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