What is the incidence of cancer in our pets?

What do we know about the occurrence of cancer in animals? Most people are very surprised to learn that dogs and cats have a higher incidence of many tumors than do humans (fig.1).

Figure 1: A chart of comparative cancer rates for humans, dogs, and cats.

Dogs have 35 times as much skin cancer as do humans, 4 times as many breast tumors, 8 times as much bone cancer, and twice as high an incidence of leukemia. The only types of cancer that are more frequently seen in humans than in small animals are not surprising: lung cancer is 7 times higher in humans, and stomach/intestinal malignancies are 13 times more frequent in man than in dogs and cats. It is clear that the higher incidence of lung cancer in man is due to the human habit of smoking---but the cause of the higher incidence of gastrointestinal malignancies in man is not so clear. Perhaps it is related to diet, but it may also be a species idiosyncrasy; humans as a species may be exceptionally susceptible to tumors of the stomach and bowel.

Through careful statistical evaluation by veterinary epidemiologists (scientists who study patterns of disease in populations), some breed predilections for cancer have been noted. Ten years ago, if a veterinarian were to have been asked the breed of dog with the highest incidence of cancer, he or she would undoubtedly reply "the Boxer." When a sick, aged Boxer was presented at the Texas A&M University Veterinary Medical Teaching Hospital, we suspected a tumor almost immediately (fig.2).

Figure 2: A boxer with skin cancer.

Since then, however, several other breeds have been noted to have an extremely high incidence of cancer much like the Boxer; the Golden Retriever, Rottweiler, and Bernese Mountain Dog currently have an inordinately high incidence of certain tumors, and these dogs seem to develop cancer at an earlier age than dogs of other breeds. Other breeds with a notably high incidence of cancer are the Boston Terrier, English Bulldog, Scottish Terrier, and Cocker Spaniel. Breeds with a relatively low incidence of cancer include the Beagle, Poodle, Collie, and Dachshund. Why do some breeds have a high incidence of cancer while some are rarely affected by it? If we knew the answer to that question, we would undoubtedly be closer to preventing cancer than we are. Probably the answer will come through the study of molecular biology and genetics; many of these dogs have probably inherited (or acquired through mutation) abnormal genes that predispose them to cancerous transformation of cells (fig.3).

Figure 3: A Golden Retriever with swelling of the leg due to osteosarcoma.

Cancer is not only a problem of purebred dogs, however; if it were, the "mutt" should have one of the lowest incidences of cancer. Yet, ironically, the dog of mixed ancestry has an average incidence of cancer, along with the Irish Setter, Schnauzer, Labrador Retriever, and dogs of many other breeds.

Some types of tumors are more prone to develop in one type of dog than in others. For example, the giant-breed dogs like the St. Bernard and the Great Dane have a much higher incidence of osteosarcoma (a very malignant bone tumor) than does the general canine population. White cats, cats with white on their faces, and dogs with white hair and thin skin are prone to develop carcinomas in their skin due to long-term exposure to the ultraviolet rays of the sun. Black dogs have a comparatively high incidence of melanomas, or pigmented skin tumors. The female dog that is not neutered has 7 times the risk of developing mammary tumors as compared to the dog that is ovariohysterectomized early in life. This fact indicates that, as is the case in human females, the hormones estrogen and progesterone are potent stimulators of breast cancer in the dog (fig.4).

Figure 4: A dog with large mammary tumor.