■ Above: Excessive exposure to ultraviolet
light can cause squamous cell carcinoma of
the skin in light-colored or white dogs.
Left and below left: Dark-colored breeds can
develop tumors of the pigment cells
(melanoma), so again, it’s best to avoid
prolonged exposure to sunlight.
Incidence in Canines
Older animals have a higher incidence
of cancer. The reason for that is
unknown, but a variety of reasons have
been suggested. First, the competence of
the immune system declines with age
and is less able to destroy cancer cells as
they arise. Also, older animals have had
prolonged exposure to environmental
carcinogens that may have resulted in
tumor development. A variety of
environmental carcinogens exist that
are responsible for the birth of tumor
cells. Those carcinogens alter the DNA
and genes of normal cells and start the
growth and division of abnormal cells.
Some types of cancer — oral
papillomas for instance — are more
common in young than old animals. It
is interesting to note that these benign
tumors of the mouth
and lips are caused by
viruses. It is possible
that the immune
system of younger dogs
is less able to cope with
viral infection.
Overall, the
incidence of tumors in
female dogs is higher
than the tumor rate in
male dogs. That is due
to the very high
incidence of breast
cancer in female dogs. Intact
(unspayed) female dogs have a seven
times greater risk of developing
mammary cancer than spayed females.
Spaying before two years of age lessens
the possibility that the dog will develop
canine mammary cancer. Spaying after
two years of age does not have the same
result, so there are good reasons for
spaying early other than the risk of
unwanted puppies. Male dogs that have
undescended, retained
testicles have a greater
risk of developing
testicular cancer in the
undescended testicle
than in the normal
testicle in the scrotum.
A variety of
environmental
carcinogens
exist that are
responsible for
the birth of
tumor cells.
Breed-Specific Cancers
The domesticated dog
has been altered and
manipulated by artificial,
selective breeding prac-
tices more than any other
type of animal. It appears that by select-
ing for certain desirable characteristics,
humans have unwittingly selected for
certain types of tumors. When a breed
becomes popular, unscrupulous breeders
may mate closely related dogs in order
to quickly increase the supply of
