Why is animal research necessary?

Research using animals is essential for understanding the biology that underpins health and disease in both humans and other animals. Without such research, we would have few of the modern medicines, antibiotics, vaccines and surgical techniques that we take for granted in both human and veterinary medicine.

Animals are used to investigate fundamental biology, to model disease and to test potential new treatments before they are tested in humans. Animal research is only undertaken where there is no alternative.

What types of animal do you use at Cambridge?

The majority of the animals we use are mice and zebrafish – they make up 97% of all procedures at Cambridge. Where these species are not suitable, we use a small number of other animals, such as xenopus frogs, rats and sheep, as well as non-human primates, namely marmosets. 

What types of animal research do you carry out?

Some of the work carried out is fundamental research, aimed at understanding how humans and animals develop and how our immune systems and brains work, for example. This knowledge is essential for underpinning our understanding of health and disease for both medical and veterinary purposes. 

Other work is aimed at tackling specific diseases, for example in helping us understand how Parkinson’s disease affects the brain and motor system and how it might be tackled, or in developing new treatments for autoimmune diseases such as type 1 diabetes and multiple sclerosis.

How many procedures are undertaken on animals at the University of Cambridge each year?

The Home Office collects data on the number of procedures carried out, rather than the number of animals used. The following numbers of procedures based on Home Office Returns were carried out on animals for research at the University of Cambridge:

Species 2022 2021 2020 2019 2018
Mouse 162,724 152,471 102,050 99,351 107,140
Rat 4,406 2,095 2,511 2,137 3,202
Zebra fish  38,602 42,354 71,789 12,456 30,589
Other fish  404 1,496 220 0 0
Primate  68 33 41 43 53
Sheep  74 122 84 110 130
Guinea pig 168 48 0 0 144
Other rodents 76 78 33 14 21
Rabbits 50 2 0 0 0
Chicken 2 0 37 20 30
Pigs 42 59 16 0 0
Xenopus 376 445 438 509 659
Total 206,992 199,203 177,219 114,640 141,968

A ‘procedure’ is regulated and counts towards our Home Office Returns if it is carried out on a protected animal (such as those species listed above) for a scientific or educational purpose and may cause that animal a level of pain, suffering, distress or lasting harm equivalent to, or higher than, that caused by inserting a hypodermic needle according to good veterinary practice.

Procedures that are regulated include modifying the genes of a protected animal if this is done for and experimental or other scientific purpose or applied for an educational purpose; for example, breeding mice with genetic alteration is a regulated procedure if the intention is to keep the animals produced beyond two-thirds of the way through their gestation period. This helps explain why the numbers of mice and zebrafish are so high: each offspring born is counted as a procedure.

How severe were these procedures?

Researchers must assign a severity classification to the procedures they plan to undertake before authority to do their work is authorised by the Home Office (prospective severity). Once their licence has been granted they must also record the actual level of suffering (actual severity), i.e. severity experienced by each animal during the course of a procedure.

The prospective severity classification of a procedure is an assessment of the degree of pain, suffering, distress or lasting harm that is expected to be experienced by an individual animal during the course of the procedure. The prospective severity classifications are defined by the EU Directive as Non-recovery, Mild, Moderate and Severe, examples of which are provided below. 

During each procedure the researcher must monitor their animals and keep a record of the level of suffering (severity) each animal experiences. After the completion of each procedure, the researcher must record the highest level of suffering and any cumulative suffering experience by an animal during the procedure. This is the actual severity for each animal. Actual severity classifications are Non-Recovery, Sub-threshold, Mild, Moderate and Severe. The table below is a record by species of the numbers of animals assigned to each actual severity classification. 


Procedures, which are performed entirely under general anaesthesia from which the animal shall not recover consciousness.


Procedures on animals as a result of which the animals are likely to experience short term mild pain, suffering or distress, as well as procedures with no significant impairment of the wellbeing or general condition of the animals.

Mild procedures include:

  • anaesthesia
  • non-invasive imaging, like and MRI scan
  • short-term social isolation
  • taking a blood sample
  • superficial non-surgical procedures e.g. ear biopsies in mice and non-surgical implantation of recording devices and minipumps


Procedures on animals as a result of which the animals are likely to experience short term moderate pain, suffering or distress, or long-lasting mild pain, suffering or distress as well as procedures that are likely to cause moderate impairment of the wellbeing or general condition of the animals.

Moderate procedures include:

  • invasive surgery under general anaesthetic e.g. surgical implantation of a catheter into a blood vessel for long term drug delivery
  • causing cancer in an animal where the tumour growth impairs normal behaviour
  • feeding a modified diet which is deficient in an essential nutrient such that it affects the health of the animal
  • exposing the animal to something that they would normally run away from, without enabling them to run away
  • the breeding of genetically altered animals where the animals health is affected, e.g. genetic models of diabetes.


Procedures on animals as a result of which the animals are likely to experience severe pain, suffering or distress, or long-lasting moderate pain, suffering or distress as well as procedures, that are likely to cause severe impairment of the wellbeing or general condition of the animals:

  • where deaths are expected and it is not easy to determine when an animal is likely to die, e.g. models of aortic aneurysm
  • testing a device that could cause pain/death if it were to fail, e.g. testing devices designed to support patients at risk of heart disease
  • inescapable electric shock treatments, e.g. to induce a model of learned helplessness
  • breeding animals with genetic disorders that are expected to experience severe and persistent impairment of general condition, birth of living offspring with severe developmental abnormalities or expected pre-weaning death

Actual severity:
The above definitions and examples also provide a good insight into what animals could have experienced when undergoing procedures and so reflect how actual severity is determined. The only difference is that in the UK the Home Office introduced a further actual severity classification known as sub-threshold. This classification therefore appears when UK annual returns of procedures are published.


This is for procedures which were originally assigned an above-threshold pain or suffering classification, but when the work was undertaken the actual level of suffering was below that which would exceed the threshold at which procedures are licenced under the Act.

Sub-threshold procedures include:

  • breeding of genetically altered animals under project licence authority but without a harmful phenotype
  • dosing with a compound in feed where the animals ate normally and suffered no consequences of being dosed

Finally, animals that are undergoing experimental procedures that are found dead and death could have been procedure related will be automatically classified as Severe unless the researcher can proved that the death was not procedure related.

Example procedures courtesy of Understanding Animal Research

Species and severity classifications Number of procedures (2022) Number of procedures (2021) Number of procedures (2020) Number of procedures (2019) Number of procedures (2018)
Mice 162,724 152,471 102,050 99,351 107,140
Non-recovery 1,356 939 1,345 1,801 1,393
Mild 53,278 56,066 36,503 32,517 38,614
Moderate 30,976 24,678 11,642 13,613 15,677
Severe 1,991 2,115 1,362 1,735 1,675
Sub-threshold 75,123 68,673 51,198 49,685 49,781
Rats 4,406 2,095 2,511 2,137 3,202
Non-recovery 161 50 68 137 164
Mild 476 440 618 552 859
Moderate 3,650 1,271 1,357 1,302 1,868
Severe 47 26 9 34 64
Sub-threshold 72 308 459 112 247
Rabbits 50 2 0 0 0
Mild 12 2 0 0 0
Moderate 38 0 0 0 0
Zebra fish 38,602 42,354 71,789 12,456 30,589
Mild 29,914 30,163 56,744 8,550 17,462
Moderate 151 10 243 93 2
Severe 70 209 23 26 2
Sub-threshold 8,467 11,972 14,779 3,787 13,123
Other fish 404 1,496 220 0 0
Mild 364 676 174 0 0
Moderate 38 210 46 0 0
Severe 0 8 0 0 0
Sub-threshold 0 600 0 0 0
Non-recovery 2 2 0 0 0
Primates 68 33 41 43 53
Mild 37 9 11 19 16
Moderate 27 21 30 23 37
Severe 4 3 0 1 0
Sheep 74 122 84 110 130
Non-recovery 0 0 8 4 5
Mild 39 38 17 11 4
Moderate 17 65 55 94 119
Severe 10 3 4 1 0
Sub-threshold 8 16 0 0 2
Pigs 42 59 16 0 0
Non-recovery 38 50 13 0 0
Mild 0 6 0 0 0
Moderate 4 3 3 0 0
Guinea-Pigs 168 48 0 0 144
Non-recovery 0 0 0 0 0
Mild 168 48 0 0 0
Moderate 0 0 0 0 144
Severe 0 0 0 0 0
Other Rodents 76 78 33 14 21
Mild 76 78 33 14 21
Domestic Fowl 2 0 37 20 30
Mild 2 0 37 20 30
Xenopus 376 445 438 509 659
Mild 376 445 438 509 658
Moderate 0 0 0 0 1
Severe 0 0 0 0 0
Total 206,992 199,203 177,219 114,640 141,968

Additional Data

In 2017 every UK establishment provided data under the EU Directive (2010/63/EU), which required Member States to collect additional data that included the number of animals bred or produced for scientific procedures that were killed without being used in procedures. The reasons for this include:

  • The animals were ‘wild type’ breeding animals;
  • They were used to produce tissue only and were not subjected to regulated procedures;
  • The animals were the wrong gender required for the studies;
  • They were a surplus resulting from breeding animals to ensure adequate supply for scientific purposes.

Only animals not reported in the annual return of procedures were counted.

Since 2017, the University of Cambridge has continued to collect these additional data annually and is publishing them in the interests of openness and transparency.

Species 2022 2021 2020 2019 2018 2017
[A1] Mice (Mus musculus) 15036 7713 14500 19227 20616 27291
[A2] Rats (Rattus norvegicus) 1420 1238 1469 734 330 266
[A3] Guinea-Pigs (Cavia porcellus)            
[A4] Hamsters (Syrian) (Mesocricetus auratus)            
[A6] Mongolian gerbil (Meriones unguiculatus)            
[A7] Other Rodents (other Rodentia)           1
[A12] Other carnivores (other Carnivora)            
[A14] Pigs (Sus scrofa domesticus)            
[A16] Sheep (Ovis aries)     3 3    
[A19] Marmoset and tamarins (eg. Callithrix jacchus)            
[A21] Rhesus monkey (Macaca mulatta)            
[A34] Zebra fish (Danio rerio) 1886 543 937 351 247 4016
[A35] Other Fish (other Pisces) 305          
Xenopus 286 19       81

How do you ensure high standards of animal welfare?

We believe that good science and good animal welfare go hand in hand.  The UK has the most rigorous animal welfare regulations in the world, and we consider adherence to these regulations as a minimum and will continue to aim for the highest possible standards of animal care.

We strongly agree with, and rigidly follow, the guiding principles emphasised by the Home Office on the need to refine protocols, keep the numbers of animals used to a minimum and replace the use of animals with other methods where possible. To this end, we encourage all staff involved in animal research and husbandry to continuously develop and improve on existing welfare standards, offering incentives to those that contribute, and rewarding those that are recognised by the laboratory animal welfare organisations.

Are you looking for alternatives to animal use?

We are committed to refining, reducing and replacing the use of animals in research - known as the 3Rs. Animals are only used where no alternatives are viable, and Cambridge scientists are leading research to find viable alternatives. Dr Laura Pellegrini (2020) and Dr Marta Shahbazi (2019) at the MRC Laboratory of Molecular Biology are among those who have recently won the UK’s annual 3Rs prize, for the scientific and technological advance with the most potential to achieve the 3Rs.

Do you test cosmetics and household products on animals?

No. It is not permitted anywhere within the UK or the European Union to test cosmetics or household products on animals. More information is available from Understanding Animal Research.