What is Cancer?

Similar to humans, our pets are living and thriving longer due to advances in medicine and an emphasis on preventive care and nutrition. So, living a longer, quality life exposes our animals to diseases of aging, especially cancer.  An estimated 6 million dogs and nearly 6 million cats will be diagnosed with cancer this year.   In many of these animals, the malignancy will look and behave much as it would in humans, such as spreading to the same organs.
Cancer occurs when the body’s immune system cannot stop cells from replicating at an abnormally fast, disorderly pace and form a mass known as a tumor. Just as in humans, companion animal cancer is not caused by any single factor. While genetics and environmental factors can play a role in the disease’s development, other variables such as toxins, radiation and tumor viruses, as well as hormones can also be responsible for causing several types of cancer. And finally, suppressed or deficient immune systems can increase an animal’s risk of developing cancer.
Dogs are affected by more forms of cancer compared to other companion-animals. According to The Veterinary Cancer Society, cancer is the leading cause of death in 47% of dogs, especially dogs over age ten, and 32% of cats. Dogs get cancer at about the same rate as humans, while cats get fewer cancers. Some breeds or families of dogs have a higher incidence for developing cancer at an earlier age, but in most cases it’s a disease found in aging animals. There are nearly 100 types of animal cancer. Cancer in pets can be found in the skin, bones, breast, head & neck, lymph system, abdomen and testicles. Most common in cats is leukemia, while the most common cancers for dogs are lymphoma and mammary gland cancer.

Types of Cancer

Approximately 1/3 of all tumors in dogs are skin tumors, and up to 20% of those are mast cell tumors. The most common location to find mast cell tumors is the skin, followed by the spleen, liver, and bone marrow. Approximately half of all skin tumors are found on the body, another 40% on the limbs (most frequently the hind limbs), and the remainder on the head or neck. Approximately 11% occur in more than one location.
The list below highlights the most common types of cancer.  Click here for a cancer reference sheet.
Skin – Skin tumors are very common in older dogs, but much less common in cats. Most skin tumors in cats are malignant, but in dogs they are often benign. Your veterinarian should examine all skin tumors in a dog or cat to determine if any are malignant.
Mammary Gland (Breast) – 50% of all breast tumors in dogs and greater than 85% of all breast tumors in cats are malignant. Spaying your female pet before 12 months of age will greatly reduce the risk of mammary gland cancer.
Head & Neck – Neoplasia of the mouth is common in dogs and less common in cats. Signs to watch for are a mass or tumor on the gums, bleeding, odor, or difficulty eating. Since many swellings are malignant, early, aggressive treatment is essential. Neoplasia may also develop inside the nose of both cats and dogs. Bleeding from the nose, breathing difficulty, or facial swelling are signs that may indicate neoplasia and should be checked by your veterinarian.
Lymphoma – Lymphoma is a common form of neoplasia in dogs and cats. It is characterized by enlargement of one or many lymph nodes in the body. A contagious feline leukemia virus can be the cause of lymphoma in some cats.
Testicles – Testicular tumors are rare in cats and common in dogs, especially those with retained testicles (testicles that did not move to their normal positions during growth, and may be located in the abdomen or between the abdomen and scrotum).
Abdominal Tumors – Tumors inside the abdomen are common but it is difficult to make an early diagnosis. Weight loss or abdominal swelling are signs of these tumors.
Bone – Bone tumors are most often seen in large breed dogs and dogs older than seven years, and rarely in cats. The leg bones, near joints, are the most common sites. Persistent pain, lameness, and swelling in the affected area are common signs of the disease.

The Role of Comparative Oncology

The human-animal bond is stronger than you might think. In fact, the study of cancer in pet animals is shaping our understanding of cancers in people and leading to additional treatment options. According to a 2007 article by Philip J. Bergman, DVM, MS, PhD, DACVIM (Oncology) and Jeffrey Toll, VMD, DACVIM, the spontaneous cancers of pets treated by veterinary oncologists are similar to those arising in people. Dogs and humans are the only two species that naturally develop lethal prostate cancers. The type of breast cancer that affects dogs spreads to bones – just as it does in women. And the most frequent bone cancer of dogs, osteosarcoma, is the same cancer that strikes teenagers.
Based on these similarities between humans and companion animals, veterinary cancer research benefits both animals and humans with cancer. In fact, The National Cancer Institute’s Center for Cancer Research has instituted a critical Comparative Oncology Program where veterinary oncologists are using naturally occurring cancers in animals to better understand and treat cancer in humans.

What’s Next?

Emotions run high when we humans hear the word cancer. Should your veterinarian suspect cancer in your pet, the first step is to obtain a definitive diagnosis, develop a treatment plan along with your vet or veterinary specialist and prepare to be an advocate for your pet by arming yourself with information. Some other online sources of information include:

An estimated 6 million dogs and nearly 6 million cats will be diagnosed with cancer this year