What are the effects of cancer on animals?

We have all seen people who are seriously debilitated by cancer, and animals are affected similarly. A malignant tumor may cause what is called "the cachexia of cancer"; the animal gradually starves to death as his tumor grows (fig.1).

Figure 1: Young Golden Retriever with emaciation due to widespread cancer.

Although tumors are not generally painful in and of themselves (they do not contain nerves), the growing tumor may produce pressure on normal tissues, such as bone; since most kinds of normal tissue contain an abundant supply of nerves, pain will ensue and may be extremely debilitating. The first sign of an osteosarcoma (bone cancer) in a large-breed dog may be lameness caused by pain in the normal bone tissue that is being invaded by the tumor (fig.2).

Figure 2: A radiograph of bony destruction caused by osteosarcoma .

With some cancers, such as bone marrow tumors, hemorrhage, fever, and infection may be seen because the normal white blood cells and platelets are being crowded out as the tumor grows. Cancers may invade regional blood vessels, leading to bleeding from the tumor and surrounding tissue. The tumor may ulcerate (have an eroded surface), bleed, and become infected on its surface. A common sign of intestinal cancer is blood loss into the bowel from the ulcerated surface of the mass; this is why occult blood tests are performed on middle-aged humans during their annual physical examination---to check for microscopic blood in the stool which might signal the presence of a cancer. An ulcerated tumor in the mouth of a dog or cat may become infected by the normal bacteria of the oral cavity, leading to the owner's complaint that his or her pet has "bad breath." The foul odor is caused by the bacterial infection on the surface of the tumor (fig.3).

Figure 3: A bacterial infection on the surface of large oral tumor.

Some tumors produce side effects in the host through the elaboration of hormones. Usually these hormones in small quantities are essential for life in the normal animal. However, the tumor may "go overboard," producing dangerously excessive quantities. For example, an insulinoma of the pancreas produces excessive insulin, and animals with this tumor will have very low blood sugar levels with resulting weakness or seizures. Thyroid adenomas (seen especially commonly in the cat) can produce excessive amounts of thyroid hormone, leading to nervousness, a rapid heart rate, weight loss, and diarrhea (fig.4).

Figure 4: A cat with severe weight loss due to hyperthyroidism.