By Dr. Kathy Boehme

Unfortunately, cancer is one of the most common causes of death in our pets. In fact, statistics show that one in four dogs and one in six cats will develop cancer during their lifetime!

In dogs over 10 years of age, 50 percent will develop cancer. In addition, large breed dogs have twice the cancer risk of small breeds. Mast cell tumors are the most common malignant tumor in dogs, while lymphoma is most common in cats.

Causes of cancer in people include genetic predisposition (although actual inherited cancer is rare), alcohol consumption and environmental exposure to toxins. It is also suggested that one third of all cancer deaths in humans are related to diet and exercise. Of course, these same factors also raise the risk of a myriad of other chronic illnesses as well.

The cause of cancer in humans is complicated and even now, there is much we do not yet know. We know even less about cancer in pets. While it is important to remember that dogs and cats are not small humans, some relationships between cancer in people and pets have been seen.

  • Genetics: We know genetics definitely play a role in cancer development, particularly in dogs.

Genetic studies have linked multiple forms of cancer with specific breeds and more will likely be discovered as genetic markers are identified. The good news is that once we have these markers, they can hopefully be “bred out” of genetic lines.

Two prominent examples include Scottish terriers, who have up to 20 times the risk of developing bladder cancer than other breeds, and golden retrievers, who may be prone to a familial form of lymphosarcoma. These inherited forms of cancer tend to present at a younger age than acquired forms.

  • Environmental pollutants: Environmental toxins have been linked to cancer in both dogs and cats.

Many studies have shown an increase in the accumulation of known carcinogens from second-hand cigarette smoke in our pets’ bodily tissues. In dogs, second-hand smoke has been linked to tumor formation in the lungs and an increase in nasal adenocarcinoma, particularly in long-nosed breeds. This type of environmental exposure has also been shown to increase the risk of two different forms of cancer in cats.

Second-hand smoke is not the only environmental toxin with the potential to cause disease, however. One study showed a strong link between the development of bladder cancer in Scottish terriers exposed to lawns and gardens treated with herbicides and insecticides. The study found that exposure to lawns and gardens treated with both herbicides and insecticides or herbicides alone increased the dogs’ risk, while exposure to lawns and gardens treated with insecticides alone did not.

In a separate study, dogs diagnosed with bladder cancer were compared with control dogs to determine the risk of cancer development with exposure to a number of different factors, including cigarette smoke, household chemicals (cleaners), topical insecticides and obesity. The study showed that cancer development was unrelated to cigarette smoke and household chemicals, but an increased risk occurred with topical insecticide use. Moreover, this increased risk was found to be significantly enhanced by obesity.

Topical exposure to older generation insecticides has also been shown to increase the risk of oral cancer in cats.

Another study conducted in Italy compared cancer rates in dogs and cats living near a waste management facility to those of pets living away from this type of exposure. It showed an overall increased risk in lymphoma in dogs, but not in cats. However, the risk of mast cell and mammary tumors between the two groups was the same.

Morris Animal Foundation’s Golden Retriever Lifetime Study, the largest and longest observational canine study conducted to date, may help shed more light on environmental risk factors linked to cancer and other chronic illnesses in this breed and allow us to better understand and prevent these conditions in all dogs.

  • Viruses: Viruses can induce tumorigenesis, or the creation of cancer.

For example, we know that cats can develop lymphoma after contracting the feline leukemia virus (FeLV). It is also possible for cats to develop fibrosarcomas at vaccine sites. The incidence of these sarcomas has been significantly reduced in recent years by altering vaccine protocols.

Dogs can develop oral skin tumors known as papillomas after an infection with a papilloma virus, which is contagious from other dogs. Fortunately, this occurs mainly in young dogs and frequently resolves without treatment.

  • Hormones: We know that sex hormones also play a role in cancer.

Female dogs and cats spayed after one or two heat cycles have an increased risk of mammary adenocarcinoma.

Currently, there is also some controversy regarding the timing of spaying and neutering due to the risk of orthopedic disease and cancer in Labrador and golden retrievers.

  • Solar radiation: Solar radiation (i.e. sun bathing) increases the risk of hemangioma and hemangiosarcoma on the skin of dogs, especially those with a sparse or lightly colored hair coat.

What about diet, exercise and body weight?

Long-term studies regarding the relationship between these factors and the development of cancer in dogs and cats are just getting started, but common sense tells me a link most likely exists.

We already know that the same hormonal and inflammatory changes that occur in overweight and sedentary humans occur in overweight pets. A study on the effects of food restriction and lower body weight on lifespan and age-related changes showed a significantly longer lifespan in dogs who remained lean throughout their lives. This study looked not only at cancer but at mortality from a variety of chronic illnesses. Overall, the cancer rate was similar between lean dogs and normal or overweight dogs, but the age they succumbed to cancer was significantly later.

Another study regarding the prevention of bladder cancer in Scottish terriers showed that eating fruits and vegetables at least three times a week decreased the incidence of the disease by 50 percent.

While we certainly have much more to learn, we can begin protecting our pets with the information we already have. In my next article, I will focus on current recommendations for cancer prevention.