What causes cancer in cats and dogs?
Cancer begins with changes (called mutations) in the genetic material called DNA. DNA is present in every cell and
encodes instructions for the cells on how to function, when to divide and when to die. Changes to DNA will, therefore, result
in a different set of instructions for the cell, which can cause abnormal behavior (i.e. the cells will not stop dividing even
though they should).

The initial change to the DNA provides the cell with an advantage for continuous growth but is not enough to cause cancer.
A number of additional changes must occur in order to promote both uncontrollable growth and acquiring the ability to
spread (
metastasize) to other parts of the body. Just like in humans, the accumulation of these changes is a slow process
and explains why cancer is usually seen in older pets.

The changes in the DNA can be inherited or caused by chronic
inflammation, hormones, viruses, cancer-causing chemical
compounds (
carcinogens), or factors present in our environment such as ultraviolet radiation from the sun.

Pesticides and Herbicides
There have been several inconclusive studies about the relationship between exposure to herbicides/pestidices present in
lawn care products and the development of cancer in pets. Because not enough studies have been conducted to clearly
prove or disprove the increased risk for cancer incidence, owners should strive to minimize  pets' access to these products.

There is substantial evidence that sunlight or ultraviolet radiation act as causative agents of skin cancer both in humans
and companion animals. Long-term exposure to sunlight can cause changes (mutations) to the genetic material of skin
cells (DNA), which can then ultimately result in development of skin tumors. Pets should be protected from direct sunlight
during the midday hours, especially if they have light skin pigmentation.  

Previous therapeutic radiation
There is some evidence suggesting that secondary tumors can develop in sites previously exposed to radiation during
cancer treatment in dogs. The concern for this effect, however, is primarily for younger dogs who are expected to enjoy
long-term survival after the treatment of their primary cancer, since these secondary tumors take years to develop.

Asbestos exposure has been shown to be associated with development of cancer called mesothelioma in people. Similar
link has been found for dogs whose owners have been associated with asbestos-related jobs or hobbies.

There has been accumulating evidence that hormones are associated with the development of breast cancer in both dogs
and cats. For example, female dogs treated with products that contain hormones had an increased incidence of breast
cancer. Furthermore, female dogs spayed before their first menstrual period and female cats spayed prior to 6 months of
age have a substantially decreased risk of developing breast cancer.

Perianal adenomas are usually benign, slow growing tumors that grow under the influence of testosterone near the rectum.
Testosterone is a male hormone produced mostly by the testes and to a lesser extent by the adrenal glands. Perianal
adenomas may occur as single or multiple masses confined to the skin and affect intact, middle-aged to older male dogs.
They are not painful but may stimulate licking of the affected area or possibly scooting.  Female dogs infrequently develop
these tumors with most affected female dogs having been spayed. No clear relationship at this time has been established
between developing hormone dependent prostate cancer and age at castration in dogs.

Papillomaviruses are infectious and contagious viruses associated with causing benign tumors in several animal species
including humans. Papillomaviruses frequently infect younger dogs, producing benign papillomas, also known as warts.
The warts are often seen in the mouth and less often on the skin and in the eye. The warts begin spontaneous regression
after 4-6 months in the mouth and 6-12 months on the skin. Papillomas are usually benign but in rare cases can transform
malignant tumors.

In contrast to dogs, papillomaviruses frequently infect older cats. Recent studies have shown that there is some
association between papillomavirus and malignant cancers in cats. The lesions caused by this virus are more like plaques
than warts and often affect haired skin rather than the mouth as seen in dogs

The cat is also affected by another class of viruses, called retroviruses. These viruses have been shown to produce a wide
spectrum of diseases, including cancer. Members of this family of viruses include the feline leukemia virus (FLV), feline
immunodeficiency virus (FIV) and feline sarcoma virus (FSV). Cats infected with FLV are at risk of developing a variety of
blood cancers, cats infected with FIV are at risk of developing several types of cancers including blood cancers, and cats
infected with FSV are risk of developing rapidly growing sarcomas.
© 2007 Pet Cancer Center. ALL RIGHTS RESERVED.
Last updated 2/19/2017
Comprehensive guide to cancer diagnosis and treatment in cats and dogs
What causes cancer?