Can the results of animal studies be applied to humans?

Image: O. Cafiero

Would it not be better to test medicines directly on humans to determine whether they are effective?

Animal experiments are necessary for the safety of humans, subject to the provision that the well-being of the animal be respected to the greatest possible extent. The declaration of the World Medical Association, which was adopted in Helsinki in 1964, clearly stipulates that the testing of new therapies directly on humans is unethical. The Declaration of Helsinki serves as the basis of much national legislation governing biomedical research.

The action of new medicines is studied, for example with human cell lines, before efficacy and safety within the organism is tested in animal experiments. If these experiments demonstrate that the risk is low, the active ingredient can also be studied on humans in clinical trials.

Animal testing is often performed with animals whose illness was artificially induced. Medicines are intended to help human beings who are ill. Is the effect on animals whose illness was artificially induced comparable with the effect on humans who fell ill naturally?

Yes, even if an illness is induced artificially in a laboratory animal, the individual processes generally follow a similar pattern as if the disease had occurred naturally. For this reason, it is possible to draw inferences for the situation in humans. Often, only part of the disease is induced in experiments with animals, which can help understand the course and processes of an illness. In this context, the role of genetically modified mice in research is steadily gaining importance.

Do animals respond to medicines in the same way as humans do?

Animals respond in the same way as humans to some medicines and differently to others. Not every animal species responds in exactly the same way, just as not all humans respond in the same way to all medicines and treatments. However, comparability is sufficient to draw conclusions on general principles of action and side-effects.

A considerable number of medicines are used in the same way in both human medicine and veterinary medicine. Many veterinary medicinal products contain the same active ingredient as the corresponding preparations used for human medicine. Examples include a variety of antibiotics, the pain-relief medicine Buprenorphine, the anaesthetics Propofol and Isoflurane, insulin, the sedative Valium, Benazepril used to treat heart failure, Carbimazole used for the control of the thyroid gland or Levothyroxine for the treatment of low thyroid activity.

For certain active ingredients, differences exist, mainly with regard to tolerability, between different animal species or even between breeds of the same species. For example, Permethrin, the active ingredient against parasites, should not be used for cats as it may be toxic for them. By contrast, the product is widely used for dogs. Many collies (and related breeds) are hypersensitive to the inhalation narcotic Isoflurane, while it can be safely used on other breeds of dogs.

Because of these differences, active ingredients intended for use in humans must be tested on at least two animal species, on rodents and non-rodents, before clinical trials with humans may be conducted.

Why are animals used to study human diseases, even though the two are completely different creatures?

Human and veterinary medicine have a wide range of commonalities. Virtually all diseases affecting human beings also occur in the same or a similar form in animals, and are treated based on the same principles. Moreover, many surgical procedures are very similar in humans and animals, such as cruciate ligament surgeries in humans and dogs.

Illnesses affecting both humans and animals:

  • Allergies, for example to dust mites and pollen: dogs, cats, rodents, rabbits
  • Cataract: dogs, cats, rodents, rabbits
  • Hearing loss, deafness: dogs, cats
  • Tooth decay: dogs, cats
  • Asthma: cats
  • Anaphylactic (allergic) shock: all animal species
  • Stomach ulcer: dogs, cats
  • Pancreatitis: dogs, cats
  • Breast cancer: dogs, cats, pets
  • Metritis: dogs, cats, rodents, rabbits
  • Bladder inflammation: dogs, cats, rodents, rabbits
  • Kidney failure: dogs, cats, various rodents
  • Immune deficiency disorders caused by viruses: cats, monkeys
  • Cardiomyopathies and other heart diseases such as cardiac insufficiency: dogs, cats, rodents, rabbits
  • Hypo- and hyperthyroidism: dogs, cats
  • Urinary calculi: dogs, cats, rabbits
  • Cancer of various organs: all animal species
  • Inflammation of various organs: all animal species
  • Epilepsy: dogs, cats, gerbils
  • Diabetes mellitus: dogs, cats, various rodents – a major concern in chinchillas
  • Senile dementia: dogs, cats
  • Various types of inflammation and infectious diseases such as toxoplasmosis, borreliosis, salmonellosis, tuberculosis, etc.

Humans develop illnesses that are often caused by their lifestyle, such as myocardial infarction or diabetes. So, does it really make sense to conduct experiments with animals?

Many illnesses are multifactorial; that is, a combination of several circumstances triggers the illness. Since different factors are involved, each of these individual factors needs to be considered in order to develop a comprehensive treatment regimen.

Not all medical conditions can be prevented with the appropriate lifestyle. Individuals who have never smoked may be diagnosed with lung cancer; young athletes become diabetic or die of a heart attack.

Illnesses are often caused by a certain gene or a metabolic component. Standardised experiments with animals are also needed in order to understand the development of illnesses. They help identify the small differences that may have a crucial impact on the illness.