cancer in dogs and cats

Cancer in Dogs and Cats

By Dr. Bruce Little

Although cancer in pets differs very little from cancer in humans, it remains a complex, misunderstood, and disheartening tragedy when our four-legged family members are diagnosed with some form of the dreaded “Big C.”  First, it might be productive to review terms that are part and parcel to these conditions.

The term “neoplasia” is reserved for a condition where there is uncontrolled, abnormal growth of cells or tissues in the body creating a neoplasm or tumor. These can be either benign neoplasms, meaning they grow but do not transfer to other tissues or organs; and malignant neoplasms, meaning they can be unpredictable and grow at various rates, invade other tissues surrounding them, or spread to other parts or organs of the body. Hippocrates first introduced the name for malignant cancer from the Greek word for crab (karkinos) because tumors resembled the claws of a crab that relentlessly and uncontrollably attained reckless growth of destructive cells that overwhelm the body as they set about their task of ultimately killing its host. The word “tumor” is often used to describe the actual swelling or mass of a neoplasm. The word “cancer” is frequently confused with neoplasia, but only malignant neoplasms are truly cancers.

What to Know About Cancer in Dogs and Cats

According to the comprehensive text book, “Canine and Feline Geriatric Oncology, 2nd Edition” written by Dr. Alice Villalobos, a veterinary oncology consultant, there are five major group divisions or types of tumors in animals. By understanding cancer cell types within this concept of major group divisions, the diagnosis, management and supportive care for cancer patients may be more approachable. Veterinary clinicians also categorize cancers according to their location in the body, such as head and neck tumors versus abdominal tumors. Many of these approaches may share treatment options; however, it is important to place each of the more than 200 known cancers in known categories. Dr. Villalobos has determined that the five major divisions of cancer are:

  • Carcinomas – Carcinomas arise from cells in the skin, cells that form the lining of body cavities, or from cells forming the inner surface of an organ.  Examples of carcinomas are mammary tumors, testicular tumors, tumors in the oral cavity or skin.
  • Sarcomas – Sarcomas are malignancies that include all connective tissues, endothelial cells, muscle tissue, bone, bone marrow, blood, mast cells and other miscellaneous cells.  Examples of sarcomas are soft tissue sarcomas on the skin, sarcomas of the bone and injection site sarcoma in the feline species.
  • Blood Cancers – Malignancies of hematopoietic cells are commonly referred to as blood cancer and include leukemia and lymphoma.
  • Central Nervous System Tumors – Tumors that arise in the brain, brainstem, and spinal cord are extremely difficult to manage in dogs and cats.  Many pets tolerate the initial growth of the CNS tumors without outward signs or symptoms. The undiagnosed tumor progresses until compression within the skull or spinal canal causes the animal to present clinical signs of neoplasia, such as behavior changes, seizures, head tilt, blindness, or paralysis.
  • Miscellaneous Tumors and Tumors of Unknown Origin – Histiocytes are cells that may multiply rapidly and infiltrate various parts of the body.   These cells then attack red blood cells and sometimes other white cells causing sickness. They may form as soft tissue tumors on the lymph nodes, viscera, eyes, skin and central nervous system.  Histiocyte tumors create a confusing set of syndromes.

Approximately 1 in 4 dogs will at some time in their life develop neoplasia. Nearly half of dogs over the age of 10 years, and one in four dogs under 10 years, will develop cancer of some kind. Dogs get cancer at a higher rate than humans and cats have a similar rate as humans (one in three). Some cancers, such as lymphoma, are more common in cats than in dogs probably related to the long-term effects of the Feline Leukemia Virus.  

There is considerable research done on cancer in dogs that may translate into benefits for diagnosing and treating cancer in humans. By the fact that dogs age much faster in years than humans (i.e., depending upon the dogs weight a 10 year old dog relates in age to a 60 to 75 year old human), it is possible to create a large number of client owned clinical research dogs that can be used as models for finding treatment modalities, genetic manipulation, medications or vaccines that stimulate or modulate the immune system in all animals to prevent the overgrowth of these neoplastic cells by utilizing the body’s own immune system.  Research is ongoing at many research institutions across the world utilizing some of these theories.

Like people, pets can develop neoplasia affecting almost any organ or tissue in their body.  The signs that may be observed vary based on the tissue involved and the severity of the neoplasm.

If you observe any of the following signs in your pet, you should consult with your veterinarian:

  • Abnormal swellings or plaques that grow on the skin
  • Abnormal masses in the oral cavity, mammary glands, testicles, vaccine or injection sites, or enlarged abdominal organs
  • Sores or ulcers that do not heal in 2 weeks on the nose, ear tips, face or non-pigmented skin
  • Sudden changes in weight
  • Pale mucous membranes, icterus, jaundice, or red blotches on mucous membranes
  • Bad breath, difficulty eating, loose teeth or bleeding from the mouth
  • Abdominal distention, bloating, masses or swelling
  • Bleeding from any location such as the nose, mouth, anus or urinary tract
  • No desire to eat snacks and favorite foods
  • Reduced energy, trouble breathing, coughing or gagging
  • Trouble eating or swallowing, salivation, vomiting, sudden voice changes
  • Chronic sneezing, unresponsive eye discharge, or unilateral nasal discharge
  • Difficulty urinating or blood in the urine
  • Diarrhea, constipation, blood in the stools
  • Lameness, painful movement, painful joints, lack of desire to exercise
  • Rapid breathing, fainting or collapsing
  • Weakness, stumbling, loss of balance, behavioral changes or pain when being picked up

Many of the signs seen with neoplasia are also seen with non-neoplastic conditions, but they still need prompt attention by a veterinarian to determine the cause. Neoplasia is frequently treatable and early diagnosis will aid your veterinarian in delivering the best care possible. Neoplasia is often suspected based on the pet’s medical history and physical examination. Tests such as x-rays, blood tests and ultrasound examinations may be necessary to confirm neoplasia. For many tumors, a biopsy is often necessary to confirm the diagnosis and help determine if the neoplasm is benign or malignant. Additional biopsies may be necessary to determine how far a malignant neoplasm or cancer has spread. Advanced imaging such as computed tomography (CT) scan, magnetic resonance imaging (MRI), or positron emission tomography (PET) scan can also improve the understanding of the tumor’s location and possible treatment options. Generally, pets with cancer are referred to veterinary oncologists for specialty consultation regarding management. This fact alone creates a grand argument for purchasing pet insurance in the event your pet acquires cancer at some point in its life.

How to Treat Neoplasia in Dogs and Cats

Depending upon the type of neoplasia identified and the overall health and condition of the animal, treatment of neoplasia may consist of one or a combination of therapies such as surgery, chemotherapy, radiation, cryosurgery (freezing), hyperthermia (heating), or immunotherapy. Diet and pain management may be instilled to accommodate the best results during these treatments. It is important to note that pets often tolerate chemotherapy better than people. Neoplasia is frequently treatable and early diagnose and categorization of the tumor will aid your veterinarian and oncologist in helping you decide which treatment is affordable and best for your pet. Some types of neoplasia can be cured, but other types can only be managed to decrease spreading and prolong your pet’s comfort and quality of life as much as possible. Many pets with advanced or untreated cancer can be helped with ”Pawspice” or pet hospice using palliative end of life care management which addresses symptoms and may restore quality of life. Often the biggest factors determining the success of treatment for neoplasia are:

  • Stage – how large the neoplasia is and how far it has spread in the body
  • Type – this indicates the chance for response to therapy, as well as both local invasion and the rate of spread to other parts of the body

The response to treatment depends on the type and extent of the neoplasia, as well as the availability and effectiveness of therapy. There is no general rule regarding an individual pet‘s response to therapy, but treatment can be successful for many pets with neoplasia. Benign neoplasms are usually easier to treat, and treatment of any type of neoplasia is more likely to be successful if the neoplasms are detected early. Despite a lack of metastasis or movement to another organ or part of the body, benign tumors can sometimes have damaging effects on the patient; for example, brain tumors are often benign but the pressure they create on the surrounding brain tissue can be life-threatening.

Euthanasia may need to be considered, especially when the type or stage of the neoplasia makes successful treatment unlikely, the cost of treatment is prohibitive for the owner, or the pet’s quality of life is poor despite treatment. Before you make your decision for treatment or euthanasia, discuss the options with your veterinarian so you can make the best choice for your pet and your family. Many times, with arresting the neoplasm by reducing its size and encroachment on other tissues or preventing its metastasis to other parts of the body, pain control and careful nursing, pets can live an acceptable quality of life for many months or even years. After all, that is the end goal to have the best life possible for our four-legged family members.

For further information, I suggest reading “Canine and Feline Geriatric Oncology – Honoring the Human-Animal Bond,” 2nd Edition written by Dr. Alice Villalobos and Laurie Kaplan.  

Dr. Villalobos also has an informative library section and Quality of Life video on her web site at http://www.pawspice.com

You can also visit the public web site of the American Veterinary Medical Association at http://www.avma.org/cancer