Cancer is very common in pets just like it is in people. In fact, it is the leading cause of death in older dogs. Owners need to be alert to signs of this dreaded disease to increase the chances of an early diagnosis and successful treatment.
What is Cancer?
Cancer, or neoplasia, is the abnormal and uncontrolled growth of cells in the body. In health, cells divide and reproduce themselves in an orderly manner, but when the cells are transformed and the normal controls fail, cancer is the result. Some types of cancer form discrete tumors, masses that can be seen or felt. Others, like leukemia, involve the overproduction of individual cells that then circulate throughout the body.
Different terms are used to describe the tendency of a cancer to act in an aggressive manner. “Benign”; means that a tumor is unlikely to invade adjacent tissues or spread to other parts of the body. The word “malignant” indicates just the opposite. If a cancer has “metastasized,” it means that it has spread from its original location to more distant parts of the body (e.g., from a mammary gland to the lungs). Malignant and/or metastatic neoplasias are more difficult to treat than are those that are benign.
Cancer can develop in almost any organ or tissue in the body, but different types of cancer can affect the same body part and have very different outcomes. For example, one dog might have a type of liver cancer that causes only mild symptoms, has not spread outside of one section of the liver, and surgery to remove the mass could be completely curative. On the other hand, another dog with a different type of liver cancer could be extremely sick, have tumors throughout his body early in the course of the disease, and not respond well to even the most advanced treatments available. In short, not all cancers, even if they arise in the same location, are alike.
To diagnose cancer and determine what type is involved, a veterinarian will need to take a tissue sample, either by removing cells using a needle and possibly a syringe or by taking a larger piece of tissue via a surgical biopsy. In some cases, the veterinarian may be able to make the diagnosis by looking at cells under a microscope in the clinic, but usually the sample is sent to a lab for a veterinary pathologist to evaluate. These tests are necessary to plan appropriate treatment and to give owners an accurate prognosis.
What Causes Cancer in Dogs
Some types of canine cancer are caused by environmental factors such as second-hand smoke, sun exposure, or contact with certain chemicals. Other cancers have a genetic basis and are more commonly diagnosed in certain breeds or run in families. For the most part though, it is impossible to determine exactly why one particular individual develops cancer, while another, under much the same conditions, does not. Surely many factors, both within the body and in the environment, play a role.
A diagnosis of cancer becomes more likely as pets age, probably because cellular damage has had a chance to accumulate over time.
Is Cancer Contagious?
There is one canine neoplasia that is caused by a contagious virus. Dogs that are infected with the canine papilloma virus develop warts, but these tend to disappear on their own without treatment. Another type of cancer, called transmissible venereal tumor, is caused by the direct spread of cancerous cells from one individual to another. So far, no infectious agent other than the tumor cells themselves has been identified. It is important to note that cancer has never been known to spread from a dog to a person, or vice-versa.
Signs of Cancer in Dogs
The symptoms associated with cancer depend on where the disease is located, what type is involved, and how advanced the disease is. None of the clinical signs listed below are unique to cancer, but all should be checked out by a veterinarian.
- Weight loss, especially if muscle mass throughout the body is also declining
- Loss of appetite
- Masses in or under the skin or in the mouth, particularly if they feel like they are attached to surrounding structures or grow or change in appearance over time
- Any asymmetry in the body (e.g., one eye is larger and looks different than the other)
- Enlarged lymph nodes. Owners may notice masses where the lower jaw meets the neck, in front of the shoulders at the base of the neck, or behind the knees.
- Rapid or difficult breathing
- Chronic sneezing
- Difficulty swallowing
- Unexplained vomiting or diarrhea
- Dark, tarry feces
- Straining to urinate or defecate
- Bleeding unassociated with trauma (including blood in the urine or stool)
- An enlarged abdomen
- A limp that worsens over time
- An inability to move one or more limbs normally
- Behavioral changes (e.g., unexplained aggression or constant circling)
- Paleness or yellowing of the mucous membranes or skin
- Wounds that don’t heal as expected
- An unusual discharge or odor from any part of the body
Some types of cancer can be difficult for owners to detect until they are quite advanced. An older pet should visit the veterinarian every six months for a physical exam and any recommended diagnostic tests.
Article by: Jennifer Coates DVM