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Tumors and Cancers in Dogs

What Causes Cancer? continued...

Anything that disrupts the genes that govern cell duplication results in the production of mutant cells. Mutant cells often reproduce at an extraordinary rate and form large masses that crowd out normal cells. Such a mass is called a cancer. Further, cancerous cells do not function as normal cells and thus do not provide needed services. If the cancer grows unchecked, it eventually replaces much of the organ while also metastasizing to other parts of the body. In time, it causes the death of the dog.

Some cancer-producing genes are inherent in the breed or genetic makeup of a dog. Bernese Mountain Dogs, for example, have a high incidence of cancers affecting all body systems. Approximately one in four Bernese Mountain Dogs will develop cancer; two of the cancer types seen in this breed-histiocytosis and mastocytoma-are known to be inherited as polygenic traits.

A number of genes have been identified as causing breast, colon, and other cancers in people and in some animals. The reason that all individuals with these genes do not develop cancer is that there are other specific genes that suppress the cancer genes. To complicate matters, there are still other genes that inhibit the suppressors. All these genes are turned on and off by external factors, such as diet, stress, and environment. Thus, cancer is a largely unpredictable phenomenon involving a complex interaction of genetics and the environment. A good example is bladder cancer in Scottish Terriers. Scotties have an increased risk of bladder cancer to begin with. If you add in exposure to lawn chemicals that contain 2,4 D, the risk increases four to seven times. In this case, genes and an environmental exposure work together to cause the cancer.

Carcinogens are environmental influences known to increase the likelihood of cancer in direct proportion to the length and intensity of exposure. Carcinogens gain access to tissue cells, cause alterations in genes and chromosomes, and disrupt the system of checks and balances that controls orderly growth. Examples of carcinogens known to increase the risk of cancer in humans are ultraviolet rays (which can cause skin cancers), X-rays (thyroid cancers), nuclear radiation (leukemia), various chemicals (aniline dyes cause bladder cancer), cigarettes and coal tars (lung, bladder, skin, and many other cancers), viruses (sarcoma in AIDS patients), and internal parasites (bladder cancers). Secondhand smoke exposure is associated with cancer in animals as well as in humans.

Injuries are sometimes implicated as causing cancers, but there is seldom a connection. Trauma causes hematomas, bruises, and contusions, but does not cause abnormal cell growth. However, an injured site is usually examined closely, and small preexisting tumors are sometimes discovered this way. Some veterinarians believe bone cancers may be more likely to develop at the site of previous fractures.

Some benign tumors, such as warts and papillomas, are clearly due to a virus. Other benign tumors simply grow for unknown reasons.

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WebMD Veterinary Reference from "Dog Owner's Home Veterinary Handbook"

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