Cancer in Dogs
September, 1999
Cancer is a major cause of death in dogs. As with people, it
is much more common in older individuals than in younger ones. As
with people, medical and surgical treatments are available. As
with people, those affected tend to have a poor long term
prognosis.
There are many different types of cancer, because every cell in
the body has the potential to become malignant. To understand
what cancer is, it is helpful to consider how the normal cells in
our bodies work. Normal cells perform a number of different
functions, but they all have one function in common - growth.
Nerve cells carry electrical impulses that cause muscle cells to
contract. Gonadal cells produce sperm or eggs for reproduction.
One type of cell in the pancreas produces enzymes for digestion,
while another produces insulin for blood sugar regulation. Normal
cells respond appropriately to a variety of stimuli by executing
their individual function. Normal cells also grow and divide at
an appropriate rate to replace the cells that age and die. Genes
inside the cell’s nucleus contain instructions for all of
the cell’s functions, including growth. A malignant cell is
formed when the genes are altered in such a way that they
instruct the cell to grow at an abnormal rate. This one function,
growth may take precedence over other functions that the cell
normally performs. In addition, all new cells formed by a
malignant cell inherit the altered genes, and are also malignant.
This leads to the formation of a tumor. Illness results when
tumor formation prevents an organ from performing its job well.
The most serious tumors are those that grow rapidly, invade
nearby tissues, or spread (metastasize) to other parts of the
body.
The causes of cancer, then, would be whatever causes these
alterations in the cell’s genes. Some known causes include
certain viruses, environmental carcinogens, and spontaneous
mutations. Heredity probably plays a role in susceptibility to
all of these.
The only known viral cause of tumors in dogs is the
papillomavirus. It causes warts, benign growths that usually
regress on their own. Viruses that cause lymphoma have been found
in other species, but not in dogs. Feline Leukemia Virus is a
well-known example of this.
Dogs are susceptible to most of the chemicals known to be
carcinogenic to humans. Exposure to ultraviolet rays of the sun
leads to squamous cell carcinoma, a skin cancer, mostly in white
cats but also likely in Collies and Shelties. Chronic lowgrade
infection or inflammation can lead to cancer. Fibrosarcoma, a
malignant growth, has been known to occur at the site of
vaccination in susceptible cats, but fortunately not in dogs.
Nasal filtration of cigarette smoke is apparently very efficient
in dogs, so naturally occurring lung cancer is rare. We
frequently find malignant tumors in dog’s lungs, but this is
when they have spread there from other sites, such as the mammary
gland or bone.
Diagnosing cancer is easy in some cases, and very difficult in
others. Most skin tumors are benign, but visual inspection doesn’t
guarantee the diagnosis. Any skin lump that grows rapidly,
changes suddenly, or bothers the dog, should be considered for
surgical removal. Definitive diagnosis is only accomplished by
sending a biopsy sample to a lab for microscopic exam. Some
exceptions to this general rule do exist. Lipomas, benign fatty
tumors, can reliably be identified by aspirating the mass with a
small needle. A rapidly growing lump in a mammary gland that
ulcerates through the skin, causing infection or bleeding, is
almost certainly cancerous. Aggressive nasal or oral tumors are
usually malignant. A routine x-ray may show typical changes of
bone cancer, but the more involved myelogram is usually needed to
see the outline of a tiny but debilitating tumor on the spinal
cord.
Cancers that affect internal organs are frequently difficult to
deal with. Routine blood tests can reliably identify problems
such as kidney failure and liver failure, but there is no blood
test to identify cancer as the cause of the organ failure.
However, the common CBC (complete blood count), may be all that
is needed to find blood cancers, or leukemias. Imaging
techniques, using X-rays or ultrasound, may show a large mass on
a spleen, but biopsy is still required to differentiate a
malignant tumor from a hematoma (pocket of blood and clots).
Tumors of the stomach or colon may be sampled using an endoscope,
but the majority of the intestinal tract can only be reached via
exploratory surgery. For tumors anywhere in the abdomen,
exploratory surgery allows an attempt at removal, at the same
time that the diagnosis is made. However, if the diagnosis is
cancer, then the problem will likely come back. Many dog owners
face the agonizing decision of either putting their companion to
sleep, or having costly procedures performed that may cause
additional pain and discomfort for the sick dog. And the decision
needs to be made with no guarantee that the animal will live
through the procedure, be cured, or just have an improved quality
of life before the disease returns.
Non-surgical cancer treatments, alone or in combination with
surgery, can also be used in dogs. Some cancers respond to
radiation, but this is usually expensive and only performed at
large teaching hospitals that are not accessible to all dog
owners. Chemotherapy, the use of drugs to kill cancer cells or
slow their growth, is frequently practiced by local
veterinarians. Sometimes this is also expensive, and side effects
in susceptible dogs may cause discontinuing the drug before it’s
had a chance to help. Even when these treatments are employed,
the goal is usually to get the cancer in remission and buy the
dog some quality time, but not to expect a cure.
There are no simple measures to take that will significantly
reduce a dog’s chance of developing all forms of cancer, but
there are some practices that will help. Obesity is a risk factor
in some cancers and can be avoided by paying attention to diet
and exercise. Spaying a female before her first heat practically
eliminates the chance of her developing mammary tumors later in
life. Each heat cycle she goes through increases her risk
dramatically. Ovarian and uterine cancers are uncommon in intact
bitches, but not seen after a spay. Similarly, neutering the male
eliminates the possibility of testicular tumors and makes
prostate cancer unlikely.