Small mammals and biomedical research

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While mice and rats are usually the most common choice for use in biomedical studies, there are several other species of small mammal (including other rodents) that can also play a key part in research studies and, in turn, medical advances. Due to their overall characteristics and sometimes greater similarities with human health and biology, these other small mammals can provide researchers with better insights into a range of different diseases.

This feature looks at three main species of small mammal – rabbits, guinea pigs and hamsters – as well as some more rarely used, but nonetheless invaluable, species that have contributed to our understanding of major health challenges such as Covid-19.

Why do we need to use small mammals in biomedical research?

Rabbits

One of the most used small mammals in biomedical research is the rabbit. They are used in particular to assess the safety of newly developed drugs and other chemical substances, such as household products. The Draize test, for example, assesses how toxic a substance is on the eyes or skin, and tends to be performed on rabbits because of the animals’ larger eyes, compared to rodents. The test has been controversial because of animal welfare concerns, and alternatives are increasingly being developed to potentially reduce or replace the need for the test on animals. Since the EU-wide cosmetics ban in 2013, rabbits along with all other animals, have not been used to test the safety of cosmetics.

Rabbits are used in disease research as there are certain human conditions that they are naturally susceptible to, such as urinary tract infections (UTIs). They have also allowed researchers to better understand how cystic fibrosis develops, by studying mucoid enteritis in rabbits, which is a similar disease. In the search for better treatments for human medical conditions, a study led by Purdue University, Indiana, USA, successfully delivered drugs to the eyes of rabbits, via a contact lens. The lens not only treated the animals’ eye disease but also caused less discomfort.

Studies with rabbits have also given us insights into many different types of diseases, from HIV/AIDS to cardiovascular disease to tuberculosis, as well as investigating surgical approaches to treat conditions and improve health. For example, they were used to understand diseases caused by the Streptococcus pneumoniae bacteria, collectively known as pneumococcal disease.

It was thanks to research in both rabbits and guinea pigs, by the Georgia Institute of Technology, USA, that a drug once considered a potential treatment for Covid-19 was discovered to have serious effects on heart rhythms, pointing to restrictions of its use on patients.

Meanwhile, a ‘pumping’ patch of heart tissue, developed by Imperial College London, UK, and designed to treat people after a heart attack, was shown to be able to repair damage after being tested on rabbits, kickstarting human trials.

Research in rabbits also seeks to improve the health of the animals themselves, particularly for treating the severe viral disease myxomatosis – that only infects rabbits – through the development of vaccines and other treatments. Thanks to studies and tests in rabbits, vaccines were also developed for rabbit haemorrhagic disease, which affects pet and wild rabbits alike.

When rabbits are used rather than mice or rats, it is usually because rabbits are easier to handle and breed and, being larger, it makes them a more suitable animal to study certain parts of the body, such as the eyes. This is also why rabbits are often preferred over rodents for studies into reproduction, because their uteruses are larger and easier to study. For instance, researchers from the Wake Forest Institute for Regenerative Medicine, USA, found a way to stimulate the repair of damaged uterus tissue in rabbits, to improve the success of pregnancy and reduce the need for uterus transplants.

With further development, this approach may provide a pathway to pregnancy for women with an abnormal uterus.

Dr Anthony Atala, Wake Forest Institute

A key research area for rabbits is in the production of antibodies, (proteins that protect you when an unwanted substance enters your body), particularly those for therapeutic purposes.

This approach has advantages in drug development as using antibodies – over small molecules or compounds, for example – means that drugs are less toxic and more specific to their target in the body. For instance, rabbit antibodies have formed the basis for drugs such as eptinezumab, which is used to treat migraines, and brolucizumab, to treat sight problems caused by retinal eye damage.

Small mammals and antibodies

This is because the comparatively larger size of a rabbit, compared to rodents, means a larger quantity of antibodies can be produced without harming the animals, and can be easily obtained by taking blood or serum under anaesthetic. Also compared to rodents, the rabbit immune system can recognise a wider range of foreign or toxic targets (called antigens) for the antibodies to attach to, and these can then help to signal to researchers the presence or absence of a specific disease.

Certain types of antibody are challenging to make without animal hosts such as rabbits and, in some cases, rabbits are in fact vital to develop therapies, such as the polyclonal antibody thymoglobulin, that is derived from rabbit serum and protects against immune rejection for organ transplants. What is more, the rabbit immune system can also identify antigens that affect humans, whereas rodent immune systems cannot.

See also the EARA feature, The vital role of antibodies in biomedical research, and the EARA/EFPIA report on animal-derived antibodies, submitted to the European Commission to highlight the continuing importance of this use of animals in research.

Guinea pigs

The valuable and long-established role of guinea pigs in biomedical research, has even led to the commonly used term ‘guinea pig’, to describe humans who take part in experiments or drug tests.

Guinea pigs and humans have similar immune systems, making them a suitable species for investigating how infections take hold and affect the body. This has helped to identify different routes to transmission – for example in a study at the University of California, Davis, USA, it was shown that viral droplets of influenza, that allow the virus to spread through the air, can also be transmitted by attaching to dust.

Guinea pigs are also highly susceptible to infections by a range of bacteria, or viruses, that also infect humans, including Mycobacterium tuberculosis that causes tuberculosis. Historically, it was thanks to research in animals, including guinea pigs, that the first effective antibiotic against the disease, streptomycin, was developed as a treatment against not only tuberculosis, but also plague and pneumonia.

Current research involving guinea pigs for infectious diseases, such as Covid-19, also holds the potential to translate to other diseases. Work at Yale University School of Medicine, USA, showed that an mRNA Covid-19 vaccine could also protect guinea pigs from Lyme disease, which is caused by bites from infected ticks.

Guinea pigs are also sensitive to certain allergens and so are helpful for looking into allergies, such as those of the skin, as well as severe reactions like anaphylactic shock that can be triggered by food, medicine or insect allergies.

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Guinea pig held by animal technician. (Credit: UAR)

Also, in relation to allergens, guinea pigs are important for studying asthma, a leading chronic disease in children, that can be caused by pollution or pollen. Compared to mice, which are the most commonly used animal to study asthma, guinea pigs have greater similarities to humans in terms of their airways, and they are also a much better model for testing the effects of asthma drugs. When exposed to allergens, their respiratory system reacts at both the early and late stages of exposure, just as humans’ do, whereas mice do not. Inhalers for asthma, that relieve asthma symptoms, came into existence thanks to early studies in guinea pigs that uncovered how chemical nerve signals can control muscles in the airways.

Meanwhile, researchers have also identified how breathing problems, caused by factors like air pollution, can affect the lungs. A recent study, at Imperial College London, demonstrated the impact of diesel fumes on guinea pigs, which in turn may worsen other underlying conditions, including asthma.

Guinea pigs are especially suited to research on hearing and hearing loss, because of the similarity in the structure of their ears to humans. A recent European Research Council project, at the Middle East Technical University, in Ankara, Turkey, has developed a fully implantable cochlear implant for people with hearing problems. It made use of hearing-impaired guinea pigs as part of its work to show that sound waves could recharge the device and make it more convenient for users. This research is now being followed up with further research, using guinea pigs, to develop a way to recharge implants using a wireless device.

Another field that guinea pigs are important in is the study of nutrition, not least because vitamin C was first discovered in guinea pigs, a crucial component of their diet, just like humans. A lack of vitamin C can lead to scurvy, in both humans and guinea pigs, and here guinea pig research has also paved the way for understanding symptoms of the disease, to provide solid evidence of the need for vitamin C to maintain health.

In major European cities like London, we are already exceeding the recommended levels for air pollution, and these findings provide another reason why we need to curb these levels.

Maria Belvisi, Imperial College London

Are small mammals being replaced or used in smaller numbers?

The most recent figures from 2020 for the use of animals in scientific procedures in the EU show that the number of small mammals has remained stable overall, year-on-year.

In 2020, 343,521 rabbits were used in research across the EU – 4.3% of the total number of animals, of all species used. In the next few years, the European Pharmacopoeia, which details the official quality standards for medicines and their ingredients in Europe, plans to completely replace the rabbit pyrogen test (RPT), which is used to detect the presence of fever-causing substances, ‘pyrogens’, in pharmaceutical products. The EU report stated that the specific use of RPT decreased by 21% in 2020.

Other small mammal species, included in the statistics, were gerbils (2,978), ferrets (1,250) and other rodents (28,186), including voles and naked mole rats, and these numbers have remained generally the same from previous years.

Guinea pigs made up 1.4% of the total animals at 111,172, while Syrian hamsters made up 0.2% (17,335). The use of Syrian hamsters had increased by two-thirds compared to 2019, which may be attributed to the role that this species played in Covid-19 research during the height of the pandemic (see below the section, Hamsters).

Hamsters

The most notable role that hamsters have played in medical advances in recent times has been for Covid-19 research and vaccine development. The most common hamster species used is the Syrian hamster, which can naturally get some infectious diseases in a similar way to humans, including Covid-19. These animals are highly susceptible to Covid, as shown in a study at the Friedrich-Loeffler-Institute, Germany, which found that pinpointing just how sensitive hamsters are to SARS-CoV-2 can help researchers fine-tune experiments that use hamsters to test for potential treatments.

Some Covid-19 symptoms, including lung damage and changes to the sense of smell, are comparable between hamsters and people, making the animals a useful model for mimicking what happens in human disease, including for different variants, such as the Omicron strain.

This also means that the use of hamsters can reflect how people may respond to certain Covid-19 treatments. For example, a German study by three EARA members, the Free University of Berlin, Charité – University Medicine Berlin, and the Max Delbrück Center, used hamsters to test a nasal vaccine that targets the mucosal membranes found in the nose, mouth, throat and lungs, which makes the immune system more effective at fighting infection. The treatment is now being prepared for trials in humans.

The same German team also used hamsters to provide insights into early mechanisms leading to lung inflammation in Covid-19, to understand how this translates to different disease severities in patients.

In addition, because hamsters are larger than mice, their internal structures are easier to visualise to assess the effects of the coronavirus on the lungs. Before Covid-19, hamsters were already used to investigate severe acute respiratory syndrome (SARS), another respiratory disease caused by a different coronavirus, that was responsible for a rapid global outbreak in 2003, while treatments for influenza have also been investigated, for the reasons outlined above.

We don’t think from these results that hamsters play a role in the dynamics of pandemic. It’s just that the virus might
ping-pong within the household if an infected person has close contact with a hamster.

Dr Anne Balkema-Buschmann, Friederich-Loeffler-Institute

Information from our current study has enabled us to close some of these gaps. This represents major progress, including in terms of a more conscious and targeted approach to the use of animals in medical research.

Dr Jakob Trimpert, Free University Berlin

Credit: NC3Rs

Disorders of the metabolism, such as diabetes, can be investigated by feeding hamsters different diets and assessing the effects. And hamsters are also good for understanding behaviour, since they exhibit natural aggressiveness and so can help probe the social behaviours that are involved in certain psychiatric and neurological conditions.

Hamsters are hibernating animals and researchers are making use of this quality to understand how to reverse the loss of synapses (which connect neurons), seen in neurodegenerative diseases such as Alzheimer's. A team at EARA member VIB-KU Leuven, Belgium, is investigating the mechanisms behind why hamsters can re-establish lost synapses during hibernation, to provide insights into human conditions and treatment.

As well as studies on the animals themselves, cells from the ovaries of Chinese hamsters (called CHO cells) have been an invaluable biological tool for carrying out genetic manipulation (to make animal cells produce important compounds) for expressing antibodies and, for commercially, producing therapeutic proteins like drugs.

The CHO cell line has had widespread use in many areas of research, particularly genetics and molecular biology, and, as such, has been the standard for growing the cells of mammals in the lab for decades. What makes Chinese hamsters the ideal animal to use for this purpose is the similarity of their cells to ours, and therefore their relevance to numerous human applications, for example to produce drugs for cancer. CHO cells are also easy to scale up to culture many cells at once.

Studies of different types of cancer, such as pancreatic, oral and renal cancers, have also benefited from the use of hamsters, for example by inducing tumours in the animals, such as in the cheek pouch model for studying oral cancer, where the effect of external carcinogens like tobacco on the inside of the mouth are investigated.

Ferrets, gerbils and voles

Ferrets, gerbils and prairie voles, though less frequently used, can provide unique insights that are not possible otherwise.

Ferrets have a greater biological similarity to humans than mice and rats, and have been useful for investigating a variety of respiratory diseases, from Covid-19 to influenza and cystic fibrosis. This is largely because the lungs of ferrets work in a similar way to ours, so that they can be used to examine how diseases infect this organ and spread to other individuals. A study at Erasmus MC, Rotterdam, Netherlands, and Columbia University Vagelos College of Physicians and Surgeons, USA, used ferrets to test and validate a nasal spray that prevents the Covid-19 virus from entering cells.

Ferrets (Credit: UAR)

Remarkably these animals also have a menstrual cycle that shares common factors with people, allowing researchers to better understand puberty, ovulation and reproduction.

The research value of naked mole rats

Naked mole rats are known for their remarkably long lifespans, as well as resistance to cancer, and this has cemented them as a key component of anti-ageing research. A team at the University of Rochester, USA, showed that the lives of mice could be extended by transferring a gene from naked mole rats.

We hope that our findings will provide the first, but not the last, example of how longevity adaptations from a long-lived species can be adapted to benefit human longevity and health.

Dr Andrei Seluanov, University of Rochester

Meanwhile, researchers at the University of Cambridge, UK, found that the interactions between the system of cells and molecules in naked mole rats may be what prevents cancer from initially developing into tumours in the animals.

The resistance of naked mole rats to oxygen starvation also means that they are valuable for investigating how to protect the brain and the heart, after a stroke or a heart attack, which is the basis of research in the lab of Prof. Gary Lewin (pictured) at EARA member the Max Delbrück Center, Berlin, Germany.

Prof Gary Lewin and naked mole rat (Credit: MDC )

Another species that is suitable for studying reproduction is the Mongolian gerbil (the most common gerbil used in research), particularly in prostate biology. These gerbils are susceptible to several bacteria, viruses and parasites that also infect people, such as Helicobacter pylori that is responsible for various types of stomach infection – gerbils can shed light on ways to tackle these diseases by studying their development in the body and identifying optimal routes to treatment.

In addition, Mongolian gerbils are able to hear a similar range of sounds to humans. This led to them being used in a study, to test a stem cell treatment for deafness, at the University of Sheffield, UK, which restored the hearing of deaf gerbils.

The research is tremendously encouraging and gives us real hope that it will be possible to fix the actual cause of some types of hearing loss in the future.

Dr Ralph Holme, Action on Hearing Loss

Studies on brain-related conditions, including stroke and epilepsy (which naturally affect gerbils), as well as brain development and behaviour, have also involved the use of this species.

The prairie vole, meanwhile, has been a key research animal for shedding light on social bonding and relationships since they usually mate for life, which is unusual for rodents. Through the study of this unique species, researchers have managed to understand the neural basis for love, as well as look at the effect of alcohol and other addictive substances on social relationships.

Small mammals in the wild

Wild species of small mammal are also used in research to provide insights into ecology (the relationship between organisms and their environment), conservation and environmental monitoring.

EARA member the University of Lisbon (ULisboa), Portugal, conducts research using wild animals, including shrews, voles and wood mice – which have been temporarily brought into the animal facilities for the studies – to investigate, for example, the effect of urbanisation on these species in order to understand how to protect the wildlife and the environment.

Another ULisboa project is focused on the effects of captivity on these animals, to ensure that the results of the experiments are transferable to nature and not influenced by the fact that the animals are in a controlled environment.

Greater white-toothed shrew (Credit: Maílis Carrilho, Faculty of Sciences, ULisboa)

Lusitanian pine vole (Credit: Ana Cerveira, Faculty of Sciences, ULisboa)

How are small mammals looked after in research?

Most of the small mammals outlined in this feature are also commonly kept as pets, which makes them straightforward to keep in a research setting as they are domesticated and used to human contact. Like mice and rats, they are also social animals and so suitable housing that keeps them in appropriately sized groups is very important, as well as ensuring that there is plenty of stimulation through play, such as with toys and tunnels.

This type of enrichment is provided to laboratory animals to improve their welfare and well-being, and there are many guidelines in place to refine the care and husbandry of small mammals.

In general, small mammals can be stressed easily, so proper care in handling, positive contact with humans, and minimal environmental disturbances are all important in maintaining their care, health and well-being – which in turn leads to better research. Rabbits, for instance, should be provided with hay for foraging, play, nest building and a varied diet, while guinea pigs should have pens that are sheltered to avoid stress.

Researchers must always follow the 3Rs (replacement, reduction, refinement) when using any animal. These principles dictate that animals are only used in studies when there is no non-animal option, and that the research community continually works towards minimising animal use wherever it is possible to do so.

In terms of replacement, with non-animal methods, alternatives to both the rabbit pyrogen test and Draize test, for example with in vitro assessments and computer modelling, are increasingly being developed, with the ultimate goal of phasing out their use. Ways are also being sought to replace small mammals used in research with different, non-mammal species, such as fish or insects, which are more readily available in the numbers needed to carry out valid biomedical research.