Medical research capitalizes on the similarities and differences among species to learn the causes of various cancers, what causes tumors to metastasize, and what kinds of interventions can be helpful in stopping disease progression.
Research examining the parallels between animals and humans leads to:
- The discovery of new treatment options to improve patient outcomes
- Enhanced disease detection, so cancers can be treated at earlier stages when they are more likely to be cured, or at minimum provide a better prognosis
- Discovering the causes/causes of various tumor types, which can help us develop new approaches to cancer prevention
- Determining genetic and environmental risk factors to explain why some individuals have an increased risk for developing cancer, are less responsive to therapy, and/or show increased susceptibility to side effects from treatment
Why Use Animals as Models of Cancer?
Cancer is a very complex disease and research geared towards learning more about its origin, progression, and treatment is intense and ever evolving. Medical researchers use animals to study cancer for a variety of reasons. Animals have shorter lifespans and more rapid generation times compared to humans, and disease progression moves forward at a more rapid pace, so results of studies using animals as models are obtained quicker.
In laboratory settings, we can control more variables for animals than would be considered ethical for humans (e.g., environment, diet, exposure to infectious agents, etc.). However, the primary reason animals are used as models is because they represent actual living systems, rather than cells growing in petri dishes or computerized models, and this hopefully will better predict what will actually occur in people.
What Are the Different Categories of Animal Models?
Broadly, when considering animal models for cancers in people, we typically think of research occurring either in the laboratory setting or clinical trials instituted at veterinary schools or large referral hospitals.
The different categories for animal models include:
- Animals who develop cancer spontaneously, without any alteration of their genes or initiation of cancer by chemical treatments (e.g., exposure to carcinogens)
- Animals who are genetically altered so that they will develop spontaneous tumors of the same types and with similar properties as the tumors that develop in humans who have those altered genes (i.e., purpose-bred laboratory animals with specific genetic mutations)
- Animals that develop spontaneous tumors if they are exposed to environmental factors, such as chemicals or radiation
- Animals whose natural, unaltered genetic makeup permits researchers to identify the genes that generate susceptibility to cancer development
The most commonly used animal cancer models in the laboratory setting are rodents (e.g., mice and rats). These animals probably encompass more than 90 percent of the animals used in medical research. Other cancer models include rabbits, dogs, cats, livestock, and fish. For these species, tumors are induced to form via direct exposure to known cancer-causing agents or direct inoculation with tumor cells, or they are purposefully bred to harbor specific genetic mutations leading to susceptibility for tumor formation.
The dogs and cats I treat in my practice on a daily basis are examples of the first category listed above. They develop their tumors spontaneously rather than as a result of exposure to cancer causing agents. In many ways, this makes our companion animals much better models than the laboratory species. But performing cancer research on pets in the clinical setting is challenging, and the least controllable in terms of outlying variables.
It’s a struggle to know that the most meaningful results could be obtained from the pets I’m seeing on a daily basis, but I’m also acutely aware of the limitations of trying to study specific aspects of their diseases.
What are Some Examples of Animal Models of Human Cancers?
The actual number of animal models of human cancers is likely unknown, however we do know animals serve as models for a variety of human tumor types including:
- Mammary cancer
- Lung cancer
- Colon cancer
- Prostate cancer
- Bladder cancer
- Ovarian cancer
- Skin cancer
- Esophageal cancer
- Head and neck cancers
- Pancreatic cancers
Revisiting the Concept of the Human-Animal Bond
Everything I’ve discussed thus far leans towards the benefits of what we can learn from animals, but sometimes we learn how to treat animals based on what happens in people as well. The best example of this I can think of is the newly developed immunotherapy vaccine treatment called Oncept ™, which is used to treat melanoma in dogs.
Melanoma is a deadly form of skin cancer in people that is highly metastatic and also highly resistant to conventional treatment with chemotherapy. Researchers at Memorial Sloan-Kettering Cancer Center, a large human oncology hospital in New York City, were working on testing a vaccine treatment for people with melanoma.The vaccine was designed to simulate the patient’s immune system to attack the cancer cells.
Melanoma can occur in dogs, but when dogs develop melanoma in the skin, unlike humans, it is typically benign. However, when this form of cancer grows in the oral cavity, it can be deadly. The similarities and differences between humans and dogs with regard to this form of cancer led to the hypothesis that the human vaccine could play a role in treating dogs.
Veterinary oncologists and human oncologists worked together to develop subsequent parallel clinical trials for a vaccine formulated for dogs. The researchers were able to refine the dosage and protocol to the current therapeutic regimen we utilize very frequently, and more excitingly, recent studies and clinical experience show the vaccine to be a very promising treatment option for what was previously considered a relatively untreatable form of cancer in both people and dogs.
The roles pets play in benefitting human life are endless, and as an oncologist, I appreciate what we can learn from their clinical presentations and response to treatments. It’s also interesting to see what we learn from people to help out our veterinary patients. It’s just another example of the wonderful nature of how the human-animal bond extends beyond pet ownership, and how much more we have to learn from each other in the future.
Dr. Joanne Intile