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Sep 06 2011

Breast cancer and pets

Breast cancer is a well-known cause of illness and death in humans. We also know that early diagnosis of breast cancer is important for both treatment success and prognosis, which is the reason for the medical recommendations for monthly self-examinations and routine mammograms. What some people do not know, however, is that breast cancer is a relatively common form of cancer in all mammals, including dogs and cats.

Breast cancer can occur in both males and females, but is much more common in females. In some species of animals, viruses are important factors in inducing mammary cancer but, as far as we are aware, this is not true in dogs or cats.

Some breeds of dogs and cats appear to have a greater tendency (genetic susceptibility) to cancer. In its early stages, breast cancer is hormone dependent, but the specific levels of hormones that induce the development of cancer are different in different species of animals.

Since mammary tumors need hormones to start growing, the incidence of breast tumors is dramatically reduced by ovariohysterectomy (spaying) before the first estrus cycle. Studies show that if a dog is spayed before the first estrus cycle, she will have less than a 0.5% risk of developing breast cancer when compared to an intact female dog; her likelihood of development of breast cancer increases rapidly, with a 26% risk of development of breast cancer compared to an intact female if she is spayed after her second heat cycle.

For some benign tumors, removal of the source of hormones (i.e. spaying) may cause the tumor to shrink. However, once a breast tumor becomes malignant, hormones no longer influence their growth. With cats, malignant (life-threatening or spreading) types of breast cancer accounts for three-quarters of mammary lumps, while pre-cancerous growths account for a little less than one-quarter and about one percent are benign. With dogs, malignant tumors of mammary glands are less common than benign ones. Most dogs and cats that develop breast cancer are middle aged to older, but malignant tumors can occur when the pet is as young as two years.

Since the progression to malignancy is unpredictable, and even benign growths may become malignant with time, early surgical removal is always essential.

The most obvious sign that a cat or dog has breast cancer is the presence of one or more lumps in or around one of the mammary glands. In some cases, the tumor may secrete a clear, milky, or bloody discharge through the teat (nipple).

The simplest way to find a mass in the mammary glands is to regularly inspect the two rows of mammary glands that run along your pet’s chest and abdomen. When your pet is relaxed, gently rub their belly and feel around each nipple for any small lumps. These lumps can be as small as a pea, and can be either soft or firm. If you feel any lumps, book an appointment to take your pet to the veterinarian.

Never watch a breast or mammary nodule to “see what happens”- always see your veterinarian as soon as possible after you notice the lump, so that your veterinarian can evaluate the lump and plan for evaluation and possible surgical removal of any lump in the mammary gland(s). As with people, early diagnosis is crucial for treatment in dogs and cats.

We encourage all pet owners to do regular breast exams of their pets. Most pets enjoy a good belly rub and you can incorporate a breast examination into your regular petting of your furry family member. Please take a moment to examine your pet now, and repeat the examination regularly. If you have any questions at all, please don’t hesitate to contact us at the veterinary clinic.


Caution: These news items, written by Lifelearn Inc., are licensed to this practice for the personal use of our clients. Any copying, printing or further distribution is prohibited without the express written permission of Lifelearn Inc. Please note that the news information presented here is NOT a substitute for a proper consultation and/or clinical examination of your pet by our clinic veterinarian.

Lifelearn Admin