The 3Rs and humane killing of animals

The “Three Rs” principles of “replacement, reduction and refinement” of laboratory animal use were first proposed by William Russell and Rex Burch in their book ‘Principles of Humane Experimental Technique’, published in 1959. Application of the principles should aim to reduce the adverse effects caused to animals, improve animal welfare and enhance the quality of the science in which the animals are used. These three principles are now widely recognised and are requirements of UK and European laws on the use of laboratory animals.


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Those killing animals for scientific purposes can implement all 3Rs in a number of different ways. Working with others in your research institution can enable tissues to be collected not only for the primary purpose of your research but also for in-vitro and ex-vivo use in other projects – replacing some use of animals. Careful selection of the method of euthanasia, so that it avoids compromising the scientific outcomes of your study allows the highest quality data to be obtained and often reduces the number of animals needed overall.

The major area for application of the 3Rs is in the field of refinement since there are many ways to minimise potential sources of pain or distress associated with euthanasia. Ensuring the animals are handled carefully, or not handled at all, during the process will reduce stress. Choosing the method that causes the least pain or distress, and ensuring that it is carried out carefully and competently can greatly contribute to improved animal welfare.

It is important to keep up-to-date with new research in this area since recommendations as to best practice are revised as new research is published. For example, current recommendations for use of carbon dioxide to kill rodents are to use a gradually rising concentration of the gas, achieved by use of filling rates of 20% of the chamber volume per minute. Recent studies suggest that faster filling rates may be better (30% for rats, Hickman et al, 20161 and 50% for mice, Boivin et al, 20162).

Updated on 12th May 2020

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