The Three Rs
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- Replacement: methods which avoid or replace the use of animals in research
- Reduction: use of methods that enable researchers to obtain comparable levels of information from fewer animals, or to obtain more information from the same number of animals.
- Refinement: use of methods that alleviate or minimize potential pain, suffering or distress, and enhance animal welfare for the animals used.
The 3Rs have a broader scope than simply encouraging alternatives to animal testing, but aim to improve animal welfare and scientific quality where the use of animals cannot be avoided. In many countries, these 3Rs are now explicit in legislation governing animal use.
It is usual to capitalise the first letter of each of the three 'R' principles (i.e. 'Replacement' rather than 'replacement') to avoid ambiguity and clarify reference to the 3Rs principles.
In 1954, the Universities Federation for Animal Welfare (UFAW) decided to sponsor systematic research on the progress of humane techniques in the laboratory. In October of that year, William Russell, described as a brilliant young zoologist who happened to be also a psychologist and a classical scholar, and Rex Burch, a microbiologist, were appointed to inaugurate a systematic study of laboratory techniques in their ethical aspects. In 1956, they prepared a general report to the Federation's committees, and this report formed the nucleus of the book which was completed at the beginning of 1958. Over much of the period they worked with a special Consultative Committee, chaired by Professor P.B. Medawar.
Scope and development
A common misconception of the 3Rs is that they refer only to replacement; however, their scope is much broader.
Replacement: In the original book, the 3Rs were restricted, arbitrarily, to vertebrates. Russell and Burch discussed the possibility of suffering with reference to sentience. They used the term "replacement technique" for any scientific method using non-sentient material to replace methods which use conscious living vertebrates. This non-sentient material included higher plants, microorganisms, and the more degenerate metazoan endoparasites which, they argued, had nervous and sensory systems that were almost atrophied. They acknowledged that the arbitrary exclusion of invertebrates meant that in several contexts, these species could be considered as possible replacements for vertebrate subjects; they termed this "comparative substitution". Russell and Burch also considered levels of replacement. In "relative replacement", animals are still required, though during an experiment they are exposed, probably or certainly, to no distress at all. In "absolute replacement", animals are not required at all at any stage.
Replacement strategies include:
- Tissue culture
- Perfused organs
- Tissue slices
- Cellular fractions
- Subcellular fractions
More recent interpretations of the replacement principle suggest the preferred use of non-animal methods over animal methods whenever it is possible to achieve the same scientific aims, i.e. invertebrates are not considered suitable replacements for vertebrates. However, others such as the National Centre for Replacement, Refinement and Reduction of Animals in Research (NC3Rs) advocate the use of some invertebrates in replacement studies.  Therefore, the term 'Replacement' can refer to the use of a less sentient species, as in "relative replacement".
Reduction: Reduction refers to methods which minimise the number of animals used per study. Russell and Burch suggested a reduction in the number of animals used could be achieved in several ways. One general way in which great reduction may occur is by the right choice of strategies in the planning and performance of whole lines of research. A second method is by controlling variation amongst the animals used in studies, and a third method is careful design and analysis of studies. With the advent, development and availability of computers since the original 3Rs, large data-sets can be used in statistical analysis, thereby reducing the numbers of animals used. In some cases, by using previously published studies, the use of animals can be totally avoided by avoiding unnecessary replication. Modern imaging techniques in conjunction with new statistical analysis methods also allow reductions in the numbers of animals used, for example, by providing greater information per animal.
Refinement: Russell and Burch wrote "Suppose, for a particular purpose, we cannot use replacing techniques. Suppose it is agreed that we shall be using every device of theory and practice to reduce to a minimum the number of animals we have to employ. It is at this point that refinement starts, and its object is simply to reduce to an absolute minimum the amount of distress imposed on those animals that are still used." Amongst areas of experiments that can be refined are the procedure to be used, the appropriateness of the species (its suitability for the procedure and its responses to a laboratory environment in general). Refinements techniques may include:
- Non-invasive techniques
- Appropriate anaesthetic and analgesic regimes for pain relief
- Training animals to voluntarily co-operate with procedures (e.g. blood sampling) so that they have greater control over the procedure reduce distress
- Provision of species-appropriate housing and environmental enrichment which meet the animals' physical and behavioural needs (e.g. providing opportunities for nesting for rodents)
The definition of Refinement has evolved from that provided by Russell and Burch. A newer definition is now commonly accepted, any approach which avoids or minimises the actual or potential pain, distress and other adverse effects experienced at any time during the life of the animals involved, and which enhances their wellbeing. Refinement encompasses not only the direct harms associated with animal use, but the indirect, or contingent harms associated with breeding, transportation, housing and husbandry.
Some have criticized the Three Rs for what they call "ambiguities" and tensions in the understanding and implementation of different prongs of the approach –Refinement, Reduction and Replacement. This is, in part, because different stakeholders (e.g. animal experimenters, institutional figures, policy makers, activists and the public) may interpret the Three Rs differently. The 3Rs principles do not address some issues, such as the ethics of using animals in research and focus instead on improving the humane use of animals which are used.
Others have noted that promotion of the 3Rs has failed to reduce the number of animals used in experiments. However, this may be the result of a misunderstanding of the definition of 'Reduction', not an absolute reduction in the number of animals used, but a reduction in the number of animals used per study. By its nature, it is difficult to estimate the number of animals not used in scientific procedures as a result of Replacement or Reduction techniques however, despite the rapid increases in medical research, animal numbers have not increased at the same rate.
In a review of dozens of articles involving mice in prolonged pain experiments, researchers found "there were no references to the "3Rs"" which in turn "raise serious questions about whether the 3Rs' principles of Replacement, Reduction, and Refinement are being appropriately implemented by researchers and institutions". The researchers continued, "That the 3Rs or any of the 3Rs' components—Replace, Reduce, or Refine—were not mentioned in any of the... studies suggests that prolonged mouse pain researchers may be unaware of or indifferent to the 3Rs framework and that this aspect is not considered relevant in the peer review process of manuscripts for scientific journals... [T]he growing proportion of the number of studies...in this paper suggests that adherence to guidelines and/or animal use committee requirements is not translating into significant progress from a reduction or replacement perspective". Following a review of the quality of experimental design in published journal articles, including the use of the 3Rs, it was found that the use and reporting of these principles was sporadic. As a result, the ARRIVE (Animal Research: Reporting of In Vivo Experiments) guidelines were developed and published in 2010. The ARRIVE guidelines present a 20-point list of items which must be reported in publications which have used animals in scientific research, including sample size calculations, explicit descriptions of the environmental enrichment employed and welfare-related assessments made during the study. Many journals now require authors to comply with the ARRIVE guidelines in the preparation of manuscripts. A follow-up review published in 2014 found that there were still low reporting levels of some elements, such as reporting of appropriate statistical methods and the avoidance of bias.
In a survey of scientists in Portugal who had recently undergone training in the Three Rs, researchers found that a "surprisingly large number of researchers were unaware of the 3Rs principle, even those who had worked with animal models for over 10 years" and that subsequent training in the Three Rs "did not change perceptions on the current and future needs for animal use in research", but did increase knowledge of the application of the 3Rs The authors found that the training they provided "appear to have little influence on researchers' acceptance of replacement alternatives to animal use".
The Home Office (UK) led the Inter-Departmental Group on Reduction, Refinement and Replacement, which aims to improve the application of the 3Rs and promote research into alternatives, reducing the need for toxicity testing through better sharing of data, and encouraging the validation and acceptance of alternatives. The Data Sharing Group drafted the Inter-Departmental Data Sharing Condordat published in August 2000 and was re-formed in June 2002 to consider the scope for the greater application of the 3Rs. Arising from the Government response to the report of the House of Lords Select Committee on animals in scientific procedures, the Group was asked to explore the scope for a UK centre for research into the 3Rs. The Group reported to Ministers that there was support for a body which would act as a means to better publicise and coordinate what is already done by way of research into the 3Rs. In May 2004, the NC3Rs was announced in the UK to act as a focal point for research into the 3Rs. Although the principles of the 3Rs were implicit in UK law under the Animals (Scientific Procedures) Act (1986), the Directive 2010/63/EU governing animal use within the European Union  makes the principles explicit and researchers must demonstrate the use of Replacement, Reduction and Refinement techniques in research involving animals.
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