Laboratory animal euthanasia

Searching for consensus on the most humane ways to kill laboratory animals.

The inevitable fate for many animals used in scientific research is that they are killed humanely, but are researchers using the best possible methods? A recent meeting in Newcastle focused on the issue of laboratory animal euthanasia, with experts from across the field of animal research attending to discuss current methods and possible alternatives, in an attempt to reach a consensus in this controversial area. NC3Rs grantholder Dr Huw Golledge, University of Newcastle, reports.

  Huw Gollage  When we talk about the number of animals used in scientific research, to all intents and purposes we mean the number of animals killed – the fate of virtually all animals used in labs is to be killed at the end of the studies in which they are used in. There is a moral imperative (backed by the law in many countries, including the UK) to ensure that the many millions of lab animals killed worldwide each year have the most humane deaths possible, yet in many cases it is not known which methods cause the least suffering.

In 2010 I received an NC3Rs strategic award to attempt to identify whether some of the techniques commonly used for killing lab rodents are humane. In addition to experimental work, disseminating the results of this research forms an important part of our programme of work and to that end, supported by the NC3Rs, I recently organised the Newcastle Consensus Meeting on Laboratory Animal Euthanasia. This meeting brought together around 90 researchers, veterinary surgeons and animal care staff, alongside representatives from regulatory bodies, research funders and animal welfare organisations from as far afield as Canada, the USA, New Zealand and Japan to discuss the results from our studies alongside the latest findings from other researchers. Chaired by Mark Prescott (NC3Rs) and Penny Hawkins (RSPCA), talks included the latest research findings on the animal welfare impacts of various methods used to kill rats, mice and fish – the three species which account for the vast majority of animals used in labs – as well as discussions on how to interpret the data and how it might be incorporated into practice. 


Continued concern over the use of Carbon Dioxide

Evidence was presented which calls into question the use of carbon dioxide (CO2), the most commonly used method to kill laboratory rats and mice. Joanna Makowska from the University of British Columbia (UBC) showcased results collected over more than a decade at UBC’s Animal Welfare Program which consistently show that mice and rats will avoid CO2 if given the choice, even if this means foregoing a highly-desired food reward or exposing themselves to an aversive stimulus.

Results from my own research confirm and extend these findings, demonstrating that CO2 causes conditioned place aversion – rats remember the place where they have been exposed to CO2 and learn to avoid it even in its absence. That rats can learn to avoid places associated with exposure to CO2 strongly suggests that CO2 causes negative affective (emotional) responses. In all likelihood breathing even relatively low levels of CO2 can make rodents fearful, as it does in humans, this suggests that CO2 cannot be used to kill rodents without at least some negative impact on their welfare.

Nicole Marquardt from the Freie Universität in Berlin, used a different approach to reach a similar conclusion. Her work shows that levels of the stress hormones adrenaline and noradrenaline are elevated in the blood of rats, mice and hamsters killed with CO2, suggesting that they may be under stress at the time they lost consciousness. 


Searching for alternatives to CO2

Studies from both the group at UBC and my work at Newcastle also suggest that some commonly suggested alternatives to CO2 are not without potential negative impacts on animal welfare. For example, the inert gas argon (an agent recently permitted as an alternative to CO2 by the new European Union Directive on the protection of animals used for scientific purposes) is highly aversive to rodents, causing an even stronger conditioned aversion than CO2. The anaesthetic isoflurane also appears to be aversive in both my experiments and in studies conducted at UBC, yet according to Makowska it may be less so than CO2, particularly where animals have not been exposed to it previously. However, on repeated exposure rats rapidly develop an aversion to isoflurane which is as strong as that to CO2.  

It is often assumed that physical techniques such as cervical dislocation are humane methods for killing small rodents, but Larry Carbone from the University of California, San Francisco described his recent study which shows that this technique has a surprisingly high failure rate, and therefore may not offer a consistently humane alternative  

The absence of a clear alternative to CO2 serves to emphasise the need for a continued search for a truly humane method to kill rodents in labs. Furthermore, the fact that rats seem to rapidly learn an aversion to isoflurane raises broader questions about the use of this agent for repeatedly anaesthetising animals (as is common in imaging studies, for instance).


Providing evidence to drive change in regulation and practice.

Zebrafish were also high on the agenda at August’s meeting with a simple welfare refinement becoming apparent as a result of research on aversion to anaesthetic agents which are often also used to kill lab fish presented by Gareth Readman from AstraZeneca’s Brixham laboratory and Bristol University, and Dan Weary of UBC. They took the complementary approaches of examining aversion to fish anaesthetics in the presence of these agents as well as learned aversion, respectively. In both cases, anaesthetic agents such as metomidate or etomidate were shown to be significantly less aversive to zebrafish than one of the most commonly used anaesthetics: MS-222 (also known as tricaine or TMS).

The meeting concluded with a session examining how government regulators and animal welfare advocates evaluate and act upon animal welfare evidence. Ngaire Dennison from the UK Home Office Animals in Science Regulation Unit (ASRU) gave some intriguing regulatory examples that opened many researchers’ eyes (including mine) to the difficult decisions regulators must take when interpreting and acting upon the scientific evidence presented to them, especially when results are conflicting, or when the available data must be extrapolated to take into account different species, strains or developmental stages.

An audience poll before and after the talks suggested some significant shifts in attitudes towards various techniques: most notably that many attendees were more concerned about the use of CO2 to kill rodents and the use of MS-222 to kill fish after hearing the presentations. Also, the majority of attendees were convinced that inert gases such as argon are not a humane replacement for CO2.

While reaching a consensus continues to be a challenge in this controversial area, results from this meeting and its predecessor in 2006 will contribute to shaping and promoting future research, and ultimately result in refinements in how the unpleasant but as yet unavoidable task of killing laboratory animals is undertaken.

A report summarising the meeting outcomes will be made available in due course.

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