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Assessing the severity of animal procedures

Any experiment or procedure conducted on an animal used in biomedical research is recorded and the level of severity (pain, suffering, distress or lasting harm) is also recorded - this is a requirement under EU Directive 2010/63 for all EU Member States.


Animal procedures can only be carried out in an authorised establishment and as part of an authorised research project.

How is the severity of animal procedures recorded?

There are four degrees of severity that are assessed:


  • Non-recovery - Procedures which are performed entirely under general anaesthetic and from which the animal does not recover consciousness.

  • Mild - Procedures where the animal is are likely to experience short-term mild pain, suffering or distress. These are also procedures with no significant impairment to the well-being or general condition of the animal. For example: anaesthesia, non-invasive imaging and scans, short-term social isolation, taking a blood sample, and superficial surgical procedures.

  • Moderate – Procedures on animals where the animals are likely to experience short-term moderate pain, suffering or distress, or long-lasting mild pain, suffering or distress. These are procedures that are likely to cause moderate impairment to the well-being or general condition of the animals. For example: surgery under general anaesthetic; causing cancer in an animal.

  • Severe – Procedures on animals where the animals are likely to experience severe pain, suffering or distress, or long-lasting moderate pain, suffering or distress. These are procedures, that are likely to cause severe impairment to the wellbeing or the general condition of the animal. For example: any test where death is the end-point, or fatalities are expected; testing a device that could cause pain/death if it were to fail; breeding animals with genetic disorders that are expected to experience severe and persistent impairment of their general condition, for example Huntington’s disease, and muscular dystrophy.

What are the levels of severity in procedures in various areas of biomedical research?

Multiple sclerosis 
In the study of multiple sclerosis, which is a condition that can affect the brain and spinal cord, and cause a wide range of potentially severe symptoms, including problems with vision, arm or leg movement, sensation or balance – the severity level is expected to be severe, as the procedure could cause severe impairment of the animal’s general wellbeing and condition.



In the study of arthritis - a common condition that causes pain and inflammation in a joint – animal models, including rats and mice, are used to study the pathogenesis of the disease and to evaluate potential anti-arthritic drugs for clinical use. As a consequence the severity level for animals is expected to be severe.

Cancer studies

Animal models are used in the development of new drugs for the treatment of cancer, in addition to computer modelling and in vitro methodologies such as cell culture assays. Once suitable compounds have been selected by non-animal methods, only those compounds that exhibit favourable characteristics are tested in animals. The severity level to maintain human cell lines in an animal model is expected to be mild and the animals are expected to experience only mild discomfort and would be killed if any health or welfare problems arise above this level.

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How do you protect animals from unnecessary experiments/pain?

The use of animals in the EU is covered by the Directive 2010/63/EU on the protection of animals used for scientific purposes. Under this directive animals can only be used in research:

  • When there is convincing scientific justification

  • When the expected benefits of the research outweigh the potential risks in terms of the suffering of the animals.

  • When the scientific objectives cannot be achieved by using non-animal alternative methods.


Only projects that fulfil these requirements are authorised.


Who assesses these levels of severity?

EU Directive 2010/63 aims to protect animals used in biomedical research and all scientific procedures are conducted under a project authorisation. When a researcher/scientist applyies for this authorisation it will need to include an estimate of the likely severity of each procedure. This informaiton is also made available in non technical summaries.

Not only do scientists work to design studies that minimise pain, suffering or distress to research animals, but there are strict regulations in place to ensure that animals in research do not suffer unnecessarily. Before an experiment can take place, a harm-benefit analysis must be conducted. This critically weighs up the potential medical or scientific benefit against the potential pain or suffering of the animal. The study will be given a severity level (the expected level of pain or suffering for the animal) which cannot be exceeded, and if necessary humane endpoints (the point at which an animal must be killed to prevent any future pain or suffering) will be included in the license.

Having considered the information provided in the application, the Competent Authority in a particular country, will assign a severity classification to each procedure. The levels of severity are constantly monitored throughout the experimental procedure, to help classify the experiment and ensure that humane endpoints when the animal is killed are established.


EU Member States must also report the actual levels of severity along with their animal use statistics.

Why are surgical procedures considered ‘moderate’ in severity?

As happens in human patients, when animals undergo surgery, they get anaesthetics. Painkillers are given after surgery where appropriate and pain must always be minimised. In order to achieve medical progress, the use of animals following the 3Rs principles, is allowed. It is reflected in the law (Directive 2010/63/EU), which allows the use of animals only under specific circumstances, and which sets out strict regulations for their use and care. Without the use of animals, scientists can only make very limited progress against disease such as cancer, heart attack, stroke, diabetes and HIV.

Do fish suffer as much pain as monkeys? Isn’t pain very subjective?

Recent evidence has shown that fish have similar pain-related physiology and behaviour to mammals, including humans. This is taken into account when measuring the severity of a study. A 2020 paper co-ordinated by the Norecopa 3Rs centre assessed the severity criteria in fish and can be found here.

Researchers do everything possible to minimise any suffering on the part of the animals they use in research, and where suffering is unavoidable they take every possible measure to reduce that suffering to an absolute minimum.


Of course, suffering is not limited to pain and stress; an animal can suffer when its biological or social needs are not met. All animals are cared for by trained animal care staff, researchers and specialised veterinarians that supervise the animals’ health and well-being.


Besides providing the animals with food, water and a clean, comfortable living environment; care is taken to enrich the lives of animals through species-appropriate enrichment – for example thicker bedding for nesting species or objects to climb on. Social animals (like rats) must be housed in groups unless doing so can be shown to jeopardise the scientific objectives, or the animals’ health and safety.

Why are experiments allowed that cause severe pain to an animal? Shouldn’t this type of experiment be banned?

In order to study and find treatments for the most serious diseases, such as Huntington’s disease or muscular dystrophy, it is likely that procedures will cause severe pain. Directive 2010/63/EU requires that experiments are always designed to cause the least pain, suffering, distress or lasting harm. The level of pain and discomfort is always kept to as low an intensity and as short a duration as possible.


Nevertheless, some procedures will involve pain or discomfort when the nature of the experiment makes it unavoidable. In some cases, the study of, and the evaluation of therapies for, painful medical conditions such as severe infection, or injury, may have the potential to result in significant levels of pain and discomfort. In these cases, efforts will be made to alleviate that pain, for example by using anaesthesia and analgesia during and after surgery, though just as with human patients it is not always possible to alleviate pain completely. 

Why are there 'severe' procedures in the creation and breeding of genetically altered (GA) animals?

Procedures for creation and breeding, will involve the breeding of GA animals whose genes have been modified. These animals are used to produce GA offspring for use in experimental procedures. The breeding of animals with genetic disorders, for diseases such as multiple sclerosis, are expected to experience severe and persistent impairment of general condition, so this would be considered a  severe procedure. 

Why are animals killed after scientific experiments?

Animals are usually humanely killed if there is a need to analyse tissue samples to further understand the nature of a disease or the effects of a experimental drug. Otherwise a procedure ends when animals are no longer being studied. The animals are then put back into their housing where they continue to be cared for. If their welfare is compromised, because the level of suffering is expected to continue to remain moderate or severe, then the animals are humanely killed. Killing is carried out by a qualified person using a humane method appropriate for the species. If the health and wellbeing of an animal is fully restored after use in mild or moderate procedures - and to keep the number of animals used in experiments to a minimum - it can be also be reused.

Is all this pain necessary?

Scientists begin their research from the standpoint of furthering human knowledge on diseases and searching for clues for new treatments for cancer, heart disease or HIV. If the only way to achieve these objectives is through the use of animals then the benefit is considered to outweigh the harm that might be caused.

By law, scientists must work to minimise the suffering of animals in laboratories. This might be by using analgesia or anaesthesia to alleviate pain during or after a procedure, or it could be providing enrichment for animals in order to encourage mental stimulation and prevent boredom. It is in a researchers’ interests to make sure animals suffer as little as possible; stressed animals are less likely to produce reliable results. All animal research must pass an ethical evaluation which weighs up its pros and cons and decides whether it is justified.

Image courtesy of: Max Planck Society

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