Why Do So Many Dogs Develop Cancer?

Cancer is common in dogs. Atleast 50% of dogs over 10 will die from cancer. If we can make an early diagnosis and treat cancer appropriately, we can change the course for many patients who would have otherwise passed due to cancer.

So why do they develop cancer in the first place?

In veterinary oncology, we typically study "risk factors" for each type of cancer. A risk factor is something that increases the likelihood that a dog will develop a certain type of cancer.

It is very difficult to study risk factors in dogs and in people. There are so many different genetic and environmental factors that can influence the development of cancer; reliably teasing out which ones led to cancer in a single patient is incredibly difficult (and expensive).

In order to be successful, researchers typically have to study a certain patient population (ex. thousands of golden retrievers) over their entire lifetime (puppy to death) and have excellent owner compliance (participate in all of the visits, which are typically every 6-12 months for the dog's entire life and agree to donate the dog's body after he passes). As you can imagine, this is complicated by many factors including people moving, other life changes, lack of funding, etc.

These studies are also incredibly expensive (millions of dollars), incredibly time consuming, and take more than a decade to complete. This funding doesn't typically exist to study cancer in dogs like it does for people.

Most of the information that we have that details potential risk factors in canine cancer are from retrospective studies. In a retrospective study researchers look at years of patient records after dogs have already developed cancer and try to figure out what the dogs had in common that might have led to cancer development. These records are typically incomplete and difficult to read/interpret (many vet records were and still are hand-written), so it makes drawing reliable conclusions very difficult. Retrospective studies are much less reliable than the prospective study described above.

The Morris Animal Foundation is currently in the process of carrying out the Golden Retriever Lifetime Study. This study is following more than 3000 golden retrievers to identify nutritional, environmental, lifestyle and genetic risk factors that might lead to the development cancer and other diseases (>60% of golden retrievers develop cancer). This is a very expensive undertaking; there is a donation link on the GRLS website if you would like to consider donating.

Risk Factors For The Development of Cancer In Dogs:

(1) Age: Every day new cells are being made in our body and in dogs' bodies. Every day there are millions of mistakes being made during this process. The vast majority of these mistakes are caught and corrected by DNA repair machinery. If a mistake is not caught and corrected, it could lead to cancer. The longer a dog lives, the more likely it is that one of these events (mutations) will lead to cancer. Simply being alive (for long enough) is a risk factor for developing cancer.

(2) Breed: In veterinary oncology, we consistently see the same types of cancers in the same breeds. Certain breed predispositions are listed in this article. There is likely a genetic component to the development of many types of canine cancers. A discussion of genetics is beyond the scope of this article; if you're interested in reading scientific publications on canine cancer genetics go to Pubmed.gov.

A few notable breed-related cancer statistics:

A - Bernese mountain dog: 25% lifetime risk of histiocytic sarcoma

B - Scottish terriers: 16 times more likely to develop transitional cell carcinoma (TCC = bladder cancer) than other breeds *Scotties fed vegetables 3 times per week (best type unknown, but carrots were used frequently) had a 30% reduced risk of TCC

C - Golden retriever: Up to 65% die due to cancer

(3) Obesity: Obese dogs are more likely to develop mammary cancer, develop it at a younger age, and develop more aggressive mammary cancer. Obese dogs are more likely to develop transitional cell carcinoma of the urinary bladder.

(4) Environmental tobacco smoke (second-hand smoke): Long-nosed dogs (dolichocephalic) are twice as likely to develop nasal cancer if they live with a smoker. The risk of nasal cancer increases by 50% for short and medium-nosed breeds that live with a smoker. One study showed that dogs that lived with a smoker developed more aggressive lymphomas than those that did not live with a smoker.

(5) Industrial environments; Use of paints/solvents: Living in an industrial environment and the use of paints/solvents by owners may be associated with the development of lymphoma and an earlier onset of lymphoma in dogs. Living in an urban environment was shown to increase the risk of mesothelioma in one study.

(6) Exposure to herbicide-treated lawns: This significantly increases the risk of transitional cell carcinoma of the urinary bladder (bladder cancer). One study showed 2,4-D (herbicide) in the urine of 4 of 8 dogs in the "untreated" households group, suggesting chemical drift from nearby treated lawns. Scottish terriers exposed to lawns and gardens treated with BOTH herbicides AND insecticides were 7.19 times more likely to develop bladder cancer (transitional cell carcinoma). Scottish terriers exposed to phenoxy-herbicide treated lawns and gardens were 4.42 times more likely to develop bladder cancer (TCC). They were not more likely to develop TCC if exposed to insecticides alone.

(7) Exposure to estrogen (not spaying female dogs): Dogs spayed before the first heat cycle have a 0.5% risk of mammary tumor development. The risk increases to 26% if they are spayed between their 2nd and 3rd heat cycle. In dogs, 50% of mammary tumors are malignant.

(8) Lack of neuter in male dogs: The incidence of testicular tumors is 27% in intact dogs. Most testicular tumors are not fatal, however scrotal mast cell tumors can be very aggressive and potentially life limiting.

(9) Spay & Neuter & Osteosarcoma: Lack of spay and neuter increases the risk for developing osteosarcoma in dogs (Colorado State study). Rottweilers spayed/neutered before 1 year of age had an increased risk (25%) of developing osteosarcoma.

*This suggests that Rottweilers should not be spayed/neutered as young as other breeds. This is a complicated breed-related discussion/decision that you should discuss with your vet/oncologist.

(10) Asbestos-related occupation or hobby: If an owner has an asbestos-related occupation or hobby, their dog is more likely to develop mesothelioma.

(11) Sun exposure: Dermal (skin) hemangiosarcoma and squamous cell carcinoma are two common solar-induced cancers in dogs. They frequently occur in light-haired dogs, thin-haired dogs (greyhound, whippet) and/or on regions that have poor hair coverage (the skin on the abdomen, inguinal area). They are also commonly found on dogs that enjoy "sun bathing".

(12) Indoor coal/kerosene heaters: Dogs that lived in an environment with an indoor coal heater were 4.2 times more likely to develop nasal cancer, and 2.2 times more likely to develop nasal cancer with a kerosene heater. Effects were most significant for long-nosed (dolichocephalic) breeds.

If you currently have a dog that doesn't have cancer, I would recommend reducing risk factors based on this list and educating yourself on the best ways to improve the chances of early detection. I would also recommend purchasing a good pet insurance policy (if you think you would treat your dog with surgery, etc. if the need arose). A good pet insurance policy allows you to take potential financial impact out of the decision-making process, which can be a huge relief.

NOTE: For a full discussion of the factors that increase and decrease cancer risk in dogs (as well as how to keep your dog as happy and healthy as possible), check out The Happy Healthy Dog Guide!

Dr. Lori Cesario

Board Certified Veterinary Oncologist

PS: I'm happy to now offer online oncology consultations. Learn more about how this service can help by visiting the Vet Cancer Consultants site.

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