enough to produce unconsciousness. Finally, the issue here isn't that there is no oxygen left in the tank (that truly would be suffocation). In such a state, the mouse would panic and hyperventilate (as it desperately sought oxygen). Instead, you want oxygen in the tank but also high levels of CO2, which produce unconsciousness and then brain cessation.

I don't think it's accurate to say that CO2 ‘suffocates’ an animal (or a human for that matter). In fact, if there is sufficient oxygen in a room or chamber, CO2 produces an effect where part of the brain shuts down and unconsciousness results. Suffocation happens when there is no oxygen or we cut off someone's ability to breathe (such as choking). Typically, what happens with CO2 is that the mouse quickly loses consciousness and actually continues to breathe for up to a minute or more before breathing stops.

If I can use an example to illustrate this, during the Apollo 13 mishap in space, at one point there was concern about excessive CO2 in the space capsule. There was sufficient oxygen for the astronauts but because the filters that removed the CO2 were saturated and the CO2 levels in the capsule were gradually building up. This would produce dulled thinking, slower reaction times and then unconsciousness and death (and this was at relatively low levels of CO2 over time). Low levels of oxygen produce headaches and hyperventilation (much like climbers at high altitudes experience). But placing CO2 in an enclosed chamber does not mean insufficient oxygen.

During the ‘orange’ alert status here in the USA (fear of Al Quaeda attack), all of this talk about finding a ‘safe’ room and taping up vents, windows, etc. has led to the worry that if one were to do that, it might lead to death by CO2. In those instances, it would NOT be a case of someone breathing all the oxygen in the room up (because it's an enclosed space). Rather, it would be because the levels of CO2 would rise to dangerous levels. And we as humans wouldn't notice it. We would just get a little slower in our thinking, then fall asleep and (unless there was an escape for the CO2) ...die. Only at incredibly high levels of CO2 would we notice that there was anything out of the ordinary (those high levels would need to be near 100% from what I understand - and levels of 25% are supposed to be sufficient for humans to become unconscious and then die).

Here are two alternative views from other people with first hand experience of using CO2:

1. ‘CO2 isn't inhumane. Being a Biology major with a Chemistry minor, I have done my fair share of research with CO2 and have heard and read a lot about this gas. The brain does shut down before the suffocation takes place (if there is oxygen present) - there have been many cases where people are working on or in their car, can't smell the CO2 and fall asleep but never wake up, so in all reality the mouse is not feeling anything. Also I don't know if this is just me, but I have had to put many of my mice to sleep so they would no longer suffer and I went to two different vet clinics here in Colorado. They took me back to the room and put my little mouse to sleep with me holding him, then they gave me my mouse, and said since the mouse is so small that they would not charge me. I didn't even pay for an office visit. So I have never had to pay to put one of my mice in a happier place when they are suffering.’

2. ‘I work at a nature centre where feeder mice/rats are raised to feed the animals, mostly snakes, but everything from coyotes to birds of prey to oppossums. We euthanise our rodents (as well as any other animals that come in that are suffering) with CO2.

I've had to euthanise these animals with CO2. It's a difficult thing to do no matter what method you use. I will say (and this is completely my opinion and not against anyone else that supports it) that I would not use this method on my own pets.’

I still find CO2 the most acceptable method and I've had to build my own tank for this process. I use compressed CO2 cartridges, put some CO2 in the tank, put the mouse in (usually with some of their bedding or a cardboard tube). I have a tube running into the tank (through a sealed hole). I then allow CO2 (into the tube - which is clamped) and then unclamp the tube so CO2 enters into the tank raising the CO2 level. The mouse usually loses consciousness within 10-30 seconds of the additional CO2. They continue breathing for up to a minute (with their heartbeat and lung movement slowing and eventually stopping). I usually leave the poor mouse in the tank for another five minutes (since there are a number of cases of mice - especially young ones - who have been frozen or exposed to near death situations who then regained consciousness). I talked to another breeder who used dry ice, which I've avoided because I find it difficult to get plus you have to rig the cage so the dry ice is at the bottom and the mouse is incapable of touching it (say, through a false bottom).

I totally agree that this (euthanasia) is not the sort of thing you try until you've thought it through (morally and operationally) and practiced it without any living creature. While my personal position is that as pet owners we sometimes have an obligation to end life before the quality is diminished (I respect those who feel otherwise), that also means we must exercise informed judgment about this act AND must do so as ‘cleanly’ as possible (in other words, this is no time for ‘amateur hour’ where a mouse is exposed to insufficient or excessive levels of anything or the effort to end their suffering only serves to prolong it).

A number of labs and authorities (Harvard's research standards for animals, Univ. of Melbourne, Berkeley, American Vet. Assoc, Canadian Council on Animal Care, ANZCCART - for Australia and New Zealand) all support or endorse the use of CO2 for euthanasia with mice. Some of these are medical labs but others are not (veterinary societies or animal welfare societies).

The Humane Society of the US has concerns with CO2 and argues that it should first be preceded by an anaesthetic gas (like Isofluorine) to produce unconsciousness. What's interesting is that the HSUS does NOT seem to recommend the double injection method for mice. In fact, the two means of euthanasia I've read about them supporting (from their papers and published positions) appear to be: CO2 (preceded with an anaesthesia) or decapitation (or variations on this). I find the last approach (decapitation and all the cervical dislocation options) totally unacceptable personally. I understand why some labs may choose this approach but I will never accept it with any mice I own.

Again, I offer these thoughts with complete respect to your position and beliefs. I think when we get into debates about animal welfare (especially when the issue of welfare and death intersect - when is it better for a pet to die rather than continue to live?), we face thorny decisions.

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